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SDCC Compiler User Guide

Table of Contents

1 Introduction
    1.1 About SDCC
    1.2 Open Source
    1.3 System Requirements
    1.4 Other Resources
2 Installation
    2.1 Linux/Unix Installation
    2.2 Windows Installation
        2.2.1 Windows Install Using a Binary Package
        2.2.2 Windows Install Using Cygwin
    2.3 Testing out the SDCC Compiler
    2.4 Install Trouble-shooting
        2.4.1 SDCC cannot find libraries or header files.
        2.4.2 SDCC does not compile correctly.
        2.4.3 What the ./configure does
        2.4.4 What the make does.
        2.4.5 What the make install command does.
    2.5 Additional Information for Windows Users
        2.5.1 Getting started with Cygwin
        2.5.2 Running SDCC as Native Compiled Executables
    2.6 SDCC on Other Platforms
    2.7 Advanced Install Options
    2.8 Components of SDCC
        2.8.1 cpp ( C-Preprocessor)
        2.8.2 asxxxx & aslink ( The assembler and Linkage Editor)
        2.8.3 SDCC - The compiler
        2.8.4 S51 - Simulator
        2.8.5 SDCDB - Source Level Debugger
3 Using SDCC
    3.1 Compiling
        3.1.1 Single Source File Projects
        3.1.2 Projects with Multiple Source Files
        3.1.3 Projects with Additional Libraries
    3.2 Command Line Options
        3.2.1 Processor Selection Options
        3.2.2 Path, Lib and Define Options
        3.2.3 MCS51 Options
        3.2.4 Optimization Options
        3.2.5 DS390 Options
        3.2.6 Other Options
        3.2.7 Intermediate Dump Options
    3.3 MCS51 Storage Class Language Extensions
        3.3.1 xdata
        3.3.2 data
        3.3.3 idata
        3.3.4 bit
        3.3.5 sfr / sbit
    3.4 Pointers
    3.5 Parameters & Local Variables
    3.6 Overlaying
    3.7 Critical Functions
    3.8 Absolute Addressing
    3.9 Interrupt Service Routines
    3.10 Startup Code
    3.11 Inline Assembler Code
    3.12 int(16 bit) and long (32 bit ) Support
    3.13 Floating Point Support
    3.14 MCS51 Memory Models
    3.15 Flat 24 bit Addressing Model
    3.16 Defines Created by the Compiler
4 SDCC Technical Data
    4.1 Optimizations
        4.1.1 Sub-expression Elimination
        4.1.2 Dead-Code Elimination
        4.1.3 Copy-Propagation
        4.1.4 Loop Optimizations
        4.1.5 Loop Reversing:
        4.1.6 Algebraic Simplifications
        4.1.7 'switch' Statements
        4.1.8 Bit-shifting Operations.
        4.1.9 Bit-rotation
        4.1.10 Highest Order Bit
        4.1.11 Peep-hole Optimizer
    4.2 Pragmas
    4.3 Library Routines
    4.4 Interfacing with Assembly Routines
    4.5 Global Registers used for Parameter Passing
        4.5.1 Assembler Routine(non-reentrant)
        4.5.2 Assembler Routine(reentrant)
    4.6 With --noregparms Option
        4.6.1 Assembler Routine Non-reentrant
        4.6.2 Assembler Routine(reentrant)
    4.7 External Stack
    4.8 ANSI-Compliance
    4.9 Cyclomatic Complexity
5 TIPS
6 Retargetting for other MCUs.
7 SDCDB - Source Level Debugger
    7.1 Compiling for Debugging
    7.2 How the Debugger Works
    7.3 Starting the Debugger
    7.4 Command Line Options.
    7.5 Debugger Commands.
        7.5.1 break [line | file:line | function | file:function]
        7.5.2 clear [line | file:line | function | file:function ]
        7.5.3 continue
        7.5.4 finish
        7.5.5 delete [n]
        7.5.6 info [break | stack | frame | registers ]
        7.5.7 step
        7.5.8 next
        7.5.9 run
        7.5.10 ptype variable 
        7.5.11 print variable
        7.5.12 file filename
        7.5.13 frame
        7.5.14 set srcmode
        7.5.15 ! simulator command
        7.5.16 quit.
    7.6 Interfacing with XEmacs.
8 Other Processors
    8.1 The Z80 and gbz80 port
9 Support
    9.1 Reporting Bugs
    9.2 Acknowledgments



1 Introduction

1.1 About SDCC

SDCC is a Free ware, retargettable, optimizing ANSI-C compiler
by Sandeep Dutta designed for 8 bit Microprocessors. The
current version targets Intel MCS51 based Microprocessors(8051,8052,
etc), Zilog Z80 based MCUs, and the Dallas 80C390 MCS51
variant. It can be retargetted for other microprocessors,
support for PIC, AVR and 186 is under development. The entire
source code for the compiler is distributed under GPL. SDCC
uses ASXXXX & ASLINK, a Freeware, retargettable assembler
& linker. SDCC has extensive language extensions suitable
for utilizing various microcontrollers underlying hardware
effectively. In addition to the MCU specific optimizations
SDCC also does a host of standard optimizations like global
sub expression elimination, loop optimizations (loop invariant,
strength reduction of induction variables and loop reversing),
constant folding & propagation, copy propagation, dead code
elimination and jumptables for 'switch' statements. For
the back-end SDCC uses a global register allocation scheme
which should be well suited for other 8 bit MCUs. The peep
hole optimizer uses a rule based substitution mechanism
which is MCU independent. Supported data-types are short
(8 bits, 1 byte), char (8 bits, 1 byte), int (16 bits, 2
bytes ), long (32 bit, 4 bytes) & float (4 byte IEEE). The
compiler also allows inline assembler code to be embedded
anywhere in a function. In addition routines developed in
assembly can also be called. SDCC also provides an option
to report the relative complexity of a function, these functions
can then be further optimized, or hand coded in assembly
if need be. SDCC also comes with a companion source level
debugger SDCDB, the debugger currently uses ucSim a freeware
simulator for 8051 and other micro-controllers. The latest
version can be downloaded from [http://sdcc.sourceforge.net/].

1.2 Open Source

All packages used in this compiler system are opensource(freeware);
source code for all the sub-packages (asxxxx assembler/linker,
pre-processor) are distributed with the package. This documentation
is maintained using a freeware word processor (LyX). 

This program is free software; you can redistribute it and/or
modify it under the terms of the GNU General Public License
as published by the Free Software Foundation; either version
2, or (at your option) any later version. This program is
distributed in the hope that it will be useful, but WITHOUT
ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
Public License for more details. You should have received
a copy of the GNU General Public License along with this
program; if not, write to the Free Software Foundation,
59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
In other words, you are welcome to use, share and improve
this program. You are forbidden to forbid anyone else to
use, share and improve what you give them. Help stamp out
software-hoarding! 

1.3 System Requirements

What do you need before you start installation of SDCC? A
computer, and a desire to compute. The preferred method
of installation is to compile SDCC from source using GNU
GCC and make. For Windows some pre-compiled binary distributions
are available for your convenience. You should have some
experience with command line tools and compiler use.

1.4 Other Resources

The SDCC home page at [http://sdcc.sourceforge.net/]
is a great place to find distribution sets. You can also
find links to the user mailing lists that offer help or
discuss SDCC with other SDCC users. Web links to other SDCC
related sites can also be found here. This document can
be found in the DOC directory of the source package as a
text or HTML file. Some of the other tools(simulator and
assembler) included with SDCC contain their own documentation
and can be found in the source distribution. If you want
the latest unreleased software, the complete source package
is available directly by anonymous CVS on www.sourceforge.net.

2 Installation

2.1 Linux/Unix Installation

1. Download the source package, it will be named something
  like sdcc-2.x.x.tgz.

2. Bring up a command line terminal, such as xterm.

3. Unpack the file using a command like: tar -xzf sdcc-2.x.x.tgz,
  this will create a sub-directory called sdcc with all
  of the sources.

4. Change directory into the main SDCC directory, for example
  type: "cd sdcc".

5. Type "./configure". This configures
  the package for compilation on your system.

6. Type "make". All of the source packages
  will compile, this can take a while.

7. Type "make install" as root.
  This copies the binary executables to the install directories.

2.2 Windows Installation

For installation under Windows you first need to pick between
a pre-compiled binary package, or installing the source
package along with the Cygwin package. The binary package
is the quickest to install, while the Cygwin package includes
all of the open source power tools used to compile the complete
SDCC source package in the Windows environment. If you are
not familiar with the Unix command line environment, you
may want to read the section on additional information for
Windows users prior to your initial installation.

2.2.1 Windows Install Using a Binary Package

1. Download the binary package and unpack it using your favorite
  unpacking tool(gunzip, WinZip, etc). This should unpack
  to a group of sub-directories. An example directory structure
  after unpacking is: c:\usr\local\bin for the executables,
  c:\usr\local\share\sdcc\include and c:\usr\local\share\sdcc\lib
  for the include and libraries.

2. Adjust your environment PATH to include the location of
  the bin directory. For example, make a setsdcc.bat file
  with the following: set PATH=c:\usr\local\bin;%PATH%

3. When you compile with sdcc, you may need to specify the
  location of the lib and include folders. For example,
  sdcc -I c:\usr\local\share\sdcc\include -L c:\usr\local\share\sdcc\lib\small
  test.c

2.2.2 Windows Install Using Cygwin

1. Download and install the cygwin package from the redhat
  site[http://sources.redhat.com/cygwin/]. Currently,
  this involved downloading a small install program which
  then automates downloading and installing selected parts
  of the package(a large 80M byte sized dowload for the
  whole thing). 

2. Bring up a Unix/Bash command line terminal from the Cygwin
  menu.

3. Follow the instructions in the preceding Linux/Unix installation
  section.

2.3 Testing out the SDCC Compiler

The first thing you should do after installing your SDCC
compiler is to see if it runs. Type "sdcc
--version" at the prompt, and the program should
run and tell you the version. If it doesn't run, or gives
a message about not finding sdcc program, then you need
to check over your installation. Make sure that the sdcc
bin directory is in your executable search path defined
by the PATH environment setting(see the Trouble-shooting
section for suggestions). Make sure that the sdcc program
is in the bin folder, if not perhaps something did not install
correctly.

SDCC binaries are commonly installed in a directory arrangement
like this:

+--------------------------------+-------------------------------------------+
| /usr/local/bin                 | Holds executables(sdcc, s51, aslink, ...) |
+--------------------------------+-------------------------------------------+
+--------------------------------+-------------------------------------------+
| /usr/local/share/sdcc/lib      | Holds common C libraries                  |
+--------------------------------+-------------------------------------------+
| /usr/local/share/sdcc/include  | Holds common C header files               |
+--------------------------------+-------------------------------------------+


Make sure the compiler works on a very simple example. Type
in the following test.c program using your favorite editor:

main()
{ int i;
i = 0;
i += 10;
}


Compile this using the following command: "sdcc
-c test.c". If all goes well, the compiler will
generate a test.asm and test.rel file. Congratulations,
you've just compiled your first program with SDCC. We used
the -c option to tell SDCC not to link the generated code,
just to keep things simple for this step.

The next step is to try it with the linker. Type in "sdcc
test.c". If all goes well the compiler will link with
the libraries and produce a test.ihx output file. If this
step fails(no test.ihx, and the linker generates warnings),
then the problem is most likely that sdcc cannot find the
usr/local/share/sdcc/lib/small directory(see the Install
trouble-shooting section for suggestions).

