+With the upcome of devices with internal xdata and flash memory devices
+ using port P2
+\begin_inset LatexCommand \index{P2 (mcs51 sfr)}
+
+\end_inset
+
+ as dedicated I/O port is becoming more popular.
+ Switching the high byte for pdata
+\begin_inset LatexCommand \index{pdata (mcs51, ds390 storage class)}
+
+\end_inset
+
+ access which was formerly done by port P2 is then achieved by a Special
+ Function Register
+\begin_inset LatexCommand \index{sfr}
+
+\end_inset
+
+.
+ In well-established MCS51 tradition the address of this
+\emph on
+sfr
+\emph default
+ is where the chip designers decided to put it.
+ Needless to say that they didn't agree on a common name either.
+ So that the startup code can correctly initialize xdata variables, you
+ should define an sfr with the name _XPAGE
+\family typewriter
+
+\begin_inset LatexCommand \index{\_XPAGE (mcs51)}
+
+\end_inset
+
+
+\family default
+ at the appropriate location if the default, port P2, is not used for this.
+ Some examples are:
+\end_layout
+
+\begin_layout Verse
+
+\family typewriter
+__sfr __at (0x85) _XPAGE; /* Ramtron VRS51 family a.k.a.
+ MPAGE */
+\end_layout
+
+\begin_layout Verse
+
+\family typewriter
+__sfr __at (0x92) _XPAGE; /* Cypress EZ-USB family, Texas Instruments (Chipcon)
+ a.k.a.
+ MPAGE */
+\end_layout
+
+\begin_layout Verse
+
+\family typewriter
+__sfr __at (0x91) _XPAGE; /* Infineon (Siemens) C500 family a.k.a.
+ XPAGE */
+\end_layout
+
+\begin_layout Verse
+
+\family typewriter
+__sfr __at (0xaf) _XPAGE; /* some Silicon Labs (Cygnal) chips a.k.a.
+ EMI0CN */
+\end_layout
+
+\begin_layout Verse
+
+\family typewriter
+__sfr __at (0xaa) _XPAGE; /* some Silicon Labs (Cygnal) chips a.k.a.
+ EMI0CN */
+\end_layout
+
+\begin_layout Standard
+For more exotic implementations further customizations may be needed.
+ See section
+\begin_inset LatexCommand \ref{sub:Startup-Code}
+
+\end_inset
+
+ for other possibilities.
+\end_layout
+
+\begin_layout Subsection
+Other Features available by SFR
+\end_layout
+
+\begin_layout Standard
+Some MCS51 variants offer features like Double DPTR
+\begin_inset LatexCommand \index{DPTR}
+
+\end_inset
+
+, multiple DPTR, decrementing DPTR, 16x16 Multiply.
+ These are currently not used for the MCS51 port.
+ If you absolutely need them you can fall back to inline assembly or submit
+ a patch to SDCC.
+\end_layout
+
+\begin_layout Subsection
+Bankswitching
+\end_layout
+
+\begin_layout Standard
+Bankswitching
+\begin_inset LatexCommand \index{Bankswitching}
+
+\end_inset
+
+ (a.k.a.
+ code banking
+\begin_inset LatexCommand \index{code banking}
+
+\end_inset
+
+) is a technique to increase the code space above the 64k limit of the 8051.
+\end_layout
+
+\begin_layout Subsubsection
+Hardware
+\end_layout
+
+\begin_layout Standard
+\begin_inset Tabular
+<lyxtabular version="3" rows="3" columns="4">
+<features>
+<column alignment="center" valignment="top" width="0">
+<column alignment="center" valignment="top" leftline="true" width="0">
+<column alignment="center" valignment="top" leftline="true" width="0">
+<column alignment="center" valignment="top" leftline="true" rightline="true" width="0">
+<row topline="true" bottomline="true">
+<cell multicolumn="1" alignment="center" valignment="top" usebox="none">
+\begin_inset Text
+
+\begin_layout Standard
+8000-FFFF
+\end_layout
+
+\end_inset
+</cell>
+<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
+\begin_inset Text
+
+\begin_layout Standard
+bank1
+\end_layout
+
+\end_inset
+</cell>
+<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
+\begin_inset Text
+
+\begin_layout Standard
+bank2
+\end_layout
+
+\end_inset
+</cell>
+<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
+\begin_inset Text
+
+\begin_layout Standard
+bank3
+\end_layout
+
+\end_inset
+</cell>
+</row>
+<row topline="true" bottomline="true">
+<cell multicolumn="1" alignment="center" valignment="top" usebox="none">
+\begin_inset Text
+
+\begin_layout Standard
+0000-7FFF
+\end_layout
+
+\end_inset
+</cell>
+<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
+\begin_inset Text
+
+\begin_layout Standard
+common
+\end_layout
+
+\end_inset
+</cell>
+<cell multicolumn="1" alignment="center" valignment="top" usebox="none">
+\begin_inset Text
+
+\begin_layout Standard
+
+\end_layout