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Need help to find VGA bug in my emulator

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First post, by hottobar

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I'm facing a graphics bug in my emulator (https://github.com/barotto/IBMulator) where a part of the framebuffer is not correctly rendered.

Here is a demo of the problem: https://imgur.com/a/XFDtR

I suspect this issue https://github.com/barotto/IBMulator/issues/9 with Rocketeer could be caused by the same bug.

My understanding of the VGA hardware is limited and frankly I don't have any idea of where to look for this bug.
Do you have any suggestions?

What's a good book about the VGA that explains its inner workings in good detail?

IBMulator shares a lot of code with Bochs and this same bug is present on that emulator too.
If I'll find and fix this maybe I could send a patch upstream.

Reply 1 of 4, by Jepael

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Looks like split screen issue, or more like no split screen support at all perhaps?

Reply 2 of 4, by hottobar

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Thank you I'll definitely look into that!
Do you know of any program or game that uses VGA split screen that I can use as a test?

Reply 3 of 4, by ripsaw8080

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Many side-scrolling games, such as Inner Worlds, use a split screen for a status area. However, some Event Horizon games, such as Veil of Darkness and The Summoning, have a split that can be moved by the mouse, and thus are particularly good for testing.

Reply 4 of 4, by hottobar

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ripsaw8080 wrote:

Veil of Darkness

Bingo! I quickly tested this game and it failed spectacularly. It seems like Bochs (and so my emu) has a very buggy support for VGA split screen.

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