The final test is to ensure sdcc can use the standard header
files and libraries. Edit test.c and change it to the following:

#include <string.h>
main()
{ char str1[10];
strcpy(str1, "testing");
}

Compile this by typing: "sdcc test.c".
This should generate a test.ihx output file, and it should
give no warnings such as not finding the string.h file.
If it cannot find the string.h file, then the problem is
that sdcc cannot find the /usr/local/share/sdcc/include
directory(see the Install trouble-shooting section for suggestions).

2.4 Install Trouble-shooting

2.4.1 SDCC cannot find libraries or header files.

The default installation assumes the libraries and header
files are located at "/usr/local/share/sdcc/lib"
and "/usr/local/share/sdcc/include".
An alternative is to specify these locations as compiler
options like this: sdcc -L /usr/local/sdcc/lib/small -I
/usr/local/sdcc/include test.c

2.4.2 SDCC does not compile correctly.

A few things to try include starting from scratch by unpacking
the .tgz source package again in an empty directory. If
this doesn't work, you could try downloading a different
version. If this doesn't work, you can re-direct the install
messages by doing the following:

$./make > dump.txt 2>&1

After this you can examine the dump.txt files to locate the
problem. Or these messages can be attached to an email that
could be helpful when requesting help from the mailing list.

2.4.3 What the "./configure"
  does

The "./configure" command is a script
that analyzes your system and performs some configuration
to ensure the source package compiles on your system. It
will take a few minutes to run, and will compile a few tests
to determine what compiler features are installed.

2.4.4 What the "make" does.

This runs the GNU make tool, which automatically compiles
all the source packages into the final installed binary
executables.

2.4.5 What the "make install"
  command does.

This will install the compiler, other executables and libraries
in to the appropriate system directories. The default is
to copy the executables to /usr/local/bin and the libraries
and header files to /usr/local/share/sdcc/lib and /usr/local/share/sdcc/include.

2.5 Additional Information for Windows Users

The standard method of installing on a Unix system involves
compiling the source package. This is easily done under
Unix, but under Windows it can be a more difficult process.
The Cygwin is a large package to download, and the compilation
runs considerably slower under Windows due to the overhead
of the Cygwin tool set. An alternative is to install a pre-compiled
Windows binary package. There are various trade-offs between
each of these methods. 

The Cygwin package allows a Windows user to run a Unix command
line interface(bash shell) and also implements a Unix like
file system on top of Windows. Included are many of the
famous GNU software development tools which can augment
the SDCC compiler.This is great if you have some experience
with Unix command line tools and file system conventions,
if not you may find it easier to start by installing a binary
Windows package. The binary packages work with the Windows
file system conventions.

2.5.1 Getting started with Cygwin

SDCC is typically distributed as a tarred/gzipped file(.tgz).
This is a packed file similar to a .zip file. Cygwin includes
the tools you will need to unpack the SDCC distribution(tar
and gzip). To unpack it, simply follow the instructions
under the Linux/Unix install section. Before you do this
you need to learn how to start a cygwin shell and some of
the basic commands used to move files, change directory,
run commands and so on. The change directory command is
"cd", the move command is "mv".
To print the current working directory, type "pwd".
To make a directory, use "mkdir".

There are some basic differences between Unix and Windows
file systems you should understand. When you type in directory
paths, Unix and the Cygwin bash prompt uses forward slashes
'/' between directories while Windows traditionally uses
'\' backward slashes. So when you work at the Cygwin bash
prompt, you will need to use the forward '/' slashes. Unix
does not have a concept of drive letters, such as "c:",
instead all files systems attach and appear as directories.

2.5.2 Running SDCC as Native Compiled Executables

If you use the pre-compiled binaries, the install directories
for the libraries and header files may need to be specified
on the sdcc command line like this: sdcc -L c:\usr\local\sdcc\lib\small
-I c:\usr\local\sdcc\include test.c if you are running outside
of a Unix bash shell.

If you have successfully installed and compiled SDCC with
the Cygwin package, it is possible to compile into native
.exe files by using the additional makefiles included for
this purpose. For example, with the Borland 32-bit compiler
you would run make -f Makefile.bcc. A command line version
of the Borland 32-bit compiler can be downloaded from the
Inprise web site.

2.6 SDCC on Other Platforms

* FreeBSD and other non-GNU Unixes - Make sure the GNU make
  is installed as the default make tool.

* SDCC has been ported to run under a variety of operating
  systems and processors. If you can run GNU GCC/make then
  chances are good SDCC can be compiled and run on your
  system.

2.7 Advanced Install Options

The "configure" command has several options.
The most commonly used option is --prefix=<directory name>,
where <directory name> is the final location for the sdcc
executables and libraries, (default location is /usr/local).
The installation process will create the following directory
structure under the <directory name> specified. 

bin/ - binary exectables (add to PATH environment variable)

     share/ 
        sdcc/include/ - include
header files 
        sdcc/lib/ - 
             small/
- Object & library files for small model library 
             large/
- Object & library files for large model library 
             ds390/
- Object & library files forDS80C390 library

The command 

'./configure --prefix=/usr/local" 

will configure the compiler to be installed in directory
/usr/local/bin.

2.8 Components of SDCC

SDCC is not just a compiler, but a collection of tools by
various developers. These include linkers, assemblers, simulators
and other components. Here is a summary of some of the components.
Note that the included simulator and assembler have separate
documentation which you can find in the source package in
their respective directories. As SDCC grows to include support
for other processors, other packages from various developers
are included and may have their own sets of documentation.

You might want to look at the various executables which are
installed in the bin directory. At the time of this writing,
we find the following programs:

sdcc - The compiler.

aslink -The linker for 8051 type processors.

asx8051 - The assembler for 8051 type processors.

sdcpp - The C preprocessor.

sdcpd - The source debugger.

s51 - The ucSim 8051 simulator.

linkz80, linkgbz80 - The Z80 and GameBoy Z80 linkers.

as-z80, as-gbz80 - The Z80 and GameBoy Z80 assemblers.

packihx - A tool to pack Intel hex files.

As development for other processors proceeds, this list will
expand to include executables to support processors like
AVR, PIC, etc.

2.8.1 cpp ( C-Preprocessor)

The preprocessor is extracted into the directory SDCCDIR/cpp,
it is a modified version of the GNU preprocessor. The C
preprocessor is used to pull in #include sources, process
#ifdef statements, #defines and so on.

2.8.2 asxxxx & aslink ( The assembler and Linkage Editor)

This is retargettable assembler & linkage editor, it was
developed by Alan Baldwin, John Hartman created the version
for 8051, and I (Sandeep) have some enhancements and bug
fixes for it to work properly with the SDCC. This component
is extracted into the directory SDCCDIR/asxxxx.

2.8.3 SDCC - The compiler

This is the actual compiler, it in turn uses the c-preprocessor
and invokes the assembler and linkage editors. All files
with the prefix SDCC are part of the compiler and are extracted
into the the directory SDCCDIR.

2.8.4 S51 - Simulator

s51 is a freeware, opensource simulator developed by Daniel
Drotos <drdani@mazsola.iit.uni-miskolc.hu>. The executable
is built as part of the build process, for more information
visit Daniel's website at <http://mazsola.iit.uni-miskolc.hu/~drdani/embedded/s51/>.

2.8.5 SDCDB - Source Level Debugger

SDCDB is the companion source level debugger . The current
version of the debugger uses Daniel's Simulator S51, but
can be easily changed to use other simulators.

3 Using SDCC

3.1 Compiling

3.1.1 Single Source File Projects

For single source file 8051 projects the process is very
simple. Compile your programs with the following command

sdcc sourcefile.c

The above command will compile ,assemble and link your source
file. Output files are as follows.

* sourcefile.asm - Assembler source file created by the compiler

* sourcefile.lst - Assembler listing file created by the
  Assembler

* sourcefile.rst - Assembler listing file updated with linkedit
  information , created by linkage editor

* sourcefile.sym - symbol listing for the sourcefile, created
  by the assembler.

* sourcefile.rel - Object file created by the assembler,
  input to Linkage editor.

* sourcefile.map - The memory map for the load module, created
  by the Linker.

* sourcefile.<ihx | s19> - The load module : ihx - Intel
  hex format (default ), s19 - Motorola S19 format when
  compiler option --out-fmt-s19 is used.

3.1.2 Projects with Multiple Source Files

SDCC can compile only ONE file at a time. Let us for example
assume that you have a project containing the following
files.

foo1.c ( contains some functions )

foo2.c (contains some more functions)

foomain.c (contains more functions and the function main)

The first two files will need to be compiled separately with
the commands

sdcc -c foo1.c

sdcc -c foo2.c

Then compile the source file containing main and link the
other files together with the following command.

sdcc foomain.c foo1.rel foo2.rel

Alternatively foomain.c can be separately compiled as well

sdcc -c foomain.c 

sdcc foomain.rel foo1.rel foo2.rel

The file containing the main function MUST be the FIRST file
specified in the command line , since the linkage editor
processes file in the order they are presented to it.

3.1.3 Projects with Additional Libraries

Some reusable routines may be compiled into a library, see
the documentation for the assembler and linkage editor in
the directory SDCCDIR/asxxxx/asxhtm.htm this describes how
to create a .lib library file, the libraries created in
this manner may be included using the command line, make
sure you include the -L <library-path> option to tell the
linker where to look for these files. Here is an example,
assuming you have the source file 'foomain.c' and a library
'foolib.lib' in the directory 'mylib'.

sdcc foomain.c foolib.lib -L mylib

Note here that 'mylib' must be an absolute path name.

The view of the way the linkage editor processes the library
files, it is recommended that you put each source routine
in a separate file and combine them using the .lib file.
For an example see the standard library file 'libsdcc.lib'
in the directory SDCCDIR/sdcc51lib.

3.2 Command Line Options

3.2.1 Processor Selection Options

-mmcs51 Generate code for the MCS51 (8051) family of processors.
This is the default processor target.

-mds390 Generate code for the DS80C390 processor.

-mz80 Generate code for the Z80 family of processors.

-mgbz80 Generate code for the GameBoy Z80 processor.

-mavr Generate code for the Atmel AVR processor(In development,
not complete).

-mpic14 Generate code for the PIC 14-bit processors(In development,
not complete).

-mtlcs900h Generate code for the Toshiba TLCS-900H processor(In
development, not complete).

3.2.2 Path, Lib and Define Options

-I<path>  The additional location where the pre processor
will look for <..h> or "..h"
files.

-D<macro[=value]> Command line definition of macros. Passed
to the pre processor.

--lib-path(-L) <absolute path to additional libraries> This
option is passed to the linkage editor, additional libraries
search path. The path name must be absolute. Additional
library files may be specified in the command line . See
section Compiling programs for more details.

3.2.3 MCS51 Options

--model-large Generate code for Large model programs see
section Memory Models for more details. If this option is
used all source files in the project should be compiled
with this option. In addition the standard library routines
are compiled with small model , they will need to be recompiled.

--model-small Generate code for Small Model programs see
section Memory Models for more details. This is the default
model.

--model-flat24 Generate code forDS80C390 24-bit flat mode.
See section Memory Models for more details.

--stack-auto All functions in the source file will be compiled
as reentrant, i.e. the parameters and local variables will
be allocated on the stack. see section Parameters and Local
Variables for more details. If this option is used all source
files in the project should be compiled with this option. 

