*** MOUSE PROGRAMMING, by Alex Warren ****************************************
    
Mouse programming is fairly easy - all you need to know are the various
interrupts involved. First you will need to start QB/etc.  with the /L option
- this loads the quick library which you will need to access the mouse
functions. In your program you will need to put the following lines near
the start:

    REM $INCLUDE: 'QB.BI'
    DIM inregs AS regtype, outregs AS regtype
    
You will now be ready to use the various mouse functions. Each mouse
function is called using interrupt 33h, like this:

    inregs.ax = n
    CALL INTERRUPT (&H33, inregs, outregs)
    
where n is the function number.
To initialize the mouse, use function 0, i.e.:

    inregs.ax = 0
    CALL INTERRUPT (&H33, inregs, outregs)
    
You will always need to execute these lines to get the mouse to work
at all. These lines also detect whether a mouse is present, with the
result returned in outregs. If outregs.ax=-1 after using the above
lines, a mouse is present.

Next you will probably want to show the mouse, so use function 1 and
you should see your mouse shown as a grey block in SCREEN 0:

    inregs.ax = 1
    CALL INTERRUPT (&H33, inregs, outregs)

In graphics modes the mouse is shown as an arrow.

Now you'll want to know how to recognise mouse button clicks and how
to find out the coordinates of the mouse pointer. To do this, use
function 3. The following things are returned:

    outregs.bx      will be the button clicked: 0 - no buttons
                                                    clicked
                                                1 - left button
                                                2 - right button
                                                3 - both buttons
    outregs.cx      the x coordinate
    outregs.dx      the y coordinate
    
The x and y coordinates are usually given as pixels, but in SCREEN 13
you'll need to divide the outregs.cx value by 2 to get the x value. To
get text coordinates, i.e. 80x25 if you're in SCREEN 0, divide
outregs.cx and outregs.dx by 8, like this:

    x = INT(outregs.cx / 8) + 1
    y = INT(outregs.dx / 8) + 1
    

Here's an example program. It waits until the user clicks a button on
the screen, and then exits. It uses the interrupt line in a SUB to
save typing and program space. Note that if you do this you will need
to make the inregs and outregs variables SHARED. (Sorry about the
split LOOP line, you'll have to make that all one line if you paste
it into QB)

    ' Sample mouse program from BASIX Fanzine Issue 9
    ' ** Remember to run QB with the /L option to load libraries **
    '$INCLUDE: 'QB.BI'
    DECLARE SUB mouse()
    DIM SHARED inregs AS REGTYPE, outregs AS REGTYPE
    SCREEN 13
    inregs.ax = 0 : mouse
    LINE(10, 10) - (50, 50), 10, B
    inregs.ax = 1 : mouse
    DO
    inregs.ax = 3 : mouse
    LOOP UNTIL outregs.bx = 1 AND (outregs.cx / 2 > 9 AND outregs.cx / 2
                            < 51 AND outregs.dx > 9 AND outregs.dx < 51)
    END
    
    SUB mouse()
    CALL INTERRUPT (&H33, inregs, outregs)
    END SUB
    

Important note about mouse programming: If you draw something to the
screen where the mouse is you'll find the mouse wipes that bit of the
screen when it is moved - try it. Use function 1 to display the mouse
in SCREEN 13, PAINT the screen green then move the mouse. It leaves a
black square behind - this is the area of the original black screen
before it was PAINTed. To stop this happening, use function 2 to hide
the mouse while making changes to the screen, then function 1 to show
it again after the changes have been made. 

You can set the mouse position using function 4, where inregs.cx is
the x co-ordinate and inregs.dx is the y co-ordinate. For example, to
set the mouse position to (30, 40) you would use:

    inregs.ax = 4
    inregs.cx = 30
    inregs.dx = 40
    CALL INTERRUPT (&H33, inregs, outregs)
    
(remember to multiply cx by 2 in screen 13)

Finally, here are two more functions. They limit where the mouse can
go so you can 'trap' it inside an area of screen. You'll need to use
inregs.cx as x1 or y1, and inregs.dx as x2 or y2. The functions are:

    7       Limit mouse on x-axis
    8       Limit on y-axis.
    
So to trap the mouse between (5, 5) and (315, 195) in SCREEN 13, use
this:

    inregs.ax = 7           ' limit x-axis
    inregs.cx = 10          ' i.e. 5*2
    inregs.dx = 630         ' i.e. 315*2
    inregs.ax = 8           ' limit y-axis
    inregs.cx = 5
    inregs.dx = 195
    mouse