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

See also: 2F121D

2F121C - DOS 3+ internal - CHECKSUM MEMORY