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

See also: 2F121C

2F121D - DOS 3+ internal - SUM MEMORY