CPM(5) File formats 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
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 (Back-
grounder 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 with-
out 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 direc-
tory 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 direc-
tory 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 modifica-
tions, 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 direc-
tory 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)
CP/M tools Oct 19, 2005 CPM(5)
