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Format of DOS 3.x disk buffer:
Offset  Size    Description
 00h    DWORD   pointer to next disk buffer, offset = FFFFh if last
                least-recently used buffer is first in chain
 04h    BYTE    drive (0=A,1=B, etc), FFh if not in use
 05h    BYTE    flags
                bit 7: ???
                bit 6: buffer dirty
                bit 5: buffer has been referenced
                bit 4: ???
                bit 3: sector in data area
                bit 2: sector in a directory, either root or subdirectory
                bit 1: sector in FAT
                bit 0: boot sector??? (guess)
 06h    WORD    logical sector number
 08h    BYTE    number of copies to write (1 for non-FAT sectors)
 09h    BYTE    sector offset between copies if multiple copies to be written
 0Ah    DWORD   pointer to DOS Drive Parameter Block (see AH=32h)
 0Eh    WORD    unused??? (almost always 0)
 10h            buffered data