CP/M File Control Block

The File Control Block is a 36-byte data structure (33 bytes in CP/M 1). It is laid out as follows:

DR 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  ...............
CR R0 R1 R2                                      ....
The bytes in it have the following meanings:

FCB+00h DR - Drive. 0 for default, 1-16 for A-P. In DOSPLUS,
             bit 7 can be set to indicate that the operation should work with
             subdirectories rather than files.
FCB+01h Fn - Filename, 7-bit ASCII. The top bits of the filename bytes
             (usually referred to as F1' to F8') have the following
               F1'-F4' - User-defined attributes. Any program can use
                        them in any way it likes. The filename in the
                        disc directory has the corresponding bits set.
               F5'-F8' - Interface attributes. They modify the 
                        behaviour of various BDOS functions or 
                        indicate error conditions. In the directory
                        these bits are always zero.
FCB+09h Tn - Filetype, 7-bit ASCII. T1' to T3' have the following
               T1' - Read-Only. 
               T2' - System (hidden). System files in user 0 can be
                    opened from other user areas.
               T3' - Archive. Set if the file has not been changed
                    since it was last copied.
FCB+0Ch EX - Set this to 0 when opening a file and then leave it to
            CP/M. You can rewind a file by setting EX, RC, S2 and CR to 0.
FCB+0Dh S1 - Reserved.   
FCB+0Eh S2 - Reserved.
FCB+0Fh RC - Set this to 0 when opening a file and then leave it to
FCB+10h AL - Image of the second half of the directory entry,
            containing the file's allocation (which disc blocks it
FCB+20h CR - Current record within extent. It is usually best to set 
            this to 0 immediately after a file has been opened and 
            then ignore it.
FCB+21h Rn - Random access record number (not CP/M 1). A 16-bit 
            value in CP/M 2 (with R2 used for overflow); an 18-bit
            value in CP/M 3. 

If you are writing an emulator at BDOS level, you need to be aware of how CP/M uses the bytes EX, S2, and CR. Some programs (such as the Digital Research linker, LINK.COM) manipulate these bytes to perform "seek" operations in files without using the random-access calls.

CR = current record,   ie (file pointer % 16384)  / 128
EX = current extent,   ie (file pointer % 524288) / 16384
S2 = extent high byte, ie (file pointer / 524288). The CP/M Plus source
    code refers to this use of the S2 byte as 'module number'.

In CP/M 2 at least, bit 7 of S2 is the 'File Write Flag'. It is set when a file is opened, and cleared if there is any change to the FCB fields (such as file length or allocation bytes) that should be written back to disk when the file is closed).

The CP/M 2 CCP uses this mechanism to remove the last record of $$$.SUB; it decreases byte 0Fh (RC) by 1, shortening the file, sets S2 to zero so the File Write Flag is cleared, and closes the file. Under CP/M 3 and later, F_TRUNCATE is used instead.

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