CP/M pagesExact file sizes in CP/M Plus
DOS Plus, CP/M 3 and some CP/M 2 clones (specifically, DOS+2.5) support exact file sizes. The following system calls support them:
BDOS function 15 - open a file
Entered with DE=address of File Control Block, C=15 (0Fh)
If the byte at FCB+32 (FCB+20h) is 255 (0FFh) then on return from this function the byte will contain the Last Record Byte Count. Remember to reset this byte to zero before attempting sequential I/O.
BDOS function 17 - search for first
Entered with DE=address of File Control Block, C=17 (11h)
Returns a pointer to the file's directory entry. The byte at ENTRY+13 (ENTRY+0Dh) contains the Last Record Byte Count.
BDOS function 18 - search for next
Entered with C=18 (12h)
Returns a pointer to the file's directory entry. The byte at ENTRY+13 (ENTRY+0Dh) contains the Last Record Byte Count.
BDOS function 30 - set file attributes
Entered with DE=address of File Control Block, C=30 (1Eh)
To set the Last Record Byte Count, store the required value at FCB+32 (FCB+20h), and set bit 7 of FCB+6.
What is the Last Record Byte Count?
From CP/M's point of view, it's just a number from 0-255 associated with the file which programs can use for any purpose whatever. The documentation defines no interpretation for it.
If we want programs to be able to share files with exact lengths, then there had better be some sort of agreement on what the numbers mean. They must satisfy:
- If the number is zero, the file uses every byte in its disc image (for compatibility with earlier versions).
- It must be possible to find the number of bytes in the last record exactly.
Unfortunately, this still leaves two plausible systems:
- The number is the number of unused bytes in the last record.
- The number is the number of bytes used in the last record; since this ranges from 1-128, 0 is not a valid value and we know it means 128.
Even more unfortunately, programs exist that use both systems:
- ISX, an ISIS-II emulator, uses the first interpretation.
- The DOS Plus operating system uses the second. So do the cpmtools utilities, MSDOS by Tilmann Reh and MSDOS by me.
- DOS+2.5 does not define any meaning for the byte, but its documentation suggests that the ISX interpretation was the intended one.