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
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 three areas:
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.
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), foreground-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.
Al stores block pointers. If the disk capacity is less than 256 blocks,
Al is interpreted as 16 byte-values, otherwise as 8 double-byte-values. 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 necessarily 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.
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
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)