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-.\" Believe it or not, reportedly there are nroffs which do not know \(en
-.if n .ds en -
-.if t .ds en \(en
-.TH CPM 5 "October 10, 2022" "CP/M tools" "File formats"
-.SH NAME \"{{{roff}}}\"{{{
-cpm \- CP/M disk and file system format
-.\"}}}
-.SH DESCRIPTION \"{{{
-.SS "Characteristic sizes" \"{{{
-Each CP/M disk format is described by the following specific sizes:
-.RS
-.sp
-Sector size in bytes
-.br
-Number of tracks
-.br
-Number of sectors
-.br
-Block size
-.br
-Number of directory entries
-.br
-Logical sector skew
-.br
-Number of reserved system tracks (optional)
-.br
-Offset to start of volume (optional and not covered by operating system,
-but disk driver specific)
-.sp
-.RE
-A block is the smallest allocatable storage unit.  CP/M supports block
-sizes of 1024, 2048, 4096, 8192 and 16384 bytes.  Unfortunately, this
-format specification is not stored on the disk and there are lots of
-formats.  Accessing a block is performed by accessing its sectors, which
-are stored with the given software skew.  \fBcpmtools\fP always counts
-sectors starting with 0, as it deals with logical sectors.  CP/M uses physical
-sectors in the skew table, which often start with 1.
-.\"}}}
-.SS "Device areas" \"{{{
-A CP/M disk contains four areas:
-.RS
-.sp
-Volume offset (optional and not covered by operating system, but disk driver specific)
-.br
-System tracks (optional)
-.br
-Directory
-.br
-Data
-.sp
-.RE
-The system tracks store the boot loader and CP/M itself.  In order to save
-disk space, there are non-bootable formats which omit those system tracks.
-The term \fIdisk capacity\fP always excludes the space for system tracks.
-Note that there is no bitmap or list for free blocks.  When accessing a
-drive for the first time, CP/M builds this bitmap in core from the directory.
-.LP
-A hard disk can have the additional notion of a \fIvolume offset\fP to
-locate the start of the drive image (which may or may not have system
-tracks associated with it).  The base unit for volume offset is byte
-count from the beginning of the physical disk, but specifiers of
-\fIK\fP, \fIM\fP, \fIT\fP or \fIS\fP may be appended to denote
-kilobytes, megabytes, tracks or sectors.  If provided, a specifier
-must immediately follow the numeric value with no whitespace.  For
-convenience upper and lower case are both accepted and only the first
-letter is significant, thus 2KB, 8MB, 1000trk and 16sec are valid
-values.  The \fBoffset\fP must appear subsequent to track, sector and sector
-length values for the sector and track units to work.
-.LP
-Note that it is possible to reserve space between the directory and
-the beginning of data.  Although typically data follows the directory,
-some systems used this to store extra data instead of using more
-system tracks (see the fields \fBALV0\fP and \fBALV1\fP in the
-DPB).
-.LP
-There are disk formats that map multiple logical tracks onto a physical
-track, which allows a little bit more capacity in case the system image
-size does not match the physical track capacity well.
-.\"}}}
-.SS "Directory entries" \"{{{
-The directory is a sequence of directory entries (also called extents),
-which contain 32 bytes of the following structure:
-.RS
-.sp
-.ta 3n 6n 9n 12n 15n 18n 21n 24n 27n 30n 33n 36n 39n 42n 45n
-St	F0	F1	F2	F3	F4	F5	F6	F7	E0	E1	E2	Xl	Bc	Xh	Rc
-.br
-Al	Al	Al	Al	Al	Al	Al	Al	Al	Al	Al	Al	Al	Al	Al	Al
-.sp
-.RE
-.\"{{{ St     = status
-\fBSt\fP is the status; possible values are:
-.RS
-.sp
-0\*(en15: used for file, status is the user number.  CP/M 2.2 only documents
-0\*(en15 and CCP and PIP only offer those, but the BDOS allows to use 0\*(en31.
