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CSM BIOS for EFI machines (Announce)

posted by Zyzzle, 03.06.2025, 04:53

> I guess it would be possible to modify the framebuffer BAR but it would
> break UEFI graphics driver without notifying about that and also there
> would need to be enough free space under 4GB. As modern VGA has over 4GB
> VRAM it might be problematic. I don't know how much space wastes the MMIO
> of modern GPU. Enginners may tend to not take much care when they have
> 64-bit address space... :P
 However, for onboard graphics using system RAM, memory allocated to VGA can be set to low amounts, like 32mb or 64mb, more than enough for VESA modes in DOS up to and beyond 2048x1536 32 bpp. But, these onboard graphics of course have no CSM. Or, at least, I've never heard of one that does have. So, you "need" a modern video card and those have at least 4GiB RAM nowadays.

MMIO on "modern" CPUs can waste quite a lot -- between 512 mb and 1GB in the 3Gib to 4GiB address range. However, as we know most DOS stuff only works in signed 32-bit memory ranges anyway (2 GiB-1).

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