--xstack Uses a pseudo stack in the first 256 bytes in the
external ram for allocating variables and passing parameters.
See section on external stack for more details.

3.2.4 Optimization Options

--nogcse Will not do global subexpression elimination, this
option may be used when the compiler creates undesirably
large stack/data spaces to store compiler temporaries. A
warning message will be generated when this happens and
the compiler will indicate the number of extra bytes it
allocated. It recommended that this option NOT be used ,
#pragma NOGCSE can be used to turn off global subexpression
elimination for a given function only.

--noinvariant Will not do loop invariant optimizations, this
may be turned off for reasons explained for the previous
option . For more details of loop optimizations performed
see section Loop Invariants.It recommended that this option
NOT be used , #pragma NOINVARIANT can be used to turn off
invariant optimizations for a given function only.

--noinduction Will not do loop induction optimizations, see
section Strength reduction for more details.It recommended
that this option NOT be used , #pragma NOINDUCTION can be
used to turn off induction optimizations for given function
only.

--nojtbound  Will not generate boundary condition check when
switch statements are implemented using jump-tables. See
section Switch Statements for more details.It recommended
that this option NOT be used , #pragma NOJTBOUND can be
used to turn off boundary checking for jump tables for a
given function only.

--noloopreverse Will not do loop reversal optimization

--noregparms By default the first parameter is passed using
global registers (DPL,DPH,B,ACC). This option will disable
parameter passing using registers. NOTE: if your program
uses the 16/32 bit support routines (for multiplication/division)
these library routines will need to be recompiled with the
--noregparms option as well.

3.2.5 DS390 Options

--stack-auto See MCS51 section for description.

--stack-10bit This option generates code for the 10 bit stack
mode of the Dallas DS80C390 part. In this mode, the stack
is located in the lower 1K of the internal RAM, which is
mapped to 0x400000. Note that the support is incomplete,
since it still uses a single byte as the stack pointer.
This means that only the lower 256 bytes of the potential
1K stack space can actually be used. However, this does
allow you to reclaim the precious 256 bytes of low RAM for
use for the DATA and IDATA segments. The compiler will not
generate any code to put the processor into 10 bit stack
mode. It is important to ensure that the processor is in
this mode before calling any re-entrant functions compiled
with this option. In principle, this should work with the
--stack-auto option, but that has not been tested. It is
incompatible with the --xstack option. It also only makes
sense if the processor is in 24 bit contiguous addressing
mode (see the --model-flat24 option).

3.2.6 Other Options

--callee-saves function1[,function2][,function3].... The
compiler by default uses a caller saves convention for register
saving across function calls, however this can cause unneccessary
register pushing & popping when calling small functions
from larger functions. This option can be used to switch
the register saving convention for the function names specified.
The compiler will not save registers when calling these
functions, extra code will be generated at the entry & exit
for these functions to save & restore the registers used
by these functions, this can SUBSTANTIALLY reduce code &
improve run time performance of the generated code. In future
the compiler (with interprocedural analysis) will be able
to determine the appropriate scheme to use for each function
call. DO NOT use this option for built-in functions such
as _muluint..., if this option is used for a library function
the appropriate library function needs to be recompiled
with the same option. If the project consists of multiple
source files then all the source file should be compiled
with the same --callee-saves option string. Also see Pragma
Directive CALLEE-SAVES. .

--debug When this option is used the compiler will generate
debug information , that can be used with the SDCDB. The
debug information is collected in a file with .cdb extension.
For more information see documentation for SDCDB.

--regextend  This option will cause the compiler to define
pseudo registers , if this option is used, all source files
in the project should be compiled with this option. See
section Register Extension for more details.

--compile-only(-c)  will compile and assemble the source
only, will not call the linkage editor.

--xram-loc<Value> The start location of the external ram,
default value is 0. The value entered can be in Hexadecimal
or Decimal format .eg. --xram-loc 0x8000 or --xram-loc 32768.

--code-loc<Value> The start location of the code segment
, default value 0. Note when this option is used the interrupt
vector table is also relocated to the given address. The
value entered can be in Hexadecimal or Decimal format .eg.
--code-loc 0x8000 or --code-loc 32768.

--stack-loc<Value> The initial value of the stack pointer.
The default value of the stack pointer is 0x07 if only register
bank 0 is used, if other register banks are used then the
stack pointer is initialized to the location above the highest
register bank used. eg. if register banks 1 & 2 are used
the stack pointer will default to location 0x18. The value
entered can be in Hexadecimal or Decimal format .eg. --stack-loc
0x20 or --stack-loc 32. If all four register banks are used
the stack will be placed after the data segment (equivalent
to --stack-after-data)

--stack-after-data This option will cause the stack to be
located in the internal ram after the data segment.

--data-loc<Value> The start location of the internal ram
data segment, the default value is 0x30.The value entered
can be in Hexadecimal or Decimal format .eg. --data-loc
0x20 or --data-loc 32.

--idata-loc<Value> The start location of the indirectly addressable
internal ram, default value is 0x80. The value entered can
be in Hexadecimal or Decimal format .eg. --idata-loc 0x88
or --idata-loc 136.

--peep-file<filename> This option can be used to use additional
rules to be used by the peep hole optimizer. See section
Peep Hole optimizations for details on how to write these
rules.

-E Run only the C preprocessor. Preprocess all the C source
files specified and output the results to standard output.

-M Tell the preprocessor to output a rule suitable for make
describing the dependencies of each object file. For each
source file, the preprocessor outputs one make-rule whose
target is the object file name for that source file and
whose dependencies are all the files `#include'd in it.
This rule may be a single line or may be continued with
`\'-newline if it is long. The list of rules is printed on
standard output instead of the preprocessed C program. `-M'
implies `-E'.

-C Tell the preprocessor not to discard comments. Used with
the `-E' option.

-MM Like `-M' but the output mentions only the user header
files included with `#include file"'. System header files
included with `#include <file>' are omitted.

-Aquestion(answer) Assert the answer answer for question,
in case it is tested with a preprocessor conditional such
as `#if #question(answer)'. `-A-' disables the standard
assertions that normally describe the target machine.

-Aquestion (answer) Assert the answer answer for question,
in case it is tested with a preprocessor conditional such
as `#if #question(answer)'. `-A-' disables the standard
assertions that normally describe the target machine.

-Umacro Undefine macro macro. `-U' options are evaluated
after all `-D' options, but before any `-include' and `-imacros'
options.

-dM Tell the preprocessor to output only a list of the macro
definitions that are in effect at the end of preprocessing.
Used with the `-E' option.

-dD Tell the preprocessor to pass all macro definitions into
the output, in their proper sequence in the rest of the
output.

-dN Like `-dD' except that the macro arguments and contents
are omitted. Only `#define name' is included in the output.

-S Stop after the stage of compilation proper; do not as-
semble. The output is an assembler code file for the input
file specified.

-Wa_asmOption[,asmOption]... Pass the asmOption to the assembler.

-Wl_linkOption[,linkOption] .. Pass the linkOption to the
linker.

--int-long-reent  Integer (16 bit) and long (32 bit) libraries
have been compiled as reentrant. Note by default these libraries
are compiled as non-reentrant. See section Installation
for more details.

--cyclomatic This option will cause the compiler to generate
an information message for each function in the source file.
The message contains some important information about the
function. The number of edges and nodes the compiler detected
in the control flow graph of the function, and most importantly
the cyclomatic complexity see section on Cyclomatic Complexity
for more details.

--float-reent  Floating point library is compiled as reentrant.See
section Installation for more details.

--out-fmt-ihx The linker output (final object code) is in
Intel Hex format. (This is the default option).

--out-fmt-s19 The linker output (final object code) is in
Motorola S19 format.

--nooverlay  The compiler will not overlay parameters and
local variables of any function, see section Parameters
and local variables for more details.

--main-return This option can be used when the code generated
is called by a monitor program. The compiler will generate
a 'ret' upon return from the 'main' function. The default
option is to lock up i.e. generate a 'ljmp .' .

--no-peep  Disable peep-hole optimization.

--peep-asm  Pass the inline assembler code through the peep
hole optimizer. Can cause unexpected changes to inline assembler
code , please go through the peephole optimizer rules defnied
in file 'SDCCpeeph.def' before using this option.

--iram-size<Value> Causes the linker to check if the interal
ram usage is within limits of the given value.

3.2.7 Intermediate Dump Options

The following options are provided for the purpose of retargetting
and debugging the compiler . These provided a means to dump
the intermediate code (iCode) generated by the compiler
in human readable form at various stages of the compilation
process. 

--dumpraw . This option will cause the compiler to dump the
intermediate code into a file of named <source filename>.dumpraw
just after the intermediate code has been generated for
a function , i.e. before any optimizations are done. The
basic blocks at this stage ordered in the depth first number,
so they may not be in sequence of execution.

--dumpgcse. Will create a dump if iCode, after global subexpression
elimination, into a file named <source filename>.dumpgcse.

--dumpdeadcode .Will create a dump if iCode, after deadcode
elimination, into a file named <source filename>.dumpdeadcode.

--dumploop. Will create a dump if iCode, after loop optimizations,
into a file named <source filename>.dumploop.

--dumprange. Will create a dump if iCode, after live range
analysis, into a file named <source filename>.dumprange.

--dumpregassign. Will create a dump if iCode, after register
assignment , into a file named <source filename>.dumprassgn.

--dumpall. Will cause all the above mentioned dumps to be
created.

Note that the files created for the dump are appended to
each time. So the files should be deleted manually , before
each dump is created. 

When reporting bugs, it can be helpful to include these dumps
along with the portion of the code that is causing the problem.

3.3 MCS51 Storage Class Language Extensions

In addition to the ANSI storage classes SDCC allows the following
MCS51 specific storage classes.

3.3.1 xdata

Variables declared with this storage class will be placed
in the extern RAM. This is the default storage class for
Large Memory model .

eg. xdata unsigned char xduc;

3.3.2 data

This is the default storage class for Small Memory model.
Variables declared with this storage class will be allocated
in the internal RAM.

eg. data int iramdata;

3.3.3 idata

Variables declared with this storage class will be allocated
into the indirectly addressable portion of the internal
ram of a 8051 .

eg.idata int idi;

3.3.4 bit

This is a data-type and a storage class specifier. When a
variable is declared as a bit , it is allocated into the
bit addressable memory of 8051.

eg.bit iFlag;

3.3.5 sfr / sbit

Like the bit keyword, sfr / sbit signifies both a data-type
and storage class, they are used to describe the special
function registers and special bit variables of a 8051. 

eg. 

sfr at 0x80 P0; /* special function register P0 at location
0x80 */

sbit at 0xd7 CY; /* CY (Carry Flag) */

3.4 Pointers

SDCC allows (via language extensions) pointers to explicitly
point to any of the memory spaces of the 8051. In addition
to the explicit pointers, the compiler also allows a _generic
class of pointers which can be used to point to any of the
memory spaces. 

Pointer declaration examples.

