CS-220, Sec.90, Week 12-B
R. Eckert


The mouse is an input device that, when moved, sends data on how far it
has moved in the horizontal (x) direction and the vertical (y) direction.
The mouse also has buttons that can be pressed or released. When the user
presses or releases any of these buttons, an interrupt is generated.

A terminate-and-stay-resident (TSR) program called the mouse driver, which
is normally loaded by the operating system when the computer is booted,
receives data sent by the mouse. The mouse driver causes a small image,
called the mouse cursor, to move on the computer's screen. The number of
pixels moved by the mouse cursor is proportional to the change in position
of the physical mouse.

The operating system provides a series of "mouse services" through
software interrupt 33h. We can use these services in our assembly language
programs to receive input information from the mouse and control it and
the mouse cursor. Some of the more inportant services are given in the
table below. (See pp. 383-396 of the Abel text book for a complete listing
of the int 33h mouse services.) In each case the service is selected by
placing a "function number" in AX register prior to executing the int 33h.

Function         In Register Values    Out Register Values
Reset mouse          AX=0            AX=0FFFFh if mouse is supported
                                     BX=number of buttons

Show mouse cursor    AX=1

Hide mouse cursor    AX=2

Poll mouse cursor,   AX=3            BX[bit-0]=1==>Left button pressed
position & button-                   BX[bit-1]=1==>Right button pressed
press status                         BX[bit-2]=1==>Middle button pressed
                                     CX=Current x virtual screen coord.
                                     DX=Current y virtual screen coord.

Set mouse cursor     AX=4
position             CX=x, DX=y

Button press info  AX=5              AX[bit-0]=1==>Left button pressed
                   BX=Button (L=0,   AX[bit-1]=1==>Right button pressed
                   R=1, M=2)         AX[bit-2]=1==>Middle button pressed
                                     BX=# of button presses since last call
                                     CX=x, DX=y at posn of last button press

Button release info  AX=6            Same as button press, but for button
                     BX=Button       releases


   2nd point flag = 0
   Initialize (reset) mouse
   (AX=0, int 33h)
   Show mouse cursor
   (AX=1, int 33h)
           | <-------------------------<-------------------------
           |                          ^                          ^
   Was Left button pressed?           |                          |
   (AX=5, BX=0, int 33h)              |                          |
   (Is BX != 0?) -------------------->                           |
           |                no                                   |
           |  yes                                                |
           |                                                     |
   Is 2nd point flag = 1? -----> 2nd point flag = 1 ----->Save x,y coords
           |                no          
           | yes                      
   Call line-drawing procedure
   (1st point coords from stored value,
    2nd point coords in CX,DX)