Format of MSDOS 5.0 UMB control block:
Offset  Size    Description
 00h    BYTE    type: 5Ah if last block in chain, 4Dh otherwise
 01h    WORD    first available paragraph in UMB if control block at start
                of UMB, 000Ah if control block at end of UMB
 03h    WORD    length in paragraphs of following UMB or locked-out region
 05h  3 BYTEs   unused
 08h  8 BYTEs   block type name: "UMB" if start block, "SM" if end block in UMB