Format of MS-DOS 5+ 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
SeeAlso: #01628,#01631