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Inspired by this question about the CPU frequency when booting the Amiga in PAL vs NTSC, I'm curious if the CPU frequency was affected by using the other display mode.

In other words, I used an ECS Amiga in North America, thus booted as NTSC. To get a little extra vertical real estate on my screen, I would run Workbench with a PAL non-interlaced mode. My A1960 monitor happily synced to the 50Hz vertical refresh rate and I enjoyed a few dozen extra lines of Workbench height.

So my question is, was my CPU running a touch slower as a result of this, or did the selection of display mode on an NTSC-booted Amiga not adjust the CPU frequency?

I'm presuming that the Denise chip's display mode wouldn't impact the CPU clock, but I'm not sure in this case. I am assuming based on the linked question's information that when I would run some games in PAL mode (I had a well worn 'PAL Booter' disk close by at all times), THAT would impact the CPU frequency. But a simple Workbench display mode selection? Presuming not...

FWIW, I had an A2630 accelerator and would be running the '030 typically, if that has any impact on the answer.

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    Isn't this already provided in an answer in the linked question?
    – Justme
    Jan 19, 2023 at 18:15
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    @Justme - no, the linked question only addresses when you boot the machine in PAL vs NTSC mode. I re-read the question and it doesn't seem to answer "if I booted as NTSC and use a PAL display mode"
    – bjb
    Jan 19, 2023 at 19:30
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    Well, quoting the answer, it has a single clock oscillator which is PAL or NTSC machine specific and thus can't change CPU frequency, it will just reprogram ECS video generating parameters. And your accelerator runs on it's own oscillator anyway so it can't change CPU frequency. I can expand or clarify if needed.
    – Justme
    Jan 19, 2023 at 19:32
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    Does this answer your question? Did the Amiga change CPU frequencies when switching between PAL and NTSC mode?
    – szulat
    Jan 19, 2023 at 19:35
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    @bjb Did you read the a answer? Changing the video mode cannot change the CPU or system clock speed so the system runs at exactly same speed, just generating different video signal timing.
    – Justme
    Jan 19, 2023 at 20:03

1 Answer 1

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Because PAL output needs to shove more data (roughly 25% more) out to the screen, it accesses chip memory longer than in NTSC mode - does this, however, more rarely (50 times/s instead of 60 times). This results in a (slightly) higher load onto chip memory - Chip RAM is simply hogged more by the video circuitry in PAL mode than in NTSC mode - Thus, it's likely that the CPU needs to wait a bit longer when it wants to get access to chip memory, and chip memory might appear slightly slower in PAL mode. Because of slightly higher resolutions, filling the screen by the CPU will definitely also take a bit longer. (That's the price you pay for a few more screen lines as Raffzahn rightly mentioned in a comment)

Any other performance aspects (especially when working on fast RAM) should not be affected at all.

However: Games that drive progress and "base speed" on the vertical retrace might appear faster, because the game is driven forward 60 times per second in NTSC than 50 times in PAL - The interrupt service routine is simply run more often - On the other hand, non-interrupt code might appear faster in PAL because it's not interrupted as often, so this perceived performance migh vary based on the application - The Workbench, which you specifically asked for, shouldn't be of the interrupt-driven type, and might in fact even appear to run a bit faster on PAL, but overall, differences should really be marginal.

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  • this doesnt seem right to me. Both Pal and NTSC have horizontal scan rates of 15khz (ntsc having fewer lines), so the time between the scan lines, is the same, and so would the data rate unless a different horizontal resolutuon were used. retrocomputing.stackexchange.com/questions/26192/…
    – camelccc
    Jan 20, 2023 at 11:50
  • @camelccc Horizontal scan line duration is basically the same on PAL and NTSC, yes - (I did not say anything about data rate) - but a PAL picture consists of more scanlines.
    – tofro
    Jan 20, 2023 at 12:40
  • @tofro: Another perhaps clearer way of explaining the difference would be to say that in PAL, a smaller fraction of the display lines are within the vertical blanking interval during which the normal chip memory accesses are waived.
    – supercat
    Jan 20, 2023 at 15:59
  • @supercat exactly the same amount of time is spent in the scanback. The only dufference I can possibly see PAL vs NTSC makung is the fact that the cpu frequency is a percent higher in NTSC, see the answer I linked in the earlier comment.
    – camelccc
    Jan 20, 2023 at 19:27
  • @camelccc: NTSC offers 60 vertical blanks per second, while PAL only has 50.
    – supercat
    Jan 20, 2023 at 19:32

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