If a VBE mode set call is made, stop using the vgabios extra stack.
This works under the premise that only a modern OS would invoke the
VBE mode changing facilities and a modern OS would always call the
vgabios with sufficient stack space.
This is an ugly hack to work around a problem Windows Vista (and
possibly later Windows releases) has with the VGA BIOS using a stack
in the e-segment.
Reported-by: Richard Laager <rlaager(a)wiktel.com>
Signed-off-by: Kevin O'Connor <kevin(a)koconnor.net>
I ran some further tests with the Windows emulator crashing problem.
I eventually came up with the targeted solution above. Basically, I
tried to find a signature that indicates Windows is running and
disable the extra stack in that case. It's a hack for sure, but it
seems to work.
For the curious, some other tests that I ran that didn't yield a
* The "concurrent dos" image I have actually uses vm86 mode and it
still expects the vgabios to only use a tiny amount of stack space.
So, just disabling the extra stack in vm86 mode breaks this
"concurrent dos" image.
* Windows faults on the first write access to the e-segment, so
testing if the extra stack is read/writable doesn't work (as the
test itself causes a fault which halts the emulation).
* Windows actually copies the first 1 Meg before starting the
emulation, so putting some kind of signature in the extra stack
doesn't help detect if it's read-only.
* When these troublesome vgabios calls are made, Windows is using a
copy of the first 1 Meg instead of accessing it in place. If a log
is written to the writable part of the first 1 Meg, then when the
log is inspected later it doesn't show any of the troublesome calls.
vgasrc/vbe.c | 1 +
vgasrc/vgabios.h | 1 +
vgasrc/vgainit.c | 15 +++++++++++++++
3 files changed, 17 insertions(+)
diff --git a/vgasrc/vbe.c b/vgasrc/vbe.c
index af3d0cc..834d7b3 100644
@@ -205,6 +205,7 @@ static void
vbe_104f02(struct bregs *regs)
dprintf(1, "VBE mode set: %x