CP/M 1.3 was designed to work with 8" 250k discs. Thus a stock CP/M 1.3 disc will be laid out in the following way:
77 tracks in total; 26 128-byte sectors per track, software skewed; 2 reserved tracks; 2 1k directory blocks, giving 64 directory entries; 240 1k data blocks, numbered 2-241.The skew table reads:
1,7,13,19,25,5,11,17,23,3,9,15,21,2,8,14,20,26,6,12,18,24,4,10,16,22The reserved tracks will contain an image of CP/M 1, used when the system is rebooted. It can therefore be deduced that CP/M 1 fits in 6.5k.
The CP/M 1.3 / 1.4 directory only has one type of entry:
CP/M pages
SS F1 F2 F3 F4 F5 F6 F7 F8 T1 T2 T3 EX S1 S2 RC .FILENAMETYP....
AL AL AL AL AL AL AL AL AL AL AL AL AL AL AL AL ................
SS = Status. 0 => File exists
0E5h => File deleted
80h => File exists and is hidden. This feature was undocumented
and does not exist in later versions of CP/M.
Fn - filename
Tn - filetype. The characters used for these are 7-bit ASCII.
EX = Extent counter. If a file grows above 16k, then it will have multiple
directory entries. The first entry has EX=0, the second has EX=1 etc.
EX ranges from 0 to 31, thus allowing files up to 512k. CP/M 1.4 only
allows 256k discs anyway.
S1 - reserved, set to 0.
S2 - reserved, set to 0.
RC - Number of records (1 record=128 bytes) used in this extent. If it is
>= 80h, this extent is full and there may be another one on the disc.
File lengths are only saved to the nearest 128 bytes.
AL - Allocation. Each AL is the number of a 1k block on the disc. If an AL
number is zero, that section of the file has no storage allocated to it
(ie it does not exist). For example, a 3k file might have allocation
5,6,8,0,0.... - the first 1k is in block 5, the second in block 6, the
third in block 8.
CP/M 1.4 is able to use other disc formats than the standard 250k — but it can only support one format at a time, and that format is used on all drives. At the start of the BDOS is an area of memory which defines the disc geometry:
xx06 JP BDOSE ;Jump to BDOS entry point xx09 DW PERSUB ;"Bad sector" error handler xx0B DW SELSUB ;"Select disk" error handler xx0D DW RODSUB ;"R/O disk" error handler xx0F-xx39 ;"TRSECT" sector translation function. Given ;a sector number in C, translates it to a ;physical sector and jumps to the BIOS SETSEC ;entry point ;The default implementation includes a ;translation table of sector numbers from ;xx1A-xx33. xx3A DB secsiz ;Sectors per track xx3B DB dirmax ;Number of directory entries - 1 xx3C DB blkshf ;Block shift, 3 => 1k, 4 => 2k, 5 => 4k etc. xx3D DB blkmsk ;Block mask, 7 => 1k, 0Fh => 2k, 1Fh => 4k etc. xx3E DB maxall ;Number of data blocks on the disc - 1 xx3F DB dirblk ;Directory allocation bitmap xx40 DB ostrks ;Number of operating system tracks
Compared to later CP/M versions, a number of limits are expressed in bytes rather than words. So under CP/M 1.4, a drive can have at most 256 tracks, 256 sectors, 256 data blocks and 256 directory entries.