INT 2F U - DOS 3+ 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
Note:   used by DOS to determine day count since 1/1/80 given a date