mirror of https://github.com/ipxe/ipxe.git
54c5d08df1
The vgabios used by bochs and qemu (and other virtualisation products)
has a bug in its implementation of INT 10,4f00 which causes the high
16 bits of %ebx and %edx to become corrupted.
The vgabios code uses a "pushaw"/"popaw" pair to preserve the low 16
bits of all non-segment registers. The vgabios code is compiled using
bcc, which generates 8086-compatible code and so never touches the
high 16 bits of the 32-bit registers. However, the function
vbe_biosfn_return_controller_information() includes the line:
size_64k = (Bit16u)((Bit32u)cur_info->info.XResolution *
cur_info->info.XResolution *
cur_info->info.BitsPerPixel) >> 19;
which generates an implicit call to the "lmulul" function. This
function is implemented in vbe.c as:
; helper function for memory size calculation
lmulul:
and eax, #0x0000FFFF
shl ebx, #16
or eax, ebx
SEG SS
mul eax, dword ptr [di]
mov ebx, eax
shr ebx, #16
ret
which modifies %eax, %ebx, and %edx (as a result of the "mul"
instruction, which places its result into %edx:%eax).
Work around this problem by marking %ebx and %edx as being clobbered
by the call to INT 10,4f00. (%eax is already used as an output
register, so does not need to be on the clobber list.)
Reported-by: Oliver Rath <rath@mglug.de>
Signed-off-by: Michael Brown <mcb30@ipxe.org>
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