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 There are disk formats that map multiple logical tracks onto a physical
63 track, which allows a little bit more capacity in case the system image
64 size does not match the physical track capacity well.
65 Directory entries
67 The directory is a sequence of directory entries (also called extents),
68 which contain 32 bytes of the following structure:
69 St F0 F1 F2 F3 F4 F5 F6 F7 E0 E1 E2 Xl Bc Xh Rc
71 Al Al Al Al Al Al Al Al Al Al Al Al Al Al Al Al
72 St is the status; possible values are:
74 0-15: used for file, status is the user number. CP/M 2.2 only
76 documents 0-15 and CCP and PIP only offer those, but the BDOS
77 allows to use 0-31.
78 16-31: used for file, status is the user number (P2DOS, CP/M
79 2.2) or used for password extent (CP/M 3 or higher)
80 32: disc label
81 33: time stamp (P2DOS)
82 0xE5: unused
83 F0-E2 are the file name and its extension. They may consist of any
86 printable 7 bit ASCII character but: < > . , ; : = ? * [ ]. The file
87 name must not be empty, the extension may be empty. Both are padded
88 with blanks. The highest bit of each character of the file name and
89 extension is used as attribute. The attributes have the following
90 meaning:
91 F0: requires set wheel byte (Backgrounder II)
93 F1: public file (P2DOS, ZSDOS), forground-only command (Back‐
94 grounder II)
95 F2: date stamp (ZSDOS), background-only commands (Backgrounder
96 II)
97 F7: wheel protect (ZSDOS)
98 E0: read-only
99 E1: system file
100 E2: archived
101 Public files (visible under each user number) are not supported by CP/M
103 2.2, but there is a patch and some free CP/M clones support them with‐
104 out any patches.
105 The wheel byte is (by default) the memory location at 0x4b. If it is
107 zero, only non-privileged commands may be executed.
108 Xl and Xh store the extent number. A file may use more than one direc‐
110 tory entry, if it contains more blocks than an extent can hold. In
111 this case, more extents are allocated and each of them is numbered se‐
112 quentially with an extent number. If a physical extent stores more
113 than 16k, it is considered to contain multiple logical extents, each
114 pointing to 16k data, and the extent number of the last used logical
115 extent is stored. Note: Some formats decided to always store only one
116 logical extent in a physical extent, thus wasting extent space. CP/M
117 2.2 allows 512 extents per file, CP/M 3 and higher allow up to 2048.
118 Bit 5-7 of Xl are 0, bit 0-4 store the lower bits of the extent number.
119 Bit 6 and 7 of Xh are 0, bit 0-5 store the higher bits of the extent
120 number.
121 Rc and Bc determine the length of the data used by this extent. The
123 physical extent is divided into logical extents, each of them being 16k
124 in size (a physical extent must hold at least one logical extent, e.g.
125 a blocksize of 1024 byte with two-byte block pointers is not allowed).
126 Rc stores the number of 128 byte records of the last used logical ex‐
127 tent. Bc stores the number of bytes in the last used record. The
128 value 0 means 128 for backward compatibility with CP/M 2.2, which did
129 not support Bc. ISX records the number of unused instead of used bytes
130 in Bc. This only applies to files with allocated blocks. For an empty
131 file, no block is allocated and Bc 0 has no meaning.
132 Al stores block pointers. If the disk capacity minus boot tracks but
134 including the directory area is less than or equal to 256 blocks, Al is
135 interpreted as 16 byte-values, otherwise as 8 double-byte-values.
136 Since the directory area is not subtracted, the directory area starts
137 with block 0 and files can never allocate block 0, which is why this
138 value can be given a new meaning: A block pointer of 0 marks a hole in
139 the file. If a hole covers the range of a full extent, the extent will
140 not be allocated. In particular, the first extent of a file does not
141 neccessarily have extent number 0. A file may not share blocks with
142 other files, as its blocks would be freed if the other files is erased
143 without a following disk system reset. CP/M returns EOF when it
144 reaches a hole, whereas UNIX returns zero-value bytes, which makes
145 holes invisible.
146 Native time stamps
148 P2DOS and CP/M Plus support time stamps, which are stored in each
149 fourth directory entry. This entry contains the time stamps for the
150 extents using the previous three directory entries. Note that you re‐
151 ally have time stamps for each extent, no matter if it is the first ex‐
152 tent of a file or not. The structure of time stamp entries is:
153 1 byte status 0x21
155 8 bytes time stamp for third-last directory entry
156 2 bytes unused
157 8 bytes time stamp for second-last directory entry
158 2 bytes unused
159 8 bytes time stamp for last directory entry
160 A time stamp consists of two dates: Creation and modification date (the
162 latter being recorded when the file is closed). CP/M Plus further al‐
163 lows optionally to record the access instead of creation date as first
164 time stamp.
165 2 bytes (little-endian) days starting with 1 at 01-01-1978
167 1 byte hour in BCD format
168 1 byte minute in BCD format
169 All time stamps are stored in local time.
171 DateStamper time stamps
173 The DateStamper software added functions to the BDOS to manage time
174 stamps by allocating a read only file with the name "!!!TIME&.DAT" in
175 the very first directory entry, covering the very first data blocks.
176 It contains one entry per directory entry with the following structure
177 of 16 bytes:
178 5 bytes create datefield
180 5 bytes access datefield
181 5 bytes modify datefield
182 1 byte magic number/checksum
183 The magic number is used for the first 7 entries of each 128-byte
185 record and contains the characters !, !, !, T, I, M and E. The check‐
186 sum is used on every 8th entry (last entry in 128-byte record) and is
187 the sum of the first 127 bytes of the record. Each datefield has this
188 structure:
189 1 byte BCD coded year (no century, so it is sane assuming any
191 year < 70 means 21st century)
192 1 byte BCD coded month
193 1 byte BCD coded day
194 1 byte BCD coded hour or, if the high bit is set, the high byte
195 of a counter for systems without real time clock
196 1 byte BCD coded minute, or the low byte of the counter
197 Disc labels
200 CP/M Plus support disc labels, which are stored in an arbitrary direc‐
201 tory entry. The structure of disc labels is:
202 1 byte status 0x20
204 F0-E2 are the disc label
205 1 byte mode: bit 7 activates password protection, bit 6 causes
206 time stamps on access, but 5 causes time stamps on modifica‐
207 tions, bit 4 causes time stamps on creation and bit 0 is set
208 when a label exists. Bit 4 and 6 are exclusively set.
209 1 byte password decode byte: To decode the password, xor this
210 byte with the password bytes in reverse order. To encode a
211 password, add its characters to get the decode byte.
212 2 reserved bytes
213 8 password bytes
214 4 bytes label creation time stamp
215 4 bytes label modification time stamp
216 Passwords
219 CP/M Plus supports passwords, which are stored in an arbitrary direc‐
220 tory entry. The structure of these entries is:
221 1 byte status (user number plus 16)
223 F0-E2 are the file name and its extension.
224 1 byte password mode: bit 7 means password required for reading,
225 bit 6 for writing and bit 5 for deleting.
226 1 byte password decode byte: To decode the password, xor this
227 byte with the password bytes in reverse order. To encode a
228 password, add its characters to get the decode byte.
229 2 reserved bytes
230 8 password bytes
231
234 mkfs.cpm(1), fsck.cpm(1), fsed.cpm(1), cpmls(1)
235CP/M tools November 2, 2022 CPM(5)