/* pointer physically in xternal ram pointing to object in
internal ram */ 
data unsigned char * xdata p;


/* pointer physically in code rom pointing to data in xdata
space */ 
xdata unsigned char * code p;


/* pointer physically in code space pointing to data in code
space */ 
code unsigned char * code p;

/* the folowing is a generic pointer physically located in
xdata space */
char * xdata p;

Well you get the idea. For compatibility with the previous
version of the compiler, the following syntax for pointer
declaration is also supported. Note the above examples will
be portable to other commercially available compilers.

unsigned char _xdata *ucxdp; /* pointer to data in external
ram */ 
unsigned char _data  *ucdp ; /* pointer
to data in internal ram */ 
unsigned char _code  *uccp ; /* pointer
to data in R/O code space */
unsigned char _idata *uccp;  /*
pointer to upper 128 bytes of ram */

All unqualified pointers are treated as 3 - byte '_generic'
pointers. These type of pointers can also to be explicitly
declared.

unsigned char _generic *ucgp;

The highest order byte of the generic pointers contains the
data space information. Assembler support routines are called
whenever data is stored or retrieved using _generic pointers.
These are useful for developing reusable library routines.
Explicitly specifying the pointer type will generate the
most efficient code. Pointers declared using a mixture of
OLD/NEW style could have unpredictable results.

3.5 Parameters & Local Variables

Automatic (local) variables and parameters to functions can
either be placed on the stack or in data-space. The default
action of the compiler is to place these variables in the
internal RAM ( for small model) or external RAM (for Large
model). They can be placed on the stack either by using
the --stack-auto compiler option or by using the 'reentrant'
keyword in the function declaration.

eg

unsigned short foo( short i) reentrant { 
... 
}

Note that when the parameters & local variables are declared
in the internal/external ram the functions are non-reentrant.
Since stack space on 8051 is limited the 'reentrant' keyword
or the --stack-auto option should be used sparingly. Note
the reentrant keyword just means that the parameters & local
variables will be allocated to the stack, it DOES NOT mean
that the function is register bank independent.

When compiled with the default option (i.e. non-reentrant
), local variables can be assigned storage classes and absolute
addresses. 

eg

unsigned short foo() { 
   xdata unsigned short i; 
   bit bvar; 
   data at 0x31 unsiged short j; 
... 
}

In the above example the variable i will be allocated in
the external ram, bvar in bit addressable space and j in
internal ram. When compiled with the --stack-auto or when
a function is declared as 'reentrant' local variables cannot
be assigned storage classes or absolute addresses.

Parameters however are not allowed any storage class, (storage
classes for parameters will be ignored), their allocation
is governed by the memory model in use , and the reentrancy
options.

3.6 Overlaying

For non-reentrant functions SDCC will try to reduce internal
ram space usage by overlaying parameters and local variables
of a function (if possible). Parameters and local variables
of a function will be allocated to an overlayable segment
if the function has no other function calls and the function
is non-reentrant and the memory model is small. If an explicit
storage class is specified for a local variable , it will
NOT be overplayed.

Note that the compiler (not the linkage editor) makes the
decision for overlaying the data items. Functions that are
called from an interrupt service routine should be preceded
by a #pragma NOOVERLAY if they are not reentrant Along the
same lines the compiler does not do any processing with
the inline assembler code so the compiler might incorrectly
assign local variables and parameters of a function into
the overlay segment if the only function call from a function
is from inline assembler code, it is safe to use the #pragma
NOOVERLAY for functions which call other functions using
inline assembler code.

Parameters and Local variables of functions that contain
16 or 32 bit multiplication or division will NOT be overlayed
since these are implemented using external functions.

eg.

#pragma SAVE 
#pragma NOOVERLAY 
void set_error( unsigned short errcd) 
{ 
    P3 = errcd; 
} 
#pragma RESTORE 
void some_isr () interrupt 2 using 1 
{ 
    ... 
    set_error(10); 
    ... 
}

In the above example the parameter errcd for the function
set_error would be assigned to the overlayable segment (if
the #pragma NOOVERLAY was not present) , this could cause
unpredictable runtime behavior. The pragma NOOVERLAY ensures
that the parameters and local variables for the function
are NOT overlayed.

3.7 Critical Functions

A special keyword may be associated with a function declaring
it as 'critical'. SDCC will generate code to disable all
interrupts upon entry to a critical function and enable
them back before returning . Note that nesting critical
functions may cause unpredictable results.

eg

int foo () critical 
{ 
... 
... 
}

The critical attribute maybe used with other attributes like
reentrant.

3.8 Absolute Addressing

Data items can be assigned an absolute address with the at
<address> keyword, in addition to a storage class.

eg. 

xdata at 0x8000 unsigned char PORTA_8255 ;

In the above example the PORTA_8255 will be allocated to
the location 0x8000 of the external ram. 

Note that is this feature is provided to give the programmer
access to memory mapped devices attached to the controller.
The compiler does not actually reserve any space for variables
declared in this way (they are implemented with an equate
in the assembler), thus it is left to the programmer to
make sure there are no overlaps with other variables that
are declared without the absolute address, the assembler
listing file (.lst) and the linker output files (<filename>.rst)
and (<filename>.map) are a good places to look for such
overlaps.

Absolute address can be specified for variables in all storage
classes.

eg.

bit at 0x02 bvar;

The above example will allocate the variable at offset 0x02
in the bit-addressable space. There is no real advantage
to assigning absolute addresses to variables in this manner
, unless you want strict control over all the variables
allocated.

3.9 Interrupt Service Routines

SDCC allows interrupt service routines to be coded in C,
with some extended keywords.

void timer_isr (void) interrupt 2 using 1 
{ 
.. 
}

The number following the 'interrupt' keyword is the interrupt
number this routine will service. The compiler will insert
a call to this routine in the interrupt vector table for
the interrupt number specified. The 'using' keyword is used
to tell the compiler to use the specified register bank
(8051 specific) when generating code for this function.
Note that when some function is called from an interrupt
service routine it should be preceded by a #pragma NOOVERLAY
(if it is not reentrant) . A special note here, int (16
bit) and long (32 bit) integer division, multiplication
& modulus operations are implemented using external support
routines developed in ANSI-C, if an interrupt service routine
needs to do any of these operations then the support routines
(as mentioned in a following section) will have to recompiled
using the --stack-auto option and the source file will need
to be compiled using the --int-long-rent compiler option.

If you have multiple source files in your project, interrupt
service routines can be present in any of them, but a prototype
of the isr MUST be present in the file that contains the
function 'main'.

Interrupt Numbers and the corresponding address & descriptions
for the Standard 8051 are listed below. SDCC will automatically
adjust the interrupt vector table to the maximum interrupt
number specified.

+--------------+--------------+----------------+
| Interrupt #  | Description  | Vector Address |
+--------------+--------------+----------------+
+--------------+--------------+----------------+
|      0       | External 0   |     0x0003     |
+--------------+--------------+----------------+
|      1       |   Timer 0    |     0x000B     |
+--------------+--------------+----------------+
|      2       | External 1   |     0x0013     |
+--------------+--------------+----------------+
|      3       |   Timer 1    |     0x001B     |
+--------------+--------------+----------------+
|      4       |   Serial     |     0x0023     |
+--------------+--------------+----------------+


If the interrupt service routine is defined without a register
bank or with register bank 0 (using 0), the compiler will
save the registers used by itself on the stack (upon entry
and restore them at exit), however if such an interrupt
service routine calls another function then the entire register
bank will be saved on the stack. This scheme may be advantageous
for small interrupt service routines which have low register
usage.

If the interrupt service routine is defined to be using a
specific register bank then only "a","b"
& "dptr" are save and restored, if such an interrupt
service routine calls another function (using another register
bank) then the entire register bank of the called function
will be saved on the stack. This scheme is recommended for
larger interrupt service routines.

Calling other functions from an interrupt service routine
is not recommended avoid it if possible.

3.10 Startup Code

The compiler inserts a jump to the C routine _sdcc__external__startup()
at the start of the CODE area. This routine can be found
in the file SDCCDIR/sdcc51lib/_startup.c, by default this
routine returns 0, if this routine returns a non-zero value
, the static & global variable initialization will be skipped
and the function main will be invoked, other wise static
& global variables will be initialized before the function
main is invoked. You could add a _sdcc__external__startup()
routine to your program to override the default if you needed
to setup hardware or perform some other critical operation
prior to static & global variable initialization.

3.11 Inline Assembler Code

SDCC allows the use of in-line assembler with a few restriction
as regards labels. All labels defined within inline assembler
code HAS TO BE of the form nnnnn$ where nnnn is a number
less than 100 (which implies a limit of utmost 100 inline
assembler labels per function). It is strongly recommended
that each assembly instruction (including labels) be placed
in a separate line ( as the example shows). When the --peep-asm
command line option is used, the inline assembler code will
be passed through the peephole optimizer, this might cause
some unexpected changes in the inline assembler code, please
go throught the peephole optimizer rules defined in file
'SDCCpeeph.def' carefully before using this option.

eg

_asm 
         mov b,#10 
00001$: 
         djnz b,00001$ 
_endasm ;

The inline assembler code can contain any valid code understood
by the assembler (this includes any assembler directives
and comment lines ) . The compiler does not do any validation
of the code within the _asm ... _endasm; keyword pair. 

Inline assembler code cannot reference any C-Labels however
it can reference labels defined by the inline assembler.

eg

foo() { 
... /* some c code */ 
_asm 
     ; some assembler code 
    ljmp $0003 
_endasm ; 
... /* some more c code */ 
clabel:   /* inline assembler cannot reference this
label */ 
_asm 
   $0003: ;label (can be reference by inline assembler
only) 
_endasm ; 
... 
}

In other words inline assembly code can access labels defined
in inline assembly. The same goes the other way, ie. labels
defines in inline assembly CANNOT be accessed by C statements.

3.12 int(16 bit) and long (32 bit ) Support

For signed & unsigned int (16 bit) and long (32 bit) variables,
division, multiplication and modulus operations are implemented
by support routines. These support routines are all developed
in ANSI-C to facilitate porting to other MCUs. The following
files contain the described routine, all of them can be
found in the directory SDCCDIR/sdcc51lib

* _mulsint.c - signed 16 bit multiplication (calls _muluint)

* _muluint.c - unsigned 16 bit multiplication

* _divsint.c - signed 16 bit division (calls _divuint)

* _divuint.c - unsigned 16 bit division.

* _modsint.c - signed 16 bit modulus (call _moduint)

* _moduint.c - unsigned 16 bit modulus.

* _mulslong.c - signed 32 bit multiplication (calls _mululong)

* _mululong.c - unsigned32 bit multiplication.

* _divslong.c - signed 32 division (calls _divulong)

* _divulong.c - unsigned 32 division.

* _modslong.c - signed 32 bit modulus (calls _modulong).

* _modulong.c - unsigned 32 bit modulus.

All these routines are compiled as non-reentrant and small
model. Since they are compiled as non-reentrant, interrupt
service routines should not do any of the above operations,
if this unavoidable then the above routines will need to
ne compiled with the --stack-auto option, after which the
source program will have to be compiled with --int-long-rent
option.

3.13 Floating Point Support

SDCC supports IEEE (single precision 4bytes) floating point
numbers.The floating point support routines are derived
from gcc's floatlib.c and consists of the following routines. 

* _fsadd.c - add floating point numbers.

* _fssub.c - subtract floating point numbers

* _fsdiv.c - divide floating point numbers

* _fsmul.c - multiply floating point numbers

* _fs2uchar.c - convert floating point to unsigned char

* _fs2char.c - convert floating point to signed char.

* _fs2uint.c - convert floating point to unsigned int.

* _fs2int.c - convert floating point to signed int.

* _fs2ulong.c - convert floating point to unsigned long.

