INT 21,4A - Modify Allocated Memory Block (SETBLOCK)

	AH = 4A
	BX = new requested block size in paragraphs
	ES = segment of the block (MCB + 1 para)

	on return:
	AX = error code if CF set  (see DOS ERROR CODES)
	BX = maximum block size possible, if CF set and AX = 8

	- modifies memory blocks allocated by  INT 21,48
	- can be used by programs to shrink or increase the size
	  of allocated memory
	- PC-DOS version 2.1 and DOS 3.x will actually allocate the largest
	  available block if CF is set.  BX will equal the size allocated.
	- see also  INT 21,49