-.br
-16\*(en31: used for file, status is the user number (P2DOS, CP/M 2.2)
-or used for password extent (CP/M 3 or higher)
-.br
-32: disc label
-.br
-33: time stamp (P2DOS)
-.br
-0xE5: unused
-.sp
-.RE
-.\"}}}
-.LP
-.\"{{{ F0-E2  = file name and extension
-\fBF0\*(enE2\fP are the file name and its extension.  They may consist of
-any printable 7 bit ASCII character but: \fB< > . , ; : = ? * [ ]\fP.
-The file name must not be empty, the extension may be empty.  Both are
-padded with blanks.  The highest bit of each character of the file name
-and extension is used as attribute.  The attributes have the following
-meaning:
-.RS
-.sp
-F0: requires set wheel byte (Backgrounder II)
-.br
-F1: public file (P2DOS, ZSDOS), forground-only command (Backgrounder II)
-.br
-F2: date stamp (ZSDOS), background-only commands (Backgrounder II)
-.br
-F7: wheel protect (ZSDOS)
-.br
-E0: read-only
-.br
-E1: system file
-.br
-E2: archived
-.sp
-.RE
-Public files (visible under each user number) are not supported by CP/M
-2.2, but there is a patch and some free CP/M clones support them without
-any patches.
-.LP
-The wheel byte is (by default) the memory location at 0x4b.  If it is
-zero, only non-privileged commands may be executed.
-.\"}}}
-.LP
-.\"{{{ Xl, Xh = extent number
-\fBXl\fP and \fBXh\fP store the extent number.  A file may use more than
-one directory entry, if it contains more blocks than an extent can hold.
-In this case, more extents are allocated and each of them is numbered
-sequentially with an extent number.  If a physical extent stores more than
-16k, it is considered to contain multiple logical extents, each pointing
-to 16k data, and the extent number of the last used logical extent
-is stored.  Note: Some formats decided to always store only one logical
-extent in a physical extent, thus wasting extent space.  CP/M 2.2 allows
-512 extents per file, CP/M 3 and higher allow up to 2048.  Bit 5\*(en7 of
-Xl are 0, bit 0\*(en4 store the lower bits of the extent number.  Bit 6
-and 7 of Xh are 0, bit 0\*(en5 store the higher bits of the extent number.
-.\"}}}
-.LP
-.\"{{{ Rc, Bc = record count, byte count
-\fBRc\fP and \fBBc\fP determine the length of the data used by this extent.  The
-physical extent is divided into logical extents, each of them being 16k
-in size (a physical extent must hold at least one logical extent, e.g. a
-blocksize of 1024 byte with two-byte block pointers is not allowed).
-Rc stores the number of 128 byte records of the last used logical extent.
-Bc stores the number of bytes in the last used record.  The value 0 means
-128 for backward compatibility with CP/M 2.2, which did not support Bc.
-ISX records the number of unused instead of used bytes in Bc.
-This only applies to files with allocated blocks.  For an empty file, no
-block is allocated and Bc 0 has no meaning.
-.\"}}}
-.LP
-.\"{{{ Al     = allocated blocks
-\fBAl\fP stores block pointers.  If the disk capacity minus boot
-tracks but including the directory area is less than or equal to 256 blocks, Al
-is interpreted as 16 byte-values, otherwise as 8 double-byte-values.
-Since the directory area is not subtracted, the directory area starts
-with block 0 and files can never allocate block 0, which is why this
-value can be given a new meaning: A block pointer of 0 marks a hole in
-the file.  If a hole covers the range of a full extent, the extent will
-not be allocated.  In particular, the first extent of a file does not
-neccessarily have extent number 0.  A file may not share blocks with other
-files, as its blocks would be freed if the other files is erased without
-a following disk system reset.  CP/M returns EOF when it reaches a hole,
-whereas UNIX returns zero-value bytes, which makes holes invisible.
