INT 33 - MS MOUSE v6.0+ - RETURN USER ALTERNATE INTERRUPT VECTOR
AX = 0019h
CX = call mask (see #03174)
Return: BX:DX = user interrupt vector
CX = call mask (0000h if not found)
Note: attempts to find a user event handler (defined by function 18h)
whose call mask matches CX
SeeAlso: AX=0018h