INT 2F U - DOS 3.0+ internal - SUM MEMORY                                       
	AX = 121Dh
	DS:SI -> memory to add up
	CX = 0000h
	DX = limit
Return: AL = byte which exceeded limit
	CX = number of bytes before limit exceeded
	DX = remainder after adding first CX bytes
	DS:SI -> byte beyond the one which exceeded the limit
Notes:	used by DOS to determine year or month given day count since 1980/1/1
	supported by DR DOS 5.0+
SeeAlso: AX=121Ch