MCB - DOS Memory Control Block Format

     Offset Size		Description

	00   byte	'M' 4Dh  member of a MCB chain, (not last)
			'Z' 5Ah  indicates last entry in MCB chain
			other values cause "Memory Allocation Failure" on exit
	01   word	PSP segment address of MCB owner (Process Id)
			possible values:
			    0 = free
			    8 = Allocated by DOS before first user pgm loaded
			    other = Process Id/PSP segment address of owner
	03   word	number of paras related to this MCB (excluding MCB)
	05 11bytes	reserved
	08  8bytes	ASCII program name, NULL terminated if less than max
			length (DOS 4.x+)
	10  nbytes	first byte of actual allocated memory block


	- to find the first MCB in the chain, use  INT 21,52
	- DOS 3.1+ the first memory block contains the DOS data segment
	  ie., installable drivers, buffers, etc
	- DOS 4.x the first memory block is divided into subsegments,
	  with their own memory control blocks; offset 0000h is the first
	- the 'M' and 'Z' are said to represent Mark Zbikowski
	- the MCB chain is often referred to as a linked list, but
	  technically isn't

DOS 4.x Initial Data Segment Subsegment Control Blocks:

     Offset Size	  Description
	00   byte	subsegment type
			'D'  device driver
			'E'  device driver appendage
			'I'  Installable File System driver
			'F'  FILES= control block storage area (for FILES>5)
			'X'  FCBS= control block storage area, if present
			'C'  BUFFERS EMS workspace area if BUFFERS /X is used
			'B'  BUFFERS= storage area
			'L'  LASTDRIVE= current directory structure array
			'S'  STACKS= code/data area, if present (see below)
	01   word	paragraph of subsegment start
	03   word	subsegment size in paragraphs
	05  3bytes	unused
	08		types "D" and "I", filename of driver loaded driver


	- see  INT 21,48  INT 21,49  INT 21,4A