* _fs2long.c - convert floating point to signed long.

* _uchar2fs.c - convert unsigned char to floating point

* _char2fs.c - convert char to floating point number

* _uint2fs.c - convert unsigned int to floating point

* _int2fs.c - convert int to floating point numbers

* _ulong2fs.c - convert unsigned long to floating point number

* _long2fs.c - convert long to floating point number.

Note if all these routines are used simultaneously the data
space might overflow. For serious floating point usage it
is strongly recommended that the Large model be used (in
which case the floating point routines mentioned above will
need to recompiled with the --model-Large option)

3.14 MCS51 Memory Models

SDCC allows two memory models for MCS51 code, small and large.
Modules compiled with different memory models should never
be combined together or the results would be unpredictable.
The library routines supplied with the compiler are compiled
as both small and large. The compiled library modules are
contained in seperate directories as small and large so
that you can link to either set. In general the use of the
large model is discouraged.

When the large model is used all variables declared without
a storage class will be allocated into the external ram,
this includes all parameters and local variables (for non-reentrant
functions). When the small model is used variables without
storage class are allocated in the internal ram.

Judicious usage of the processor specific storage classes
and the 'reentrant' function type will yield much more efficient
code, than using the large-model. Several optimizations
are disabled when the program is compiled using the large
model, it is therefore strongly recommdended that the small
model be used unless absolutely required.

3.15 Flat 24 bit Addressing Model

This option generates code for the 24 bit contiguous addressing
mode of the Dallas DS80C390 part. In this mode, up to four
meg of external RAM or code space can be directly addressed.
See the data sheets at www.dalsemi.com for further information
on this part.

In older versions of the compiler, this option was used with
the MCS51 code generator (-mmcs51). Now, however, the '390
has it's own code generator, selected by the -mds390 switch.
This code generator currently supports only the flat24 model,
but the --model-flat24 switch is still required, in case
later versions of the code generator support other models
(such as the paged mode of the '390). The combination of
-mmcs51 and --model-flat24 is now depracated.

Note that the compiler does not generate any code to place
the processor into24 bitmode (it defaults to 8051 compatible
mode). Boot loader or similar code must ensure that the
processor is in 24 bit contiguous addressing mode before
calling the SDCC startup code.

Like the --model-large option, variables will by default
be placed into the XDATA segment. 

Segments may be placed anywhere in the 4 meg address space
using the usual --*-loc options. Note that if any segments
are located above 64K, the -r flag must be passed to the
linker to generate the proper segment relocations, and the
Intel HEX output format must be used. The -r flag can be
passed to the linker by using the option -Wl-r on the sdcc
command line.

3.16 Defines Created by the Compiler

The compiler creates the following #defines .

* SDCC - this Symbol is always defined.

* SDCC_STACK_AUTO - this symbol is defined when --stack-auto
  option is used.

* SDCC_MODEL_SMALL - when small model is used.

* SDCC_MODEL_LARGE - when --model-large is used.

* SDCC_USE_XSTACK - when --xstack option is used.

4 SDCC Technical Data

4.1 Optimizations

SDCC performs a a host of standard optimizations in addition
to some MCU specific optimizations. 

4.1.1 Sub-expression Elimination

The compiler does local and global common subexpression elimination.

eg. 

i = x + y + 1; 
j = x + y;

will be translated to

iTemp = x + y 
i = iTemp + 1 
j = iTemp

Some subexpressions are not as obvious as the above example.

eg.

a->b[i].c = 10; 
a->b[i].d = 11;

In this case the address arithmetic a->b[i] will be computed
only once; the equivalent code in C would be.

iTemp = a->b[i]; 
iTemp.c = 10; 
iTemp.d = 11;

The compiler will try to keep these temporary variables in
registers.

4.1.2 Dead-Code Elimination

eg.

int global; 
void f () { 
  int i; 
  i = 1;    /* dead store
*/ 
  global = 1; /* dead store */ 
  global = 2; 
  return; 
  global = 3; /* unreachable */ 
}

will be changed to

int global; void f () 
{     
 global = 2;     
 return; 
}

4.1.3 Copy-Propagation

eg.

int f() { 
   int i, j; 
   i = 10; 
   j = i; 
   return j; 
}

will be changed to 

int f() { 
    int i,j; 
    i = 10; 
    j = 10; 
    return 10; 
}

Note: the dead stores created by this copy propagation will
be eliminated by dead-code elimination .

4.1.4 Loop Optimizations

Two types of loop optimizations are done by SDCC loop invariant
lifting and strength reduction of loop induction variables.In
addition to the strength reduction the optimizer marks the
induction variables and the register allocator tries to
keep the induction variables in registers for the duration
of the loop. Because of this preference of the register
allocator , loop induction optimization causes an increase
in register pressure, which may cause unwanted spilling
of other temporary variables into the stack / data space
. The compiler will generate a warning message when it is
forced to allocate extra space either on the stack or data
space. If this extra space allocation is undesirable then
induction optimization can be eliminated either for the
entire source file ( with --noinduction option) or for a
given function only (#pragma NOINDUCTION).

* Loop Invariant:

eg

for (i = 0 ; i < 100 ; i ++) 
     f += k + l;

changed to

itemp = k + l; 
for ( i = 0; i < 100; i++ ) f += itemp;

As mentioned previously some loop invariants are not as apparent,
all static address computations are also moved out of the
loop.

* Strength Reduction :

This optimization substitutes an expression by a cheaper
expression.

eg.

for (i=0;i < 100; i++) ar[i*5] = i*3;

changed to

itemp1 = 0; 
itemp2 = 0; 
for (i=0;i< 100;i++) { 
     ar[itemp1] = itemp2; 
     itemp1 += 5; 
     itemp2 += 3; 
}

The more expensive multiplication is changed to a less expensive
addition.

4.1.5 Loop Reversing:

This optimization is done to reduce the overhead of checking
loop boundaries for every iteration. Some simple loops can
be reversed and implemented using a "decrement
and jump if not zero" instruction. SDCC
checks for the following criterion to determine if a loop
is reversible (note: more sophisticated compiers use data-dependency
analysis to make this determination, SDCC uses a more simple
minded analysis).

* The 'for' loop is of the form 
  "for ( <symbol> = <expression> ; <sym> [< | <=] <expression>
  ; [<sym>++ | <sym> += 1])
         <for body>"

* The <for body> does not contain "continue"
  or 'break".

* All goto's are contained within the loop.

* No function calls within the loop.

* The loop control variable <sym> is not assigned any value
  within the loop

* The loop control variable does NOT participate in any arithmetic
  operation within the loop.

* There are NO switch statements in the loop.

Note djnz instruction can be used for 8-bit values ONLY,
therefore it is advantageous to declare loop control symbols
as either 'char' or 'short', ofcourse this may not be possible
on all situations.

4.1.6 Algebraic Simplifications

SDCC does numerous algebraic simplifications, the following
is a small sub-set of these optimizations.

eg 
i = j + 0 ; /* changed to */ i = j; 
i /= 2; /* changed to */ i >>= 1; 
i = j - j ; /* changed to */ i = 0; 
i = j / 1 ; /* changed to */ i = j;

Note the subexpressions given above are generally introduced
by macro expansions or as a result of copy/constant propagation.

4.1.7 'switch' Statements

SDCC changes switch statements to jump tables when the following
conditions are true. 

* The case labels are in numerical sequence , the labels
  need not be in order, and the starting number need not
  be one or zero.

eg 

switch(i) {       
             
   switch (i) { 
case 4:...        
             
   case 1: ... 
case 5:...        
             
   case 2: ... 
case 3:...        
             
   case 3: ... 
case 6:...        
             
   case 4: ... 
}             
             
       }

Both the above switch statements will be implemented using
a jump-table.

* The number of case labels is at least three, since it takes
  two conditional statements to handle the boundary conditions.

* The number of case labels is less than 84, since each label
  takes 3 bytes and a jump-table can be utmost 256 bytes
  long. 

Switch statements which have gaps in the numeric sequence
or those that have more that 84 case labels can be split
into more than one switch statement for efficient code generation.

eg

switch (i) { 
case 1: ... 
case 2: ... 
case 3: ... 
case 4: ... 
case 9: ... 
case 10: ... 
case 11: ... 
case 12: ... 
}

If the above switch statement is broken down into two switch
statements

switch (i) { 
case 1: ... 
case 2: ... 
case 3: ... 
case 4: ... 
}

switch (i) { 
case 9: ... 
case 10: ... 
case 11: ... 
case 12:... 
}

then both the switch statements will be implemented using
jump-tables whereas the unmodified switch statement will
not be .

4.1.8 Bit-shifting Operations.

Bit shifting is one of the most frequently used operation
in embedded programming . SDCC tries to implement bit-shift
operations in the most efficient way possible.

eg.

unsigned short i;

... 
i>>= 4; 
..

generates the following code.

mov a,_i 
swap a 
anl a,#0x0f 
mov _i,a

In general SDCC will never setup a loop if the shift count
is known. Another example

unsigned int i; 
... 
i >>= 9; 
...

will generate

mov a,(_i + 1) 
mov (_i + 1),#0x00 
clr c 
rrc a 
mov _i,a

Note that SDCC stores numbers in little-endian format (i.e.
lowest order first)

4.1.9 Bit-rotation

A special case of the bit-shift operation is bit rotation,
SDCC recognizes the following expression to be a left bit-rotation.

unsigned char i; 
... 
i = ( ( i << 1) | ( i >> 7)); 
...

will generate the following code.

mov a,_i 
rl a 
mov _i,a

SDCC uses pattern matching on the parse tree to determine
this operation .Variations of this case will also be recognized
as bit-rotation i.e i = ((i >> 7) | (i << 1)); /* left-bit
rotation */

4.1.10 Highest Order Bit

It is frequently required to obtain the highest order bit
of an integral type (int,long,short or char types). SDCC
recognizes the following expression to yield the highest
order bit and generates optimized code for it.

eg 
unsigned int gint; 
foo () { 
unsigned char hob; 
   ... 
   hob = (gint >> 15) & 1; 
   .. 
}

Will generate the following code.

                            
61 ;  hob.c 7 
   000A E5*01               
62         mov 
a,(_gint + 1) 
   000C 33                  
63         rlc 
a 
   000D E4                  
64         clr 
a 
   000E 13                  
65         rrc 
a 
   000F F5*02               
66         mov 
_foo_hob_1_1,a

Variations of this case however will NOT be recognized .
It is a standard C expression , so I heartily recommend
this be the only way to get the highest order bit, (it is
portable). Of course it will be recognized even if it is
embedded in other expressions.

eg.

xyz = gint + ((gint >> 15) & 1);

will still be recognized.

4.1.11 Peep-hole Optimizer

The compiler uses a rule based , pattern matching and re-writing
mechanism for peep-hole optimization . It is inspired by
'copt' a peep-hole optimizer by Christopher W. Fraser (cwfraser@microsoft.com).
A default set of rules are compiled into the compiler, additional
rules may be added with the --peep-file <filename> option.
The rule language is best illustrated with examples.

replace { 
mov %1,a 
mov a,%1 } by { mov %1,a }

The above rule will the following assembly sequence

mov r1,a 
mov a,r1

to

mov r1,a

Note: All occurrences of a '%n' ( pattern variable ) must
denote the same string. With the above rule, the assembly
sequence

mov r1,a 
mov a,r2

will remain unmodified. Other special case optimizations
may be added by the user (via --peep-file option), eg. some
variants of the 8051 MCU allow only 'AJMP' and 'ACALL' ,
the following two rules will change all 'LJMP' & 'LCALL'
to 'AJMP' & 'ACALL'.

replace { lcall %1 } by { acall %1 } 
replace { ljmp %1 } by { ajmp %1 }

The inline-assembler' code is also passed through the peep
hole optimizer, thus the peephole optimizer can also be
used as an assembly level macro expander. The rules themselves
are MCU dependent whereas the rule language infra-structure
is MCU independent. Peephole optimization rules for other
MCU can be easily programmed using the rule language.

