The myth of the vertical retrace interrupt on EGA/VGA

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

Postby Scali » 2019-3-04 @ 20:31

SirNickity wrote:Results are mixed enough that you can't count on it being supported, so it wouldn't make much sense to write software that relies on it.


That is the takeaway.
Until I wrote these experimental tools and had people run them on a wide variety of hardware, I had no idea of how broad the support was for this feature.
Now we know that it is indeed not something you can generally count on. I mean, if it was say one in 20 video cards that wouldn't support it, especially if those would be the cheap brands like Trident or Realtek, I could justify using it.
But it seems that even many popular and high-end cards don't support it (or the chips do, but it's not connected on the PCB).

The only thing that's still missing is the results of a real IBM PS/2.
I mean, with 8088 MPH, our justification was that at least all our code worked perfectly on any true blue IBM (5150, 5155 and 5160).
If this feature works relibably on a PS/2, then at least I could create some kind of '8088 MPH' for the PS/2, and use this as one of its defining features.
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Re: The myth of the vertical retrace interrupt on EGA/VGA

Postby Error 0x7CF » 2019-3-05 @ 21:31

I could run it on my IBM PS/1 Model 2011 sometime Friday if you'd like. I'm pretty sure it's VGA and not MCGA.

What I can tell you right now is it doesn't generate interrupts on my Thinkpad 755CS.
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Re: The myth of the vertical retrace interrupt on EGA/VGA

Postby Error 0x7CF » 2019-3-08 @ 21:43

Zero IRQs out of my PS/1. Then again, it was a value machine so it may not be connected for similar reasoning as IBM's ISA card.
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Re: The myth of the vertical retrace interrupt on EGA/VGA

Postby Scali » 2019-3-08 @ 21:53

Error 0x7CF wrote:Zero IRQs out of my PS/1. Then again, it was a value machine so it may not be connected for similar reasoning as IBM's ISA card.


Thanks for testing, and yes, perhaps you're right.
I suppose the most interesting test would be a true PS/2, ideally the first model to ever introduce VGA (a model 30, 50, 60 or 80 apparently).
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Re: The myth of the vertical retrace interrupt on EGA/VGA

Postby SirNickity » 2019-3-12 @ 21:25

I just won an auction for a Model 30 286. I'll give it a try when I can. Will probably have to wait a couple weeks for shipping, cleaning, and PSU recapping.
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Re: The myth of the vertical retrace interrupt on EGA/VGA

Postby VileRancour » 2019-4-07 @ 23:11

Sort of relevant here - an interesting 1986 writeup about EGA's vertical refresh interrupt (some of which might explain why it never proved to be very useful):

https://archive.org/details/PC_Tech_Jou ... /page/n133
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Re: The myth of the vertical retrace interrupt on EGA/VGA

Postby elianda » 2019-4-08 @ 12:40

The only thing that's still missing is the results of a real IBM PS/2.


Done

I have tested it on my IBM Model 80 8580 and it shows:
IRQs: 0
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Re: The myth of the vertical retrace interrupt on EGA/VGA

Postby Scali » 2019-4-08 @ 13:03

elianda wrote:Done

I have tested it on my IBM Model 80 8580 and it shows:
IRQs: 0


Okay, now it REALLY gets interesting!
Did IBM deliberately disconnect the IRQ line? Or is there some kind of jumper or BIOS configuration option or such to enable it?
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Re: The myth of the vertical retrace interrupt on EGA/VGA

Postby SirNickity » 2019-4-09 @ 18:21

Just got my Model 30 286 and finished the PSU recap. Hard drive is failing (shocking...), but the floppy drive works fine. I'll give this a try as well.
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Re: The myth of the vertical retrace interrupt on EGA/VGA

Postby canthearu » 2019-4-21 @ 14:55

I have one of these cards.

https://computer-retro.de/Bilder/Grafik ... n-1991.jpg

IRQ2 is not actually connected to the trident chip, but the pin on the ISA connector is routed to the top, along with the IRQ pin from the trident. The traces end at 2 separated via's next to the top middle jumper. So IRQ2 must have been thought of during the design of this board, but decided fairly late on they weren't going to connect it and just broke the trace into 2 parts.

Putting a tiny jumper between the 2 vias reconnects IRQ2 to the trident chip and suddenly IRQs work again.
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Re: The myth of the vertical retrace interrupt on EGA/VGA

Postby SirNickity » 2019-4-26 @ 23:55

So I tried this on my PS/2 Model 30 286, with onboard VGA. By default, no interrupts. But, I couldn't remember if there were any CLI switches, so I ran it as vretirq /? and, while it didn't show a help screen, it did start counting interrupts and the white bar scrolled up screen about at a rate of about one roll from bottom to top per second. Re-running without the switch went back to 0 interrupts. The switch brought it back. Not sure what to make of that, but hope it helps.
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