CP/M 3.1 uses a very similar system to CP/M 2.2, but with even more formats supported. The disc statistics are stored in a parameter block (the DPB), which contains the following information:
DEFW spt ;Number of 128-byte records per track DEFB bsh ;Block shift. 3 => 1k, 4 => 2k, 5 => 4k.... DEFB blm ;Block mask. 7 => 1k, 0Fh => 2k, 1Fh => 4k... DEFB exm ;Extent mask, see later DEFW dsm ;(no. of blocks on the disc)-1 DEFW drm ;(no. of directory entries)-1 DEFB al0 ;Directory allocation bitmap, first byte DEFB al1 ;Directory allocation bitmap, second byte DEFW cks ;Checksum vector size, 0 or 8000h for a fixed disc. ;No. directory entries/4, rounded up. DEFW off ;Offset, number of reserved tracks DEFB psh ;Physical sector shift, 0 => 128-byte sectors ;1 => 256-byte sectors 2 => 512-byte sectors... DEFB phm ;Physical sector mask, 0 => 128-byte sectors ;1 => 256-byte sectors, 3 => 512-byte sectors... The directory allocation bitmap is interpreted as: al0 al1 b7b6b5b4b3b2b1b0 b7b6b5b4b3b2b1b0 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 - ie, in this example, the first 4 blocks of the disc contain the directory.
The CP/M 3.1 directory has four types of entry:
0U 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 ................ 0U = User number. 0-15. The user number allows multiple files of the same name to coexist on the disc. User number = 0E5h => File deleted Fn - filename Tn - filetype. The characters used for these are 7-bit ASCII. The top bit of T1 (often referred to as T1') is set if the file is read-only. T2' is set if the file is a system file (this corresponds to "hidden" on other systems). System files with user number 0 can be read from any user number. T3' is set if the file has been backed up. EX = Extent counter, low byte - takes values from 0-31 S2 = Extent counter, high byte. An extent is the portion of a file controlled by one directory entry. If a file takes up more blocks than can be listed in one directory entry, it is given multiple entries, distinguished by their EX and S2 bytes. The formula is: Entry number = ((32*S2)+EX) / (exm+1) where exm is the extent mask value from the Disc Parameter Block. S1 - Last Record Byte Count RC - Number of records (1 record=128 bytes) used in this extent, low byte. The total number of records used in this extent is (EX & exm) * 128 + RC If RC is >= 80h, this extent is full and there may be another one on the disc. File lengths are optionally saved exactly (using the S1 byte) but this system is hardly ever used. AL - Allocation. Each AL is the number of a 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. AL numbers can either be 8-bit (if there are fewer than 256 blocks on the disc) or 16-bit (stored low byte first).
20 F1 F2 F3 F4 F5 F6 F7 F8 T1 T2 T3 LB PB RR RR LABENAMETYP.... P1 P2 P3 P4 P5 P6 P7 P8 D1 D1 D1 D1 D2 D2 D2 D2 ................ 20h - Characteristic number of a disc label F1-F8, T1-T3 - Label name, 7-bit ASCII LB - Label byte. Bit 0 set => Label exists Bit 4 set => Time stamp on create --+ Bit 5 set => Time stamp on update +--These 2 are mutually Bit 6 set => Time stamp on access --+ exclusive Bit 7 set => Password protection enabled PB - Used to decode the label password RR - Reserved, set to zero. P1-P8 - password, rather feebly encrypted. D1 - Label create datestamp D2 - Label update datestamp
21 D1 D1 D1 D1 D2 D2 D2 D2 M1 00 D3 D3 D3 D3 D4 !............... D4 D4 D4 M2 00 D5 D5 D5 D5 D6 D6 D6 D6 M3 00 00 ................ 21h - Characteristic number of a date stamp. D1 - File 1 create OR access date D2 - File 1 update date D3 - File 2 create OR access date D4 - File 2 update date D5 - File 3 create OR access date D6 - File 3 update date M1 - File 1 password mode M2 - File 2 password mode M3 - File 3 password mode 00 - Reserved. The format of a date stamp is: DW day ;Julian day number, stored low byte first. ;Day 1 = 1 Jan 1978. DB hour ;BCD hour, eg 13h => 13:xx DB min ;BCD minute
1U F1 F2 F3 F4 F5 F6 F7 F8 T1 T2 T3 PM PB RR RR .FILENAMETYP.... P1 P2 P3 P4 P5 P6 P7 P8 RR RR RR RR RR RR RR RR ................ 1U = 16+User number (ie 16-31). The user number will be the number of the file to which the password belongs. F1-F8 - Filename of the file to which the password belongs T1-T3 - Filetype of the file to which the password belongs PM - Password mode byte Bit 7 set => Password required to read from file Bit 6 set => Password required to write to file Bit 5 set => Password required to delete file PB - Used to decode the password P1-P8 - The password, rather feebly encrypted. RR - Reserved, set to 0.
This system is extremely simple:
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