Int 33 03H 

Mouse interrupt 33 function 03H to get mouse position |  location and status of the buttons. 


mov ax,03
int 33h

RETURNS

BX=1 if left button is down
BX-2 if right button is down
BX-3 if both buttons are down
 CX = X ( Row )
DX = Y ( Coloumn ) 

Well in graphical mode 320 x 200 the value of cx is doubled. so handle this in code.