-.\"}}}
-.\"}}}
-.SS "Native time stamps" \"{{{
-P2DOS and CP/M Plus support time stamps, which are stored in each fourth
-directory entry.  This entry contains the time stamps for
-the extents using the previous three directory entries.  Note that you
-really have time stamps for each extent, no matter if it is the first
-extent of a file or not.  The structure of time stamp entries is:
-.RS
-.sp
-1 byte status 0x21
-.br
-8 bytes time stamp for third-last directory entry
-.br
-2 bytes unused
-.br
-8 bytes time stamp for second-last directory entry
-.br
-2 bytes unused
-.br
-8 bytes time stamp for last directory entry
-.sp
-.RE
-A time stamp consists of two dates: Creation and modification date (the
-latter being recorded when the file is closed).  CP/M Plus further
-allows optionally to record the access instead of creation date as first
-time stamp.
-.RS
-.sp
-2 bytes (little-endian) days starting with 1 at 01-01-1978
-.br
-1 byte hour in BCD format
-.br
-1 byte minute in BCD format
-.sp
-.RE
-All time stamps are stored in local time.
-.\"}}}
-.SS "DateStamper time stamps" \"{{{
-The DateStamper software added functions to the BDOS to manage
-time stamps by allocating a read only file with the name "!!!TIME&.DAT"
-in the very first directory entry, covering the very first data
-blocks.  It contains one entry per directory entry with the
-following structure of 16 bytes:
-.RS
-.sp
-5 bytes create datefield
-.br
-5 bytes access datefield
-.br
-5 bytes modify datefield
-.br
-1 byte magic number/checksum
-.sp
-.RE
-The magic number is used for the first 7 entries of each 128-byte record
-and contains the characters \fB!\fP, \fB!\fP, \fB!\fP, \fBT\fP, \fBI\fP,
-\fBM\fP and \fBE\fP.  The checksum is used on every 8th entry (last entry
-in 128-byte record) and is the sum of the first 127 bytes of the record.
-Each datefield has this structure:
-.RS
-.sp
-1 byte BCD coded year (no century, so it is sane assuming any year < 70
-means 21st century)
-.br
-1 byte BCD coded month
-.br
-1 byte BCD coded day
-.br
-1 byte BCD coded hour or, if the high bit is set, the high byte of a
-counter for systems without real time clock
-.br
-1 byte BCD coded minute, or the low byte of the counter
-.sp
-.DE
-.\"}}}
-.SS "Disc labels" \"{{{
-CP/M Plus support disc labels, which are stored in an arbitrary directory
-entry.
-The structure of disc labels is:
-.RS
-.sp
-1 byte status 0x20
-.br
-\fBF0\*(enE2\fP are the disc label
-.br
-1 byte mode: bit 7 activates password protection, bit 6 causes time stamps on
-access, but 5 causes time stamps on modifications, bit 4 causes time stamps on
-creation and bit 0 is set when a label exists.  Bit 4 and 6 are exclusively set.
-.br
-1 byte password decode byte: To decode the password, xor this byte with the password
-bytes in reverse order.  To encode a password, add its characters to get the
-decode byte.
-.br
-2 reserved bytes
-.br
-8 password bytes
-.br
-4 bytes label creation time stamp
-.br
-4 bytes label modification time stamp
-.sp
-.RE
-.\"}}}
-.SS "Passwords" \"{{{
-CP/M Plus supports passwords, which are stored in an arbitrary directory
-entry.
-The structure of these entries is:
-.RS
-.sp
-1 byte status (user number plus 16)
-.br
-\fBF0\*(enE2\fP are the file name and its extension.
-.br
-1 byte password mode: bit 7 means password required for reading, bit 6 for writing
-and bit 5 for deleting.
-.br
-1 byte password decode byte: To decode the password, xor this byte with the password
-bytes in reverse order.  To encode a password, add its characters to get the
-decode byte.
-.br
-2 reserved bytes
-.br
-8 password bytes
-.sp
-.RE
-.\"}}}
-.\"}}}
-.SH "SEE ALSO" \"{{{
-.IR mkfs.cpm (1),
-.IR fsck.cpm (1),
-.IR fsed.cpm (1),
-.IR cpmls (1)
-.\"}}}