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v86 does not emulate 'chunky' VGA graphics mode.
Version: ccb5e1ce (Sep 20, 2025 11:09)
v86 does not emulate the 'chunky' VGA graphics mode as seen in the NEW KERNEL/VGA demo (https://github.com/pritamzope/OS.git)
Instead of outputting pixels, it stays in the VGA text mode, but displaying an array of whatever should be onscreen as text. The text mode in v86 is incorrectly set to: 'Resolution: 80x480'.
Coincidentally this is also BIOS-related. Like in your other issue, it works using Bochs BIOS (or to be more specific, its VGA BIOS):
https://github.com/user-attachments/assets/da146783-4c9c-4f9a-898d-e9ebe28a7ce7
I only vaguely know about screen mode differences between the two BIOSes (Bochs and SeaBIOS), but I'm certain that mode 320x200x8 is supported by both, maybe you can find a different way to set the video mode (for example, BIOS interrupt 0x10 with mode 0x13, IIRC).
To build the CDROM image VGA_Simple_OS.iso I simply ran OS/VGA/kernel_c/Simple/run.sh.
So, is this still a v86 issue? It does not come with the Bochs BIOS by default, yet it fixes most things. Should it be bundled by default?
Bochs BIOS is bundled with V86.
It's not used in any of the presets, and I think that a small change to V86 would be needed to support presetting BIOS and VGABIOS to Bochs.
Pragmatically, Bochs could be seen as a band-aid until this is also supported in V86 when using SeaBIOS.
No, a simple qemu-system-i386 VGA_Simple_OS.iso does not have this issue.
Exactly! I was sure QEMU used Seabios, that is why I thought all these issues were v86 related. How could the same BIOS have such different and broken affects under different emulators?
Bochs with built-in BIOS works as expected: bochsout-bochs.txt, bochsrc-bochs.txt
screenshot
However, it doesn't work on SeaBIOS, taken from v86: bochsout-v86.txt, bochsrc-v86.txt
screenshot
...and compiled from SeaBIOS source code: bochsout-seabios.txt, bochsrc-seabios.txt, seabios-config.txt
And for interest, ET4000AX VGA BIOS gives the correct image in v86:
screenshot
Bochs commit: https://github.com/bochs-emu/Bochs/commit/84b37f5faab6a552c97692b01db830f046572c81 SeaBIOS commit: https://github.com/coreboot/seabios/commit/0026c353eb4e220af29750fcf000d48faf8d4ab3
I found a few more pieces to this puzzle.
Mixing SeaBIOS BIOS with Bochs VGABIOS
We do not need Bochs BIOS, we can use V86's default SeaBIOS BIOS and mix it with Bochs VGABIOS, I tested it and that's enough to make it work.
Guest OS "pritamzope/OS"
The code we tested in VGA/kernel_c/* doesn't actually use the BIOS to interface the VGA adapter, instead it writes directly to the VGA registers to setup the video mode (for example see VGA/kernel_c/Simple/vga.c, it's the same for all 4 projects in this subtree).
This means that the only difference here in regard to the VGA BIOS is the register state of the VGA adapter after the BIOS has setup the initial video mode at early boot. To my understanding, V86 loads BIOS and VGABIOS, starts the BIOS, and then the BIOS uses the VGABIOS to setup the initial VGA state. The potentially different inital VGA states might explain why the direct register access works with Bochs VGABIOS and not with SeaBIOS' VGABIOS.
Note that the code in /NEW KERNEL/VESA VBE/* uses the VBE BIOS extensions instead to setup the VGA video mode, and this works as expected under V86 with the default BIOSes, I compared the output to qemu's to make sure:
Screenshots VESA_VBE20 under V86 and qemu
Download: VESA_VBE20.zip
Qemu's default VGABIOS
From https://www.qemu.org/docs/master/system/qemu-manpage.html:
QEMU uses the PC BIOS from the Seabios project and the Plex86/Bochs LGPL VGA BIOS.
The qemu VGABIOS (Bochs, actually) project appears to be at https://github.com/bochs-emu/VGABIOS, and the VGABIOS image file it builds is VGABIOS-lgpl-latest.bin, and I'm 99% sure that this is the VGABIOS used by qemu. So far I couldn't confirm this on my Debian 12 (still looking), maybe someone else can help me to confirm this? I don't know enough about qemu.
So I tested our default SeaBIOS together with that qemu VGABIOS image file and guest OS "pritamzope/OS" works as expected under V86.
If this is indeed the default VGABIOS used by qemu, and if it should be compatible with all V86 profiles and tests, then simply switching to this VGABIOS might be a proper fix here.
Download VGABIOS-lgpl-latest.bin
Interesting, I tried using bios.bin and vgabios-stdvga.bin (default in QEMU) from https://gitlab.com/qemu-project/qemu/-/tree/master/pc-bios and they don't work with VGA_Simple_OS.iso on v86 as expected.
Interesting, I tried using
bios.binandvgabios-stdvga.bin(default in QEMU) from https://gitlab.com/qemu-project/qemu/-/tree/master/pc-bios and they don't work withVGA_Simple_OS.isoon v86 as expected.
Yeah, I can confirm. Did you see the README:
SeaBIOS (bios.bin) is the successor of pc bios.
See http://www.seabios.org/ for more information.The VGA BIOS and the Cirrus VGA BIOS come from the LGPL VGA bios
project (http://www.nongnu.org/vgabios/).
The second link points to Bochs, it's confusing.
This leaves the main question unanswered. I checked the VGA logs of both VGABIOSes by booting both without any bootable media, this logs all VGA things that the BIOS is doing during startup.
Unsurprisingly, these logs look completely different, Bochs is about 1/3rd longer.
If I find time I will try to compare the registers states of V86's VGAAdapter at the end of these boot sequences, maybe we have a bad or missing default. That would assume that this guest does not initialize the entire VGA adapter.