The syntax for a rule is as follows ,

rule := replace [ restart ] '{' <assembly sequence> '\n' 
               
            '}' by '{' '\n'

               
             
  <assembly sequence> '\n' 
               
            '}' [if <functionName>
] '\n' 
<assembly sequence> := assembly instruction (each instruction
including labels must be on a separate line).   

The optimizer will apply to the rules one by one from the
top in the sequence of their appearance, it will terminate
when all rules are exhausted. If the 'restart' option is
specified, then the optimizer will start matching the rules
again from the top, this option for a rule is expensive
(performance), it is intended to be used in situations where
a transformation will trigger the same rule again. A good
example of this the following rule.

replace restart { 
pop %1 
push %1 } by { 
; nop 
}

Note that the replace pattern cannot be a blank, but can
be a comment line. Without the 'restart' option only the
inner most 'pop' 'push' pair would be eliminated. i.e.

pop ar1 
pop ar2 
push ar2 
push ar1

would result in

pop ar1 
; nop 
push ar1

with the 'restart' option the rule will be applied again
to the resulting code and the all the 'pop' 'push' pairs
will be eliminated to yield

; nop 
; nop

A conditional function can be attached to a rule. Attaching
rules are somewhat more involved, let me illustrate this
with an example.

replace { 
     ljmp %5 
%2:} by { 
     sjmp %5 
%2:} if labelInRange

The optimizer does a look-up of a function name table defined
in function 'callFuncByName' in the source file SDCCpeeph.c
, with the name 'labelInRange', if it finds a corresponding
entry the function is called. Note there can be no parameters
specified for these functions, in this case the use of '%5'
is crucial, since the function labelInRange expects to find
the label in that particular variable (the hash table containing
the variable bindings is passed as a parameter). If you
want to code more such functions , take a close look at
the function labelInRange and the calling mechanism in source
file SDCCpeeph.c. I know this whole thing is a little kludgey
, may be some day we will have some better means. If you
are looking at this file, you will also see the default
rules that are compiled into the compiler, you can your
own rules in the default set there if you get tired of specifying
the --peep-file option.

4.2 Pragmas

SDCC supports the following #pragma directives. This directives
are applicable only at a function level.

* SAVE - this will save all the current options .

* RESTORE - will restore the saved options from the last
  save. Note that SAVES & RESTOREs cannot be nested. SDCC
  uses the same buffer to save the options each time a SAVE
  is called.

* NOGCSE - will stop global subexpression elimination.

* NOINDUCTION - will stop loop induction optimizations .

* NOJTBOUND - will not generate code for boundary value checking
  , when switch statements are turned into jump-tables.

* NOOVERLAY - the compiler will not overlay the parameters
  and local variables of a function.

* NOLOOPREVERSE - Will not do loop reversal optimization

* EXCLUDE NONE | {acc[,b[,dpl[,dph]]] - The exclude pragma
  disables generation of pair of push/pop instruction in
  ISR function (using interrupt keyword). The directive
  should be placed immediately before the ISR function definition
  and it affects ALL ISR functions following it. To enable
  the normal register saving for ISR functions use "#pragma
  EXCLUDE none"

* CALLEE-SAVES function1[,function2[,function3...]] - The
  compiler by default uses a caller saves convention for
  register saving across function calls, however this can
  cause unneccessary register pushing & popping when calling
  small functions from larger functions. This option can
  be used to switch the register saving convention for the
  function names specified. The compiler will not save registers
  when calling these functions, extra code will be generated
  at the entry & exit for these functions to save & restore
  the registers used by these functions, this can SUBSTANTIALLY
  reduce code & improve run time performance of the generated
  code. In future the compiler (with interprocedural analysis)
  will be able to determine the appropriate scheme to use
  for each function call. If --callee-saves command line
  option is used, the function names specified in #pragma
  CALLEE-SAVES is appended to the list of functions specified
  inthe command line.

The pragma's are intended to be used to turn-off certain
optimizations which might cause the compiler to generate
extra stack / data space to store compiler generated temporary
variables. This usually happens in large functions. Pragma
directives should be used as shown in the following example,
they are used to control options & optimizations for a given
function; pragmas should be placed before and/or after a
function, placing pragma's inside a function body could
have unpredictable results.

eg

#pragma SAVE   /* save the current settings
*/ 
#pragma NOGCSE /* turnoff global subexpression elimination
*/ 
#pragma NOINDUCTION /* turn off induction optimizations */

int foo () 
{ 
    ... 
    /* large code */ 
    ... 
} 
#pragma RESTORE /* turn the optimizations back on */

The compiler will generate a warning message when extra space
is allocated. It is strongly recommended that the SAVE and
RESTORE pragma's be used when changing options for a function.

4.3 Library Routines

The following library routines are provided for your convenience.

stdio.h - Contains the following functions printf & sprintf
these routines are developed by Martijn van Balen <balen@natlab.research.philips.com>. 

%[flags][width][b|B|l|L]type

           flags:
-        left justify output
in specified field width 
                
+        prefix output with
+/- sign if output is signed type 
                
space    prefix output with a blank if
it's a signed positive value 
          width:         
specifies minimum number of characters outputted for numbers

                         
or strings. 
                         
- For numbers, spaces are added on the left when needed.

                           
If width starts with a zero character, zeroes and used 
                           
instead of spaces. 
                         
- For strings, spaces are are added on the left or right
(when 
                           
flag '-' is used) when needed. 
                         

          b/B:           
byte argument (used by d, u, o, x, X) 
          l/L:           
long argument (used by d, u, o, x, X)
          type: 
d        decimal number 
                
u        unsigned decimal number

                
o        unsigned octal number

                
x        unsigned hexadecimal
number (0-9, a-f) 
                
X        unsigned hexadecimal
number (0-9, A-F) 
                
c        character 
                
s        string (generic pointer)

                
p        generic pointer (I:data/idata,
C:code, X:xdata, P:paged) 
                
f        float (still to be
implemented)

Also contains a very simple version of printf (printf_small).
This simplified version of printf supports only the following
formats.

format     output type     argument-type

%d         decimal
      int 
%ld        decimal       long

%hd        decimal       short/char

%x        hexadecimal    int

%lx       hexadecimal    long

%hx       hexadecimal    short/char

%o         octal         int

%lo        octal         long

%ho        octal         short/char

%c        character      char/short

%s        character     _generic
pointer

The routine is very stack intesive , --stack-after-data parameter
should be used when using this routine, the routine also
takes about 1K of code space .It also expects an external
function named putchar(char ) to be present (this can be
changed). When using the %s format the string / pointer
should be cast to a generic pointer. eg.

printf_small("my str %s, my int %d\n",(char
_generic *)mystr,myint);

* stdarg.h - contains definition for the following macros
  to be used for variable parameter list, note that a function
  can have a variable parameter list if and only if it is
  'reentrant'

  va_list, va_start, va_arg, va_end.

* setjmp.h - contains defintion for ANSI setjmp & longjmp
  routines. Note in this case setjmp & longjmp can be used
  between functions executing within the same register bank,
  if long jmp is executed from a function that is using
  a different register bank from the function issuing the
  setjmp function, the results may be unpredictable. The
  jump buffer requires 3 bytes of data (the stack pointer
  & a 16 byte return address), and can be placed in any
  address space.

* stdlib.h - contains the following functions.

  atoi, atol.

* string.h - contains the following functions.

  strcpy, strncpy, strcat, strncat, strcmp, strncmp, strchr,
  strrchr, strspn, strcspn, strpbrk, strstr, strlen, strtok,
  memcpy, memcmp, memset.

* ctype.h - contains the following routines.

  iscntrl, isdigit, isgraph, islower, isupper, isprint, ispunct,
  isspace, isxdigit, isalnum, isalpha.

* malloc.h - The malloc routines are developed by Dmitry
  S. Obukhov (dso@usa.net). These routines will allocate
  memory from the external ram. Here is a description on
  how to use them (as described by the author).

  //Example: 
       //    
  #define DYNAMIC_MEMORY_SIZE 0x2000 
       //    
  ..... 
       //    
  unsigned char xdata dynamic_memory_pool[DYNAMIC_MEMORY_SIZE];
  
       //    
  unsigned char xdata * current_buffer; 
       //    
  ..... 
       //    
  void main(void) 
       //    
  { 
       //        
  ... 
       //        
  init_dynamic_memory(dynamic_memory_pool,DYNAMIC_MEMORY_SIZE);
  
       //        
  //Now it's possible to use malloc. 
       //        
  ... 
       //        
  current_buffer = malloc(0x100); 
       //

* serial.h - Serial IO routines are also developed by Dmitry
  S. Obukhov (dso@usa.net). These routines are interrupt
  driven with a 256 byte circular buffer, they also expect
  external ram to be present. Please see documentation in
  file SDCCDIR/sdcc51lib/serial.c . Note the header file
  "serial.h" MUST be included in the file containing
  the 'main' function.

* ser.h - Alternate serial routine provided by Wolfgang Esslinger
  <wolfgang@WiredMinds.com> these routines are more compact
  and faster. Please see documentation in file SDCCDIR/sdcc51lib/ser.c

* ser_ir.h - Another alternate set of serial routines provided
  by Josef Wolf <jw@raven.inka.de> , these routines do not
  use the external ram.

* reg51.h - contains register definitions for a standard
  8051

* reg552.h - contains register definitions for 80C552.

* float.h - contains min, max and other floating point related
  stuff.

All library routines are compiled as --model-small , they
are all non-reentrant, if you plan to use the large model
or want to make these routines reentrant, then they will
have to be recompiled with the appropriate compiler option.

Have not had time to do the more involved routines like printf,
will get to them shortly.

4.4 Interfacing with Assembly Routines

4.5 Global Registers used for Parameter Passing

By default the compiler uses the global registers "DPL,DPH,B,ACC"
to pass the first parameter to a routine, the second parameter
onwards is either allocated on the stack (for reentrant
routines or --stack-auto is used) or in the internal / external
ram (depending on the memory model). 

4.5.1 Assembler Routine(non-reentrant)

In the following example the function cfunc calls an assembler
routine asm_func, which takes two parameters.

extern int asm_func( unsigned short, unsigned short);

 
int c_func (unsigned short i, unsigned short j) 
{ 
        return asm_func(i,j); 
} 
int main() 
{ 
   return c_func(10,9); 
}

The corresponding assembler function is:-

        .globl _asm_func_PARM_2 
        .globl _asm_func 
        .area OSEG 
_asm_func_PARM_2:      
.ds      1 
        .area CSEG 
_asm_func: 
        mov    
a,dpl 
        add    
a,_asm_func_PARM_2 
        mov    
dpl,a 
        mov    
dpl,#0x00 
        ret

Note here that the return values are placed in 'dpl' - One
byte return value, 'dpl' LSB & 'dph' MSB for two byte values.
'dpl', 'dph' and 'b' for three byte values (generic pointers)
and 'dpl','dph','b' & 'acc' for four byte values.

