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 meanings: 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 meanings: 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 CP/M. FCB+10h AL - Image of the second half of the directory entry, containing the file's allocation (which disc blocks it owns). 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'.
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