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Forums > C64 Coding > Best practice IRQ recovery
2021-06-10 20:30

Registered: Jul 2010
Posts: 208
Best practice IRQ recovery


Here's a little newbie question. Sorry, I'm still learning this shit and it's really complicated :(

I have kernel off ($01=$35) and I am running IRQ's using the normal $fffe/$ffff vectors.
I want to exit from this and call a prepacked piece of code (in this case something packed with TinyCrunch).

I tried restoring the IRQ vectors and jump to the packer. However, it just hangs. I tried some other things but all gave the same result. The only thing that worked was when I did this:

lda #$36
sta $01
jsr $ff81
jmp unpacker

The problem of course is that it resets the VIC which isn't really great for my situation.

So, my question:

What is the correct/proper way to exit from a part and go to the next? preferably not using kernal routines :|

Thank you.

2021-06-10 20:54

Registered: Mar 2009
Posts: 944
F...ing around with $01 reg just to use KERNAL routines is hardly ever worth the trouble if you don't want KERNAL banked in other parts of your code.
Just look into KERNAL ROM routines to see what they do, for example here: http://unusedino.de/ec64/technical/aay/c64/krnromma.htm and then take the few things you need, normally that would be what you find at $EA81 = restoring accumulator and x/y registers and use them somewhere in RAM.
2021-06-10 21:09

Registered: Apr 2002
Posts: 259
Where does it hang, before or after the depacker starts?
2021-06-10 21:34

Registered: Dec 2001
Posts: 9913
What is the correct/proper way to exit from a part and go to the next?

that raises the question what you are doing there... a trackmo? classic "press space" stuff?
2021-06-10 21:46

Registered: Jul 2010
Posts: 208
It crashes before it gets to the unpacking
2021-06-10 21:53

Registered: Dec 2001
Posts: 1007
Set a breakpoint at unpacker and then step through the code and see where it crashes? There's no one answer here, it makes some assumption about things and you'll have to figure out what it is.
2021-06-10 21:56

Registered: Dec 2001
Posts: 9913
the first thing that comes to mind is... don't use SEI. disable the irq sources instead. then set the pointers to their defaults.
2021-06-10 21:58

Registered: Apr 2002
Posts: 1916
Without further information, my bet is on pending interrupts that wreak havoc as soon as the I flag is cleared. =)

Then wrong vectors/memory configuration usually cause crashes, or with the ROM interrupt handler restored, an infinite IRQ-handling loop (as the ROM interrupt handler only acknowledges CIA1 interrupts, but not VIC interrupts).

Bottom line being that properly disabling any interrupt sources (not just executing SEI) is good practice for cleanly exiting a demo part (in a classical spacemo, that is).

Edit: Oh yeah, what Groepaz said. :D
2021-06-10 22:03

Registered: Jul 2010
Posts: 208
Well, I'm just learning stuff right now :)

ok, I will re-visit setting the IRQ vectors back. What would be the perfect sequence for this?

Should I just modify the running IRQ code to go to the default IRQ handler and then wait a few frames for it to catch it?
2021-06-10 22:12

Registered: Apr 2002
Posts: 1916
It's common practice to check for space during an interrupt handler that's executed once a video frame. The I flag is usually still set (because you're in an interrupt) and the interrupt itself was triggered by VIC.

When detecting the keypress, just write 0 to $d01a to disable any VIC interrupts, then write $ff to $d019 to acknowledge any pending VIC interrupts. Resetting any interrupt vectors should not be required. Directly exit to whatever code to unpack and run the next part.
2021-06-10 22:21

Registered: Jul 2010
Posts: 208

set IRQ vector to default handler
wait a frame or two
go to unpacker

Is that what you meant?
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