Hi,

I guess I fit a common profile (at least here): I am "getting back to my roots" by trying to code a c64 game in asm (a.k.a. revisiting an incomplete childhood dream). This is my first experiment (I'm using KickAssembler and Vice), and so far I'm having a blast.. However, here is the outline of a problem I can't seem to solve by myself:

(a) I have an event that must occur at regular intervals (e.g. every second)

(b) I also want to react on certain key presses, and have the key event continue to trigger as long as the key is held

My current solution is as follow:

(1) I set the key autorepeat mode:

Code:

lda #128 sta $028a

(2) I set an interrupt handler:

Code:

sei lda #<interrupt_handler sta $0314 lda #>interrupt_handler sta $0315 cli

(3) In this handler, I keep a counter, which, every time it reaches 60, runs the required timing event (a) (i.e., every second, since the interrupt is called every 60th of a sec); this seems to be working rather well (although I don't know if it's a very accurate way to handle a timer)

(4) In this same interrupt (directly following the previous code), I also call, at smaller intervals, the GETIN kernal routine ($ffe4), and that's where my problems begin.

Every time a certain key is detected, some additional code is run. I noticed that the "time" (or rather complexity) of this code seems to be important, because with the experiments I made, when it is below a certain threshold, everything runs fine. But when it's over it, strange things start to happen, and my program basically crashes.

This thread seems to agree with this "theory":

http://www.lemon64.com/forum/viewtopic.php?t=38309

I see recommendations that events of this type should be "run off the IRQ", but I don't understand enough about interrupts for this to be really meaningful. I seem to understand that the mechanism I'm currently using is the timer signal from the CIA chip (not sure about this), and that there other types of interrupt I could be using (the raster interrupt in particular is often mentioned). Ideally, I would require an event triggered simply on key presses, but I don't know if such a thing is possible, or simple enough to do.

Thanks in advance for any help!

For an action type game relying on keys you'd be way better off scanning the keyboard yourself.
But if e.g. it's just an RPG or something where you press certain keys singularly then letting the OS do it should be fine.

The timer IRQ does odd bits of system housekeeping too. The deficiency of the OS keyscan is that it's subject to ghosting and lost keys due to either multiple keypresses or someone using the joystick.

You can solve multi key presses and mostly eliminate joystick interference if you scan for individual keys yourself.

Many thanks guys! THEWIZ's solution was easy to implement, and seems to be working very well.. I learned something!