SeaVGABIOS ignores preserve-memory flag - SeaBIOS

12 Jul 2020


      Hello, here’s another defect I’ve found, this time in SeaVGABIOS.
Video BIOS interrupt 0x10 subfunction 0 accepts a flag that determines 
whether video memory should be preserved after mode change. SeaVGABIOS 
ignores this flag. The reason is a buggy implementation of the interrupt 
call in vgasrc/vgabios.c:
...
static void
handle_1000(struct bregs *regs)
{
    int mode = regs->al & 0x7f;
// Set regs->al
if (mode > 7)
    regs->al = 0x20;
else if (mode == 6)
    regs->al = 0x3f;
else
    regs->al = 0x30;

int flags = MF_LEGACY | (GET_BDA(modeset_ctl) & 

(MF_NOPALETTE|MF_GRAYSUM));
    if (regs->al & 0x80)
        flags |= MF_NOCLEARMEM;
vga_set_mode(mode, flags);

}
The AL register’s structure member is overwritten by the chained if 
before the flag is checked, which means the flag is never picked up. 
This is a regression from commit 
5108c69c47e18244206593c0c7918711311d8ef3, which removed the noclearmem 
variable that stashed the flag for later.[0]
Sloppy refactoring aside, there was no explanation for this condition 
chain, so I became puzzled as to why the register was being overwritten 
in the first place. It seems this was meant to mimic a behaviour 
described in Ralf Brown’s:
...
Return: AL = video mode flag (Phoenix, AMI BIOS)
            20h mode > 7
            30h modes 0-5 and 7
            3Fh mode 6
        AL = CRT controller mode byte (Phoenix 386 BIOS v1.10)
I have checked this with an Intel video BIOS on a physical machine, and 
it left the register untouched, so this behaviour seems hardly 
universal. Not even DOSBox emulates this, and I think they do have some 
incentives to emulate these calls as faithfully (to old hardware) as 
possible, so that is saying something. (Not to mention RBIL itself 
describes two conflicting behaviours here.) So I doubt that this 
condition chain needs to be kept in the first place. But even if it is 
kept, I think it should be moved a little later. And probably commented 
too.
[0]: 
https://git.seabios.org/cgit/seabios.git/commit/vgasrc/vgabios.c?id=5108c69c...
—
Regards,
f