INT 2F U - DOS 3.0+ internal - CHECKSUM MEMORY                                  
	AX = 121Ch
	DS:SI -> start of memory to checksum
	CX = number of bytes
	DX = initial checksum
	SS = DOS DS (must be using a DOS internal stack)
Return: AX, CX destroyed
	DX = checksum
	DS:SI -> first byte after checksummed range
Notes:	used by DOS to determine day count since 1980/1/1 given a date
	supported by DR DOS 5.0+
SeeAlso: AX=121Dh