1CPM(5) File formats CPM(5)
2
6 cpm - CP/M disk and file system format
7
9 Characteristic sizes
10 Each CP/M disk format is described by the following specific sizes:
11 Sector size in bytes
13 Number of tracks
14 Number of sectors
15 Block size
16 Number of directory entries
17 Logical sector skew
18 Number of reserved system tracks (optional)
19 Offset to start of volume (optional and not covered by operating
20 system, but disk driver specific)
21 A block is the smallest allocatable storage unit. CP/M supports block
23 sizes of 1024, 2048, 4096, 8192 and 16384 bytes. Unfortunately, this
24 format specification is not stored on the disk and there are lots of
25 formats. Accessing a block is performed by accessing its sectors,
26 which are stored with the given software skew. cpmtools always counts
27 sectors starting with 0, as it deals with logical sectors. CP/M uses
28 physical sectors in the skew table, which often start with 1.
29 Device areas
31 A CP/M disk contains four areas:
32 Volume offset (optional and not covered by operating system, but
34 disk driver specific)
35 System tracks (optional)
36 Directory
37 Data
38 The system tracks store the boot loader and CP/M itself. In order to
40 save disk space, there are non-bootable formats which omit those system
41 tracks. The term disk capacity always excludes the space for system
42 tracks. Note that there is no bitmap or list for free blocks. When
43 accessing a drive for the first time, CP/M builds this bitmap in core
44 from the directory.
45 A hard disk can have the additional notion of a volume offset to locate
47 the start of the drive image (which may or may not have system tracks
48 associated with it). The base unit for volume offset is byte count
49 from the beginning of the physical disk, but specifiers of K, M, T or S
50 may be appended to denote kilobytes, megabytes, tracks or sectors. If
51 provided, a specifier must immediately follow the numeric value with no
52 whitespace. For convenience upper and lower case are both accepted and
53 only the first letter is significant, thus 2KB, 8MB, 1000trk and 16sec
54 are valid values. The offset must appear subsequent to track, sector
55 and sector length values for the sector and track units to work.
56 Note that it is possible to reserve space between the directory and the
58 beginning of data. Although typically data follows the directory, some
59 systems used this to store extra data instead of using more system
60 tracks (see the fields ALV0 and ALV1 in the DPB).
61 Directory entries
63 The directory is a sequence of directory entries (also called extents),
64 which contain 32 bytes of the following structure:
65 St F0 F1 F2 F3 F4 F5 F6 F7 E0 E1 E2 Xl Bc Xh Rc
67 Al Al Al Al Al Al Al Al Al Al Al Al Al Al Al Al
68 St is the status; possible values are:
70 0-15: used for file, status is the user number. CP/M 2.2 only
72 documents 0-15 and CCP and PIP only offer those, but the BDOS
73 allows to use 0-31.
74 16-31: used for file, status is the user number (P2DOS, CP/M
75 2.2) or used for password extent (CP/M 3 or higher)
76 32: disc label
77 33: time stamp (P2DOS)
78 0xE5: unused
79 F0-E2 are the file name and its extension. They may consist of any
82 printable 7 bit ASCII character but: < > . , ; : = ? * [ ]. The file
83 name must not be empty, the extension may be empty. Both are padded
84 with blanks. The highest bit of each character of the file name and
85 extension is used as attribute. The attributes have the following
86 meaning:
87 F0: requires set wheel byte (Backgrounder II)
89 F1: public file (P2DOS, ZSDOS), forground-only command (Back‐
90 grounder II)
91 F2: date stamp (ZSDOS), background-only commands (Backgrounder
92 II)
93 F7: wheel protect (ZSDOS)
94 E0: read-only
95 E1: system file
96 E2: archived
97 Public files (visible under each user number) are not supported by CP/M
99 2.2, but there is a patch and some free CP/M clones support them with‐
100 out any patches.
101 The wheel byte is (by default) the memory location at 0x4b. If it is
103 zero, only non-privileged commands may be executed.
104 Xl and Xh store the extent number. A file may use more than one direc‐
106 tory entry, if it contains more blocks than an extent can hold. In
107 this case, more extents are allocated and each of them is numbered se‐
108 quentially with an extent number. If a physical extent stores more
109 than 16k, it is considered to contain multiple logical extents, each
110 pointing to 16k data, and the extent number of the last used logical
111 extent is stored. Note: Some formats decided to always store only one
112 logical extent in a physical extent, thus wasting extent space. CP/M
113 2.2 allows 512 extents per file, CP/M 3 and higher allow up to 2048.
114 Bit 5-7 of Xl are 0, bit 0-4 store the lower bits of the extent number.
115 Bit 6 and 7 of Xh are 0, bit 0-5 store the higher bits of the extent
116 number.
