CP/M 3.1 disc formats

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.

CP/M 3.1 directory

The CP/M 3.1 directory has four types of entry:

Files:


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).

Disc label


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

Date stamps

If date stamps are in use, then every fourth directory entry will be a date stamp entry, containing stamps for the preceding three entries.


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 disc label

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

Password control


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.

Password encryption system

This system is extremely simple:

When making the password, add all 8 bytes together (packing with spaces if necessary). This becomes PB (the decode byte). XOR each byte with PB and store them backwards in the directory (ie the last byte becomes P1).
To decode the password, XOR PB with the 8 bytes of the password and read it off backwards.