[Coco] IRQ question

Salvador Garcia salvadorgarciav at yahoo.com
Thu Feb 2 11:16:57 EST 2017

Although I have no experience with 6809 code, I have done a few ISRs for i86 computers and when I read the part about the ISR being "big" that also caught my attention.For debug only, reduce the ISR size by 50% (or more), removing or commenting out code that isn't critical to the game's functionality (if possible) and see if that improves behavior. Salvador

      From: Arthur Flexser <flexser at fiu.edu>
 To: CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com> 
 Sent: Wednesday, February 1, 2017 10:20 PM
 Subject: Re: [Coco] IRQ question
You say your IRQ service routine is big.  Could it possibly be SO big that
servicing takes more than a 60th of a second on the CoCo 3?  That would
certainly cause some IRQ servicings to be skipped.  Or even if servicing
took slightly under a 60th of a second, the next IRQ would hit almost
immediately after return from the previous service, so little code could be
executed in between, causing a substantial slowup.

You could count cycles in the service routine to closely estimate how long
servicing should take in the CoCo 3 double-speed mode.


On Wed, Feb 1, 2017 at 9:55 PM, Glen Hewlett <glen.hewlett at sympatico.ca>

> Hi All,
> I need some help from a CoCo3 IRQ expert.
> I thought I had this figured out but something is not right with my IRQ
> setting for my Pac Man transcode project.  I’ve gotten to the first attract
> mode and the animation is running about 1/4 the speed of the real machine.
> I was getting worried it was a problem with the CPU not being fast enough
> but it looks like the IRQ just isn’t kicking in at the correct time.  It
> seems to update the IRQ on the CoCo 3 every 4 frames and not when the
> VBLANK occurs at least according to MAME in debug mode.  What I need is it
> to be triggered on every VBLANK or (VSYNC)
> On Pac man hardware the IRQ kicks in approximately 60 times a second and
> using MAME debugger option I can see it in action, every IRQ it does is one
> frame.  On the CoCo 3 under MAME debugger running my code it seems to kick
> in around every 4th frame and definitely not when the beamy is at 241 like
> pac man shows under MAME debug mode.  I know MAME might not be perfect but
> there is definitely something wrong with my IRQ settings.  I’ve setup the
> following code for my IRQ thinking it should trigger on the VBORD, is this
> the same as VBLANK?
>        LDA    #%00110100              *
>        STA    $FF01                  * HSYNC IRQ Disabled, IRQ Polarity
> Flag falling Edge, Data Direction Normal, Select Line LSB = 0, HSYNC Flag =
> 0
>        STA    $FF03                  * VSYNC IRQ Disabled, IRQ Polarity
> Flag falling Edge, Data Direction Normal, Select Line MSB = 0, VSYNC Flag =
> 0
>        STA    $FF21                  * CONTROL OF CD FIRQ* TO CPU
> DISABLED, IRQ Polarity Falling Edge of CD, CD Flag off
>        STA    $FF23                  * CONTROL OF Cart FIRQ* TO CPU
> DISABLED, IRQ Polarity Falling Edge of Cart, Cart Flag off
>        LDA    #%01101100              *
>        STA    $FF90                  * CoCo 3 Mode, MMU Enabled, GIME
> IRQ Enabled, GIME FIRQ Disabled, Vector RAM at FEXX enabled, Standard SCS
> Normal, ROM Map 16k Int, 16k Ext
>        LDA    #%00000000              *
>        STA    $FF91                  * Mem Type 64k chips, 63.695 usec
> timer, MMU Task 0 - $FFA0-$FFA7
>        LDA    #%10000000              *
>        STA    $FF98                  * Graphics mode, Colour output, 60
> hz, max vertical res
>        LDA    #%00001000              * $08
>        STA    $FF92                  * Enable only the Vertical Border
> Sync (VBORD) Interrupt
> The IRQ routine itself is big… Other then running normal code I’m only
> doing the following in the IRQ routine itself:
> IRQ starts…
>        No special code here
> At the end of the IRQ I have:
>        LDA    $FF92                  * Reading this makes the IRQ
> enabled again
>        RTI
> Does the above look OK for setting the IRQ to hit 60 times a second on the
> VBLANK signal?  If so then maybe the CoCo 3 just isn’t fast enough to run
> my Pac Man translated code.  :(
> Thanks for looking,
> Glen
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