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	buffer flags (see #01651)
 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 #01395 at AH=32h)
 0Eh	WORD	unused??? (almost always 0)
 10h		buffered data
SeeAlso: #01649,#01652,#01653,#01655