Format of DOS 2.x disk buffer:

Offset  Size    Description
 00h    DWORD   pointer to next disk buffer, offset = FFFFh if last
 04h    BYTE    drive (0=A, 1=B, etc), FFh if not in use
 05h  3 BYTEs   unused??? (seems always to be 00h 00h 01h)
 08h    WORD    logical sector number
 0Ah    BYTE    number of copies to write (1 for non-FAT sectors)
 0Bh    BYTE    sector offset between copies if multiple copies to be written
 0Ch    DWORD   pointer to DOS Device Control Block (see AH=32h)
 10h            buffered data