117 Rc and Bc determine the length of the data used by this extent. The
119 physical extent is divided into logical extents, each of them being 16k
120 in size (a physical extent must hold at least one logical extent, e.g.
121 a blocksize of 1024 byte with two-byte block pointers is not allowed).
122 Rc stores the number of 128 byte records of the last used logical ex‐
123 tent. Bc stores the number of bytes in the last used record. The
124 value 0 means 128 for backward compatibility with CP/M 2.2, which did
125 not support Bc. ISX records the number of unused instead of used bytes
126 in Bc. This only applies to files with allocated blocks. For an empty
127 file, no block is allocated and Bc 0 has no meaning.
128 Al stores block pointers. If the disk capacity minus boot tracks but
130 including the directory area is less than 256 blocks, Al is interpreted
131 as 16 byte-values, otherwise as 8 double-byte-values. Since the direc‐
132 tory area is not subtracted, the directory area starts with block 0 and
133 files can never allocate block 0, which is why this value can be given
134 a new meaning: A block pointer of 0 marks a hole in the file. If a
135 hole covers the range of a full extent, the extent will not be allo‐
136 cated. In particular, the first extent of a file does not neccessarily
137 have extent number 0. A file may not share blocks with other files, as
138 its blocks would be freed if the other files is erased without a fol‐
139 lowing disk system reset. CP/M returns EOF when it reaches a hole,
140 whereas UNIX returns zero-value bytes, which makes holes invisible.
141 Native time stamps
143 P2DOS and CP/M Plus support time stamps, which are stored in each
144 fourth directory entry. This entry contains the time stamps for the
145 extents using the previous three directory entries. Note that you re‐
146 ally have time stamps for each extent, no matter if it is the first ex‐
147 tent of a file or not. The structure of time stamp entries is:
148 1 byte status 0x21
150 8 bytes time stamp for third-last directory entry
151 2 bytes unused
152 8 bytes time stamp for second-last directory entry
153 2 bytes unused
154 8 bytes time stamp for last directory entry
155 A time stamp consists of two dates: Creation and modification date (the
157 latter being recorded when the file is closed). CP/M Plus further al‐
158 lows optionally to record the access instead of creation date as first
159 time stamp.
160 2 bytes (little-endian) days starting with 1 at 01-01-1978
162 1 byte hour in BCD format
163 1 byte minute in BCD format
164 All time stamps are stored in local time.
166 DateStamper time stamps
168 The DateStamper software added functions to the BDOS to manage time
169 stamps by allocating a read only file with the name "!!!TIME&.DAT" in
170 the very first directory entry, covering the very first data blocks.
171 It contains one entry per directory entry with the following structure
172 of 16 bytes:
173 5 bytes create datefield
175 5 bytes access datefield
176 5 bytes modify datefield
177 1 byte magic number/checksum
178 The magic number is used for the first 7 entries of each 128-byte
180 record and contains the characters !, !, !, T, I, M and E. The check‐
181 sum is used on every 8th entry (last entry in 128-byte record) and is
182 the sum of the first 127 bytes of the record. Each datefield has this
183 structure:
184 1 byte BCD coded year (no century, so it is sane assuming any
186 year < 70 means 21st century)
187 1 byte BCD coded month
188 1 byte BCD coded day
189 1 byte BCD coded hour or, if the high bit is set, the high byte
190 of a counter for systems without real time clock
191 1 byte BCD coded minute, or the low byte of the counter
192 Disc labels
195 CP/M Plus support disc labels, which are stored in an arbitrary direc‐
196 tory entry. The structure of disc labels is:
197 1 byte status 0x20
199 F0-E2 are the disc label
200 1 byte mode: bit 7 activates password protection, bit 6 causes
201 time stamps on access, but 5 causes time stamps on modifica‐
202 tions, bit 4 causes time stamps on creation and bit 0 is set
203 when a label exists. Bit 4 and 6 are exclusively set.
204 1 byte password decode byte: To decode the password, xor this
205 byte with the password bytes in reverse order. To encode a
206 password, add its characters to get the decode byte.
207 2 reserved bytes
208 8 password bytes
209 4 bytes label creation time stamp
210 4 bytes label modification time stamp
211 Passwords
214 CP/M Plus supports passwords, which are stored in an arbitrary direc‐
215 tory entry. The structure of these entries is:
216 1 byte status (user number plus 16)
218 F0-E2 are the file name and its extension.
219 1 byte password mode: bit 7 means password required for reading,
220 bit 6 for writing and bit 5 for deleting.
221 1 byte password decode byte: To decode the password, xor this
222 byte with the password bytes in reverse order. To encode a
223 password, add its characters to get the decode byte.
224 2 reserved bytes
225 8 password bytes
226
229 mkfs.cpm(1), fsck.cpm(1), fsed.cpm(1), cpmls(1)
230CP/M tools Jan 22, 2021 CPM(5)