cpm(5)
NAME
cpm - CP/M disk and file system format
DESCRIPTION
Characteristic sizes
Each CP/M disk format is described by the following specific sizes:
Sector size in bytes
Number of tracks
Number of sectors
Block size
Number of directory entries
Logical sector skew
Number of reserved system tracks (optional)
Offset to start of volume (optional and not covered by operating
system, but disk driver specific)
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.
Device areas
A CP/M disk contains four areas:
Volume offset (optional and not covered by operating system, but
disk driver specific)
System tracks (optional)
Directory
Data
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 disk capacity 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.
A hard disk can have the additional notion of a volume offset 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 K, M, T or S
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 offset must appear subsequent to track, sector
and sector length values for the sector and track units to work.
Directory entries
The directory is a sequence of directory entries (also called extents),
which contain 32 bytes of the following structure:
St F0 F1 F2 F3 F4 F5 F6 F7 E0 E1 E2 Xl Bc Xh Rc
Al Al Al Al Al Al Al Al Al Al Al Al Al Al Al Al
St is the status; possible values are:
0-15: used for file, status is the user number
16-31: used for file, status is the user number (P2DOS) or used
for password extent (CP/M 3 or higher)
32: disc label
33: time stamp (P2DOS)
0xE5: unused
F0-E2 are the file name and its extension. They may consist of any
printable 7 bit ASCII character but: < > . , ; : = ? * [ ]. 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:
F0: requires set wheel byte (Backgrounder II)
F1: public file (P2DOS, ZSDOS), forground-only command
(Backgrounder II)
F2: date stamp (ZSDOS), background-only commands (Backgrounder
II)
F7: wheel protect (ZSDOS)
E0: read-only
E1: system file
E2: archived
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.
The wheel byte is (by default) the memory location at 0x4b. If it is
zero, only non-privileged commands may be executed.
Xl and Xh 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 5-7 of Xl are 0, bit 0-4 store the lower bits of the extent number.
Bit 6 and 7 of Xh are 0, bit 0-5 store the higher bits of the extent
number.
Rc and Bc 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.
Al stores block pointers. If the disk capacity minus boot tracks but
including the directory area is less than 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.
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:
1 byte status 0x21
8 bytes time stamp for third-last directory entry
2 bytes unused
8 bytes time stamp for second-last directory entry
2 bytes unused
8 bytes time stamp for last directory entry
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.
2 bytes (little-endian) days starting with 1 at 01-01-1978
1 byte hour in BCD format
1 byte minute in BCD format
All time stamps are stored in local time.
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:
5 bytes create datefield
5 bytes access datefield
5 bytes modify datefield
1 byte magic number/checksum
The magic number is used for the first 7 entries of each 128-byte
record and contains the characters !, !, !, T, I, M and E. 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:
1 byte BCD coded year (no century, so it is sane assuming any
year < 70 means 21st century)
1 byte BCD coded month
1 byte BCD coded day
1 byte BCD coded hour or, if the high bit is set, the high byte
of a counter for systems without real time clock
1 byte BCD coded minute, or the low byte of the counter
Disc labels
CP/M Plus support disc labels, which are stored in an arbitrary
directory entry. The structure of disc labels is:
1 byte status 0x20
F0-E2 are the disc label
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.
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.
2 reserved bytes
8 password bytes
4 bytes label creation time stamp
4 bytes label modification time stamp
Passwords
CP/M Plus supports passwords, which are stored in an arbitrary
directory entry. The structure of these entries is:
1 byte status (user number plus 16)
F0-E2 are the file name and its extension.
1 byte password mode: bit 7 means password required for reading,
bit 6 for writing and bit 5 for deleting.
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.
2 reserved bytes
8 password bytes
SEE ALSO
mkfs.cpm(1), fsck.cpm(1), fsed.cpm(1), cpmls(1)
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