The parameter naming convention is _<function_name>_PARM_<n>,
where n is the parameter number starting from 1, and counting
from the left. The first parameter is passed in "dpl"
for One bye parameter, "dptr"
if two bytes, "b,dptr"
for three bytes and "acc,b,dptr"
for four bytes, the varaible name for the second parameter
will be _<function_name>_PARM_2.

Assemble the assembler routine with the following command.

asx8051 -losg asmfunc.asm

Then compile and link the assembler routine to the C source
file with the following command,

sdcc cfunc.c asmfunc.rel

4.5.2 Assembler Routine(reentrant)

In this case the second parameter onwards will be passed
on the stack , the parameters are pushed from right to left
i.e. after the call the left most parameter will be on the
top of the stack. Here is an example.

extern int asm_func( unsigned short, unsigned short);

 

int c_func (unsigned short i, unsigned short j) reentrant

{ 
        return asm_func(i,j); 
} 
int main() 
{ 
   return c_func(10,9); 
}

The corresponding assembler routine is.

        .globl _asm_func 
_asm_func: 
        push  _bp 
        mov  _bp,sp 
        mov  r2,dpl
        mov  a,_bp 
        clr  c 
        add  a,#0xfd 
        mov  r0,a 
        add  a,#0xfc
        mov  r1,a 
        mov  a,@r0 
        add  a,r2
        mov  dpl,a 
        mov  dph,#0x00 
        mov  sp,_bp 
        pop  _bp 
        ret

The compiling and linking procedure remains the same, however
note the extra entry & exit linkage required for the assembler
code, _bp is the stack frame pointer and is used to compute
the offset into the stack for parameters and local variables.

4.6 With --noregparms Option

When the source is compiled with --noregparms option , space
is allocated for each of the parameters passed to a routine.

4.6.1 Assembler Routine Non-reentrant

In the following example the function cfunc calls an assembler
routine asm_func, which takes two parameters.

extern int asm_func( unsigned short, unsigned short);

 
int c_func (unsigned short i, unsigned short j) 
{ 
        return asm_func(i,j); 
} 
int main() 
{ 
   return c_func(10,9); 
}

The corresponding assembler function is:-

        .globl _asm_func_PARM_1 
        .globl _asm_func_PARM_2 
        .globl _asm_func 
        .area OSEG 
_asm_func_PARM_1:      
.ds     1 
_asm_func_PARM_2:      
.ds      1 
        .area CSEG 
_asm_func: 
        mov    
a,_asm_func_PARM_1 
        add    
a,_asm_func_PARM_2 
        mov    
dpl,a 
        mov    
dpl,#0x00 
        ret

Note here that the return values are placed in 'dpl' - One
byte return value, 'dpl' LSB & 'dph' MSB for two byte values.
'dpl', 'dph' and 'b' for three byte values (generic pointers)
and 'dpl','dph','b' & 'acc' for four byte values.

The parameter naming convention is _<function_name>_PARM_<n>,
where n is the parameter number starting from 1, and counting
from the left. i.e. the left-most parameter name will be
_<function_name>_PARM_1.

Assemble the assembler routine with the following command.

asx8051 -losg asmfunc.asm

Then compile and link the assembler routine to the C source
file with the following command,

sdcc cfunc.c asmfunc.rel

4.6.2 Assembler Routine(reentrant)

In this case the parameters will be passed on the stack ,
the parameters are pushed from right to left i.e. after
the call the left most parameter will be on the top of the
stack. Here is an example.

extern int asm_func( unsigned short, unsigned short);

 

int c_func (unsigned short i, unsigned short j) reentrant

{ 
        return asm_func(i,j); 
} 
int main() 
{ 
   return c_func(10,9); 
}

The corresponding assembler routine is.

        .globl _asm_func 
_asm_func: 
        push  _bp 
        mov  _bp,sp 
        mov  a,_bp 
        clr  c 
        add  a,#0xfd 
        mov  r0,a 
        mov  a,_bp 
        clr  c 
        add  a,#0xfc 
        mov  r1,a 
        mov  a,@r0 
        add  a,@r1 
        mov  dpl,a 
        mov  dph,#0x00 
        mov  sp,_bp 
        pop  _bp 
        ret

The compiling and linking procedure remains the same, however
note the extra entry & exit linkage required for the assembler
code, _bp is the stack frame pointer and is used to compute
the offset into the stack for parameters and local variables.

4.7 External Stack

The external stack is located at the start of the external
ram segment , and is 256 bytes in size. When --xstack option
is used to compile the program, the parameters and local
variables of all reentrant functions are allocated in this
area. This option is provided for programs with large stack
space requirements. When used with the --stack-auto option,
all parameters and local variables are allocated on the
external stack (note support libraries will need to be recompiled
with the same options).

The compiler outputs the higher order address byte of the
external ram segment into PORT P2, therefore when using
the External Stack option, this port MAY NOT be used by
the application program.

4.8 ANSI-Compliance

Deviations from the compliancy.

1. functions are not always reentrant.

2. structures cannot be assigned values directly, cannot be
  passed as function parameters or assigned to each other
  and cannot be a return value from a function.

  eg

struct s { ... }; 
struct s s1, s2; 
foo() 
{ 
... 
s1 = s2 ; /* is invalid in SDCC although allowed in ANSI
*/ 
... 
}

struct s foo1 (struct s parms) /* is invalid in SDCC although
allowed in ANSI */ 
{ 
struct s rets; 
... 
return rets;/* is invalid in SDCC although allowed in ANSI
*/ 
}

1. 'long long' (64 bit integers) not supported.

2. 'double' precision floating point not supported.

3. integral promotions are suppressed. What does this mean
  ? The compiler will not implicitly promote an integer
  expression to a higher order integer, exception is an
  assignment or parameter passing. 

4. No support for setjmp and longjmp (for now).

5. Old K&R style function declarations are NOT allowed.

foo( i,j) /* this old style of function declarations */ 
int i,j; /* are valid in ANSI .. not valid in SDCC */ 
{ 
... 
}

1. functions declared as pointers must be dereferenced during
  the call.

  int (*foo)();

   ... 
   /* has to be called like this */ 
   (*foo)();/* ansi standard allows calls to be made like
'foo()' */

4.9 Cyclomatic Complexity

Cyclomatic complexity of a function is defined as the number
of independent paths the program can take during execution
of the function. This is an important number since it defines
the number test cases you have to generate to validate the
function . The accepted industry standard for complexity
number is 10, if the cyclomatic complexity reported by SDCC
exceeds 10 you should think about simplification of the
function logic.

Note that the complexity level is not related to the number
of lines of code in a function. Large functions can have
low complexity, and small functions can have large complexity
levels. SDCC uses the following formula to compute the complexity.

complexity = (number of edges in control flow graph) - 
             (number
of nodes in control flow graph) + 2;

Having said that the industry standard is 10, you should
be aware that in some cases it may unavoidable to have a
complexity level of less than 10. For example if you have
switch statement with more than 10 case labels, each case
label adds one to the complexity level. The complexity level
is by no means an absolute measure of the algorithmic complexity
of the function, it does however provide a good starting
point for which functions you might look at for further
optimization.

5 TIPS

Here are a few guide-lines that will help the compiler generate
more efficient code, some of the tips are specific to this
compiler others are generally good programming practice.

* Use the smallest data type to represent your data-value.
  If it is known in advance that the value is going to be
  less than 256 then use a 'short' or 'char' instead of
  an 'int'.

* Use unsigned when it is known in advance that the value
  is not going to be negative. This helps especially if
  you are doing division or multiplication.

* NEVER jump into a LOOP.

* Declare the variables to be local whenever possible, especially
  loop control variables (induction).

* Since the compiler does not do implicit integral promotion,
  the programmer should do an explicit cast when integral
  promotion is required.

* Reducing the size of division , multiplication & modulus
  operations can reduce code size substantially. Take the
  following code for example.

  foobar( unsigned int p1, unsigned char ch)
  {
      unsigned char ch1 = p1 % ch ;
      ....    
  }

  For the modulus operation the variable ch will be promoted
  to unsigned int first then the modulus operation will
  be performed (this will lead to a call to a support routine).
  If the code is changed to 

  foobar( unsigned int p1, unsigned char ch)
  {
      unsigned char ch1 = (unsigned char)p1
  % ch ;
      ....    
  }

  It would substantially reduce the code generated (future
  versions of the compiler will be smart enough to detect
  such optimization oppurtunities).

Notes on MCS51 memory layout(Trefor@magera.freeserve.co.uk)

The 8051 family of micro controller have a minimum of 128
bytes of internal memory which is structured as follows

- Bytes 00-1F - 32 bytes to hold up to 4 banks of the registers
R7 to R7 

- Bytes 20-2F - 16 bytes to hold 128 bit variables and 

- Bytes 30-7F - 60 bytes for general purpose use.

Normally the SDCC compiler will only utilise the first bank
of registers, but it is possible to specify that other banks
of registers should be used in interrupt routines. By default,
the compiler will place the stack after the last bank of
used registers, i.e. if the first 2 banks of registers are
used, it will position the base of the internal stack at
address 16 (0X10). This implies that as the stack grows,
it will use up the remaining register banks, and the 16
bytes used by the 128 bit variables, and 60 bytes for general
purpose use.

By default, the compiler uses the 60 general purpose bytes
to hold "near data". The compiler/optimiser may also declare
some Local Variables in this area to hold local data. 

If any of the 128 bit variables are used, or near data is
being used then care needs to be taken to ensure that the
stack does not grow so much that it starts to over write
either your bit variables or "near data". There is no runtime
checking to prevent this from happening.

The amount of stack being used is affected by the use of
the "internal stack" to save registers before a subroutine
call is made, - --stack-auto will declare parameters and
local variables on the stack - the number of nested subroutines.

If you detect that the stack is over writing you data, then
the following can be done. --xstack will cause an external
stack to be used for saving registers and (if --stack-auto
is being used) storing parameters and local variables. However
this will produce more and code which will be slower to
execute. 

--stack-loc will allow you specify the start of the stack,
i.e. you could start it after any data in the general purpose
area. However this may waste the memory not used by the
register banks and if the size of the "near data" increases,
it may creep into the bottom of the stack.

--stack-after-data, similar to the --stack-loc, but it automatically
places the stack after the end of the "near data". Again
this could waste any spare register space.

--data-loc allows you to specify the start address of the
near data. This could be used to move the "near data" further
away from the stack giving it more room to grow. This will
only work if no bit variables are being used and the stack
can grow to use the bit variable space.

Conclusion.

If you find that the stack is over writing your bit variables
or "near data" then the approach which best utilised the
internal memory is to position the "near data" after the
last bank of used registers or, if you use bit variables,
after the last bit variable by using the --data-loc, e.g.
if two register banks are being used and no data variables,
--data-loc 16, and - use the --stack-after-data option.

If bit variables are being used, another method would be
to try and squeeze the data area in the unused register
banks if it will fit, and start the stack after the last
bit variable.

6 Retargetting for other MCUs.

The issues for retargetting the compiler are far too numerous
to be covered by this document. What follows is a brief
description of each of the seven phases of the compiler
and its MCU dependency.

