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Thread: The myth of the vertical retrace interrupt on EGA/VGA

  1. #11
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    Quote Originally Posted by Agent Orange View Post
    Question: What was the upshot?
    Upshot of what?

  2. #12
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    Quote Originally Posted by Scali View Post
    Upshot of what?
    Connecting the IRQ2 line. What was the end result?

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    Quote Originally Posted by Agent Orange View Post
    Connecting the IRQ2 line. What was the end result?
    That it behaved correctly, and signaled an interrupt at the end of the visible area every frame, as per the IBM EGA, VGA and XGA specification.

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    Quote Originally Posted by Scali View Post
    That it behaved correctly, and signaled an interrupt at the end of the visible area every frame, as per the IBM EGA, VGA and XGA specification.
    Not to nit-pick, but what would the observer see? Would performance or viewing be noticeably enhanced?

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    Quote Originally Posted by Agent Orange View Post
    Not to nit-pick, but what would the observer see? Would performance or viewing be noticeably enhanced?
    I don't think the user would notice a difference. It seems that because of the poor support of this functionality on clone VGA systems, that it was rarely used by software, if at all.
    My intention with this thread is two-fold:
    1) To see if we can get a decent idea of how well-supported this feature really is, and whether developers in the past were right to avoid this functionality.
    2) If it is supported widely enough, I might use it in my future DOS demos. The PC platform has a nasty shortage of timers. Being able to use the 8253 PIT for music and timing the graphics on the vertical retrace would make some things a lot easier/more accurate.

  6. #16
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    I've updated the code: https://www.dropbox.com/s/bzobfyhmpp...etirq.zip?dl=0
    It now supports an optional parameter, so you can specify the IRQ (between 0 and 15):
    Code:
    vretirq 11
    If you don't specify anything, it still defaults to IRQ 2, as before (on a standard AT, it should work with both IRQ2 and IRQ9, since the AT BIOS has a default IRQ9 handler which redirects to a software IRQ2, to remain backward-compatible with the PC and XT).

    Interestingly enough, some modern Plug&Play systems do actually still support the vertical retrace IRQ, but it is just not configured at IRQ2/9 where you expect it. It may be at IRQ10, 11 or wherever the BIOS saw fit. But it does get generated properly, so it can be used.

  7. #17
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    Another update:
    Someone over at Vogons had a PVGA1A card as well, and connected a wire from pin 35 on the PLCC to pin B4 on the ISA slot (IRQ2). Vertical retrace interrupt works fine, just like the Trident.
    He also tested an ET3000... and it is the first card that actually seems broken. As in, it generates a few IRQs, and then just stops for no apparent reason.
    All other cards tested so far, either worked just fine, or didn't work at all (where some just lacked a trace on the PCB and could be patched with a simple wire).

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