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Format of DOS 4.x disk buffer:

Offset  Size    Description
 00h    WORD    forward ptr, offset only, to next least recently used buffer
 02h    WORD    backward ptr, offset only
 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 in use???
                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: ???
 06h    DWORD   logical sector number
 0Ah    BYTE    number of copies to write
                for FAT sectors, same as number of FATs
                for data and directory sectors, usually 1
 0Bh    WORD    offset in sectors between copies to write for FAT sectors
 0Dh    DWORD   pointer to DOS Device Control Block (see AH=32h)
 11h  3 BYTEs   ??? zero or unset garbage
 14h            buffered data


Note:   All buffered sectors which have the same address modulo NDBCH are on
        the same doubly-linked circular chain.  The links consist of offset
        addresses only, the segment being the same for all buffers in the
        chain.