Re: Subject: Re: [Qemu-devel] VGA BIOS source code

[Bartosz Fabianowski]

Alright, after digging into QEMU's graphics card emulation and into the 
VGA BIOS source, I came to a few conclusions:

a) SDD 6.53 is doing something utterly wrong. It switches the graphics 
card into a banked graphics mode (it specifically requests banked and 
not LFB) and then simply "forgets" to switch banks. I checked and 
double-checked, there is simply no call to the bank switching function. 
I guess this also means the problem with SDD 6.53 can't be fixed by 
tinkering with QEMU and/or the BIOS.

c) The fact that SDD does something wrong doesn't mean that only SDD is 
at fault if graphics modes don't work as advertised. The entire VESA VBE 
implementation is rather broken, shaky and incomplete.

b) The way the graphics card is emulated by QEMU (and Bochs and plex86 
for that matter) is totally inefficient, error-prone and overly 
complicated. The card's logic is split between a few C files (vl.c, 
hw/vga.c) that get compiled and executed natively and a bunch of C files 
containing heaps of x86 assembly that get compiled into a VGA BIOS. The 
host emulator and the BIOS work together in a very complicated way and 
it is never obvious why a certain function is implemented in the BIOS or 
on the host.

d) Finally, as a conclusion from a) to c), I believe it would be best to 
ditch the entire VGA BIOS and implement all functions of the graphics 
card in C, natively, inside QEMU. This will be more efficient and less 
error prone. Now, I haven't seen the patches mentioned by Fabrice that 
emulate a CLGD54xx. But I certainly hope it is all native and not a 
half-native half-emulated solution like the current one.

- Bartosz