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

   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 1/1/80
   supported by DR-DOS 5.0+

See Also: AX=121Ch