CP/M 2.2 disc formats

CP/M 2.2 works with a much larger range of discs than CP/M 1.4. 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 for a fixed disc

                        ;No. directory entries/4, rounded up.

        DEFW    off     ;Offset, number of reserved tracks



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 reserved tracks will contain an image of CP/M 2.2, used when the system is rebooted. Discs can be formatted as data only discs, in which case they have no system tracks and cannot be used to reboot the system.

CP/M 2.2 directory

The CP/M 2.2 directory has only one type of entry:


UU 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   ................



UU = User number. 0-15 (on some systems, 0-31). 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).

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 - reserved, set to 0.

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 only saved to the nearest 128 bytes.

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).

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