Apple II
Technical Notes
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                                                  Developer Technical Support


Mouse
#1:    Interrupt Environment with the Mouse

Revised by:    Matt Deatherage                                  November 1988
Revised by:    Rilla Reynolds                                   November 1985

This Technical Note describes the interrupt environment one should take into 
account when programming mouse-based applications on the Apple II family of 
computers.
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Software developers who are writing mouse-based programs in assembly language 
need to be concerned about the computer's interrupt environment, even if they 
are using the mouse in passive mode.  Listed below are several conditions 
which assembly language programmers should take into account if their programs 
are to run on the Apple II family of computers.

o    Do not disable interrupts unless absolutely necessary.  If you 
     disable them, be sure to re-enable them.
o    Disable interrupts when calling any mouse routine.  Always use PHP 
     and SEI to disable interrupts, then use PLP to re-enable them.  
     This method preserves the state of interrupts (enabled or 
     disabled).
o    Do not re-enable interrupts (PLP) after a call to ReadMouse until 
     X and Y data have been removed from the screen holes.
o    Disable interrupts (PHP and SEI) before placing position 
     information in the screen holes (PosMouse or ClampMouse).
o    Enter all mouse routines (except ServeMouse) with the X register 
     set to $Cn and Y register set to $n0, where n = the slot number.
o    Some programs need to disable interrupts for purposes other than 
     reading the mouse.  If interrupts are disabled then re-enabled, 
     the first call to ReadMouse could return incorrect values; 
     subsequent calls to ReadMouse will return correct values until 
     interrupts are disabled and re-enabled again.  Disabling 
     interrupts for mouse calls does not create this problem.  If you 
     watch numbers from the mouse while moving it in a direction which 
     would increase values, you would see something similar to: 6, 7, 
     8, 9, 8, 9, 10.  In practice, this momentary "glitch" in the 
     stream of data has little importance.  If you feel you must avoid 
     this glitch altogether, do not disable interrupts for more than 40 
     microseconds or make sure that at least one mouse interrupt takes 
     place after re-enabling interrupts.