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REP + String instructions behavior

I have a few questions about the behavior of an 8088 with REP-prefixed string instructions. What is the state of IP when executing a REP STOSB, for example? Does it point at the REP prefix or the STOSB opcode itself? The intel docs say: A repeating string operation can be suspended by an exception …

Re: IBM VGA BIOS initialization

mkarcher wrote on 2022-09-18, 21:01: I don't understand what you mean by "2T4". Maybe you are talking about GFX, index 9, bit 7? Sorry, using the field names from the Ferraro book. It's a bit 7 in the Maximum Scanline Register. I really appreciate you taking the time to reply in such depth.

Re: IBM VGA BIOS initialization

Great info, and all that makes sense. I am a little puzzled as to why the Maximum Scan Line is set differently in the CGA 320x200 mode (1) vs EGA 320x200 mode (0). Or what the purpose of the '2T4' bit is if it's set for those modes but not mode 13h.

Re: IBM VGA BIOS initialization

I got things more or less working, I think. One surprising thing in Mode 13h, is that the CRTC is set up for 80 characters and 400 lines instead of the expected 320x200. I don't see any of the bits that would affect the clock enabled. How does the VGA know to double the resolution?

Re: IBM VGA BIOS initialization

A frame should actually be htotal (converted to pixel clocks, so times 8 or 9 dot clocks) times vtotal (in lines). The lines past horizontal/vertical display end are counted just like normally displayed lines, just not rendering anything at that point. So 800 horizontal clocks times the 500ish …

Re: IBM VGA BIOS initialization

I seem to recall that "Vertical Retrace" (both the status bit and the optional interrupt) is triggered by the scanline counter reaching Vertical Displayed End. The intention must have been to inform the program "okay now you can write to video memory without worrying about screen tearing". …

Re: IBM VGA BIOS initialization

superfury wrote on 2022-09-12, 20:40: You can run UniPCemu and dump the VGA state when it's running the retrace test. I'm afraid I haven't had much luck getting UniPCemu to run at all... but say I get it running where would this VGA state dump feature be found?

IBM VGA BIOS initialization

Hello all, I've been writing yet another hobby IBM PC emulator, and several posts here have been incredibly useful resources on CGA timings, etc. I was a bit hesitant to ask but this seems like the place to get the answer, so, here's my first post. I've encountered a bit of a puzzling roadblock …

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