Int 2F Fn 121C  U - DOS 3+ Internal - Checksum Memory                      [D]

   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 1/1/80 given a date
   supported by DR-DOS 5.0+

See Also: AX=121Dh