1. Parsing the source and building the annotated parse tree.
  This phase is largely MCU independent (except for the
  language extensions). Syntax & semantic checks are also
  done in this phase , along with some initial optimizations
  like back patching labels and the pattern matching optimizations
  like bit-rotation etc.

2. The second phase involves generating an intermediate code
  which can be easy manipulated during the later phases.
  This phase is entirely MCU independent. The intermediate
  code generation assumes the target machine has unlimited
  number of registers, and designates them with the name
  iTemp. The compiler can be made to dump a human readable
  form of the code generated by using the --dumpraw option.

3. This phase does the bulk of the standard optimizations
  and is also MCU independent. This phase can be broken
  down into several sub-phases.

  * Break down intermediate code (iCode) into basic blocks.

  * Do control flow & data flow analysis on the basic blocks.

  * Do local common subexpression elimination, then global
    subexpression elimination

  * dead code elimination

  * loop optimizations

  * if loop optimizations caused any changes then do 'global
    subexpression elimination' and 'dead code elimination'
    again.

4. This phase determines the live-ranges; by live range I
  mean those iTemp variables defined by the compiler that
  still survive after all the optimizations. Live range
  analysis is essential for register allocation, since these
  computation determines which of these iTemps will be assigned
  to registers, and for how long.

5. Phase five is register allocation. There are two parts
  to this process .

  (a) The first part I call 'register packing' (for lack of
    a better term) . In this case several MCU specific expression
    folding is done to reduce register pressure.

  (b) The second part is more MCU independent and deals with
    allocating registers to the remaining live ranges. A
    lot of MCU specific code does creep into this phase
    because of the limited number of index registers available
    in the 8051.

6. The Code generation phase is (unhappily), entirely MCU
  dependent and very little (if any at all) of this code
  can be reused for other MCU. However the scheme for allocating
  a homogenized assembler operand for each iCode operand
  may be reused.

7. As mentioned in the optimization section the peep-hole
  optimizer is rule based system, which can reprogrammed
  for other MCUs.

7 SDCDB - Source Level Debugger

SDCC is distributed with a source level debugger. The debugger
uses a command line interface, the command repertoire of
the debugger has been kept as close to gdb ( the GNU debugger)
as possible. The configuration and build process is part
of the standard compiler installation, which also builds
and installs the debugger in the target directory specified
during configuration. The debugger allows you debug BOTH
at the C source and at the ASM source level.

7.1 Compiling for Debugging

The --debug option must be specified for all files for which
debug information is to be generated. The complier generates
a .cdb file for each of these files. The linker updates
the .cdb file with the address information. This .cdb is
used by the debugger .

7.2 How the Debugger Works

When the --debug option is specified the compiler generates
extra symbol information some of which are put into the
the assembler source and some are put into the .cdb file,
the linker updates the .cdb file with the address information
for the symbols. The debugger reads the symbolic information
generated by the compiler & the address information generated
by the linker. It uses the SIMULATOR (Daniel's S51) to execute
the program, the program execution is controlled by the
debugger. When a command is issued for the debugger, it
translates it into appropriate commands for the simulator .

7.3 Starting the Debugger

The debugger can be started using the following command line.
(Assume the file you are debugging has

the file name foo).

>sdcdb foo

The debugger will look for the following files.

1. foo.c - the source file.

2. foo.cdb - the debugger symbol information file.

3. foo.ihx - the intel hex format object file.

7.4 Command Line Options.

* --directory=<source file directory> this option can used
  to specify the directory search list. The debugger will
  look into the directory list specified for source , cdb
  & ihx files. The items in the directory list must be separated
  by ':' , e.g. if the source files can be in the directories
  /home/src1 and /home/src2, the --directory option should
  be --directory=/home/src1:/home/src2 . Note there can
  be no spaces in the option. 

* -cd <directory> - change to the <directory>.

* -fullname - used by GUI front ends.

* -cpu <cpu-type> - this argument is passed to the simulator
  please see the simulator docs for details.

* -X <Clock frequency > this options is passed to the simulator
  please see simulator docs for details.

* -s <serial port file> passed to simulator see simulator
  docs for details.

* -S <serial in,out> passed to simulator see simulator docs
  for details.

7.5 Debugger Commands.

As mention earlier the command interface for the debugger
has been deliberately kept as close the GNU debugger gdb
, as possible, this will help int integration with existing
graphical user interfaces (like ddd, xxgdb or xemacs) existing
for the GNU debugger.

7.5.1 break [line | file:line | function | file:function]

Set breakpoint at specified line or function.

sdcdb>break 100 
sdcdb>break foo.c:100
sdcdb>break funcfoo
sdcdb>break foo.c:funcfoo

7.5.2 clear [line | file:line | function | file:function ]

Clear breakpoint at specified line or function.

sdcdb>clear 100
sdcdb>clear foo.c:100
sdcdb>clear funcfoo
sdcdb>clear foo.c:funcfoo

7.5.3 continue

Continue program being debugged, after breakpoint.

7.5.4 finish

Execute till the end of the current function.

7.5.5 delete [n]

Delete breakpoint number 'n'. If used without any option
clear ALL user defined break points.

7.5.6 info [break | stack | frame | registers ]

* info break - list all breakpoints

* info stack - show the function call stack.

* info frame - show information about the current execution
  frame.

* info registers - show content of all registers.

7.5.7 step

Step program until it reaches a different source line.

7.5.8 next

Step program, proceeding through subroutine calls.

7.5.9 run

Start debugged program.

7.5.10 ptype variable 

Print type information of the variable.

7.5.11 print variable

print value of variable.

7.5.12 file filename

load the given file name. Note this is an alternate method
of loading file for debugging.

7.5.13 frame

print information about current frame.

7.5.14 set srcmode

Toggle between C source & assembly source.

7.5.15 ! simulator command

Send the string following '!' to the simulator, the simulator
response is displayed. Note the debugger does not interpret
the command being sent to the simulator, so if a command
like 'go' is sent the debugger can loose its execution context
and may display incorrect values.

7.5.16 quit.

"Watch me now. Iam going Down. My name is Bobby Brown"

7.6 Interfacing with XEmacs.

Two files are (in emacs lisp) are provided for the interfacing
with XEmacs, sdcdb.el and sdcdbsrc.el. These two files can
be found in the $(prefix)/bin directory after the installation
is complete. These files need to be loaded into XEmacs for
the interface to work, this can be done at XEmacs startup
time by inserting the following into your '.xemacs' file
(which can be found in your HOME directory) (load-file sdcdbsrc.el)
[ .xemacs is a lisp file so the () around the command is
REQUIRED), the files can also be loaded dynamically while
XEmacs is running, set the environment variable 'EMACSLOADPATH'
to the installation bin directory [$(prefix)/bin], then
enter the following command ESC-x load-file sdcdbsrc . To
start the interface enter the following command ESC-x sdcdbsrc
, you will prompted to enter the file name to be debugged. 

The command line options that are passed to the simulator
directly are bound to default values in the file sdcdbsrc.el
the variables are listed below these values maybe changed
as required.

* sdcdbsrc-cpu-type '51

* sdcdbsrc-frequency '11059200

* sdcdbsrc-serial nil

The following is a list of key mapping for the debugger interface.

 
;; Current Listing :: 
;;key               binding                      Comment

;;---               -------                      -------

;; 
;; n              
sdcdb-next-from-src          SDCDB
next command 
;; b              
sdcdb-back-from-src          SDCDB
back command 
;; c              
sdcdb-cont-from-src          SDCDB
continue command
;; s              
sdcdb-step-from-src          SDCDB
step command 
;; ?              
sdcdb-whatis-c-sexp          SDCDB
ptypecommand for data at 
;;                                          
buffer point 
;; x              
sdcdbsrc-delete              SDCDB
Delete all breakpoints if no arg 
;;                                              given
or delete arg (C-u arg x) 
;; m              
sdcdbsrc-frame               SDCDB
Display current frame if no arg, 
;;                                              given
or display frame arg 
;;                                             buffer
point 
;; !              
sdcdbsrc-goto-sdcdb          Goto
the SDCDB output buffer 
;; p              
sdcdb-print-c-sexp           SDCDB
print command for data at 
;;                                          
buffer point 
;; g              
sdcdbsrc-goto-sdcdb          Goto
the SDCDB output buffer 
;; t              
sdcdbsrc-mode                Toggles
Sdcdbsrc mode (turns it off) 
;; 
;; C-c C-f        
sdcdb-finish-from-src        SDCDB
finish command 
;; 
;; C-x SPC        
sdcdb-break                  Set
break for line with point 
;; ESC t          
sdcdbsrc-mode                Toggle
Sdcdbsrc mode 
;; ESC m          
sdcdbsrc-srcmode            
Toggle list mode 
;; 


8 Other Processors

8.1 The Z80 and gbz80 port

SDCC can target both the Zilog Z80 and the Nintendo Gameboy's
Z80-like gbz80. The port is incomplete - long support is
incomplete (mul, div and mod are unimplimented), and both
float and bitfield support is missing, but apart from that
the code generated is correct.

As always, the code is the authoritave reference - see z80/ralloc.c
and z80/gen.c. The stack frame is similar to that generated
by the IAR Z80 compiler. IX is used as the base pointer,
HL is used as a temporary register, and BC and DE are available
for holding varibles. IY is currently unusued. Return values
are stored in HL. One bad side effect of using IX as the
base pointer is that a functions stack frame is limited
to 127 bytes - this will be fixed in a later version.

9 Support

SDCC has grown to be large project, the compiler alone (without
the Assembler Package, Preprocessor) is about 40,000 lines
of code (blank stripped). The open source nature of this
project is a key to its continued growth and support. You
gain the benefit and support of many active software developers
and end users. Is SDCC perfect? No, that's why we need your
help. The developers take pride in fixing reported bugs.
You can help by reporting the bugs and helping other SDCC
users. There are lots of ways to contribute, and we encourage
you to take part in making SDCC a great software package.

9.1 Reporting Bugs

Send an email to the mailing list at 'user-sdcc@sdcc.sourceforge.net'
or 'devel-sdcc@sdcc.sourceforge.net'. Bugs will be fixed
ASAP. When reporting a bug, it is very useful to include
a small test program which reproduces the problem. If you
can isolate the problem by looking at the generated assembly
code, this can be very helpful. Compiling your program with
the --dumpall option can sometimes be useful in locating
optimization problems.

9.2 Acknowledgments

Sandeep Dutta(sandeep.dutta@usa.net) - SDCC, the compiler,
MCS51 code generator, Debugger, AVR port
Alan Baldwin (baldwin@shop-pdp.kent.edu) - Initial version
of ASXXXX & ASLINK. 
John Hartman (jhartman@compuserve.com) - Porting ASXXX &
ASLINK for 8051
Dmitry S. Obukhov (dso@usa.net) - malloc & serial i/o routines.

Daniel Drotos <drdani@mazsola.iit.uni-miskolc.hu> - for his
Freeware simulator
Malini Dutta(malini_dutta@hotmail.com) - my wife for her
patience and support.
Unknown - for the GNU C - preprocessor.
Michael Hope - The Z80 and Z80GB port, 186 development
Kevin Vigor - The DS390 port.
Johan Knol - DS390/TINI libs, lots of fixes and enhancements.
Scott Datallo - PIC port.
(Thanks to all the other volunteer developers who have helped
with coding, testing, web-page creation, distribution sets,
etc. You know who you are :-)


This document initially written by Sandeep Dutta

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respective companies.