[Coco] Timer interrupt frequency (50Hz vs 60 Hz)

[Alex Evans]

You could use something like the sync instruction with interrupts masked.

On Thu, Mar 20, 2025, 21:17 RETRO Innovations via Coco <coco at maltedmedia.com>
wrote:

> On 3/20/2025 7:51 PM, Alex Evans via Coco wrote:
> > The trouble I see is that both hsync and hsync are IRQ though you can
> check
> > the pia to see which one fired.
> Apologies, I thought one was IRQ and the other FIRQ.  But yes, you could
> check PIA.
> > There is also the issue that servicing an
> > IRQ is not really fast because all of the registers are preserved.
>
> Well, my thought is that this code would only run once for at most 1/25
> of a second (worst case situation is that fsync irq is enabled
> immediately following an FSYNC IRQ, so it would run one frame without
> counting, and then the next it would count.  But, the below code can be
> optimized if needed.
>
> My main is whether it would give different values on NTSC versus PAL.  I
> assume so, since there would be 262 or 263 hsyncs in NTSC, but 312/313
> in PAL.
>
> >
> > On Thu, Mar 20, 2025, 20:33 RETRO Innovations via Coco <
> coco at maltedmedia.com>
> > wrote:
> >
> >> On 3/20/2025 7:07 PM, Alex Evans via Coco wrote:
> >>> Yes, you can interrupt (IRQ) on hsync on any coco.
> >> So if you write:
> >>
> >> prep:
> >>
> >> //put 0 in NUM
> >>
> >> // put 0 in START
> >>
> >> //enable fsync
> >>
> >> // wait for START to equal 2
> >>
> >> // use NUM value to calc PAL or NTSC
> >>
> >>
> >> fsync_IRQ
> >>
> >> // increment START
> >>
> >> // if START = 1
> >>
> >> // zero out NUM, enable hsync, and return
> >>
> >> // else disable fsync and hsync and return
> >>
> >> hsync_IRQ:
> >>
> >> // increment NUM
> >>
> >> // return
> >>
> >> Would the above give you different numbers in NUM you could use to
> >> determine PAL/NTSC?
> >>
> >> Jim
> >>
> >> --
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