Format of DOS 4.x disk buffer chain head (array, one entry per chain):

Offset  Size    Description
 00h    WORD    EMS logical page number in which chain is resident, garbage if
                not using EMS
 02h    DWORD   pointer to least recently used buffer header.  All buffers on
                this chain are in the same segment.
 06h    WORD    number of dirty buffers on this chain???
Note:   Buffered disk sectors are assigned to chain N where N is the sector's
        address modulo NDBCH,  0 <= N <= NDBCH-1.  Each chain resides
        completely within one EMS page.  This structure is in main memory even
        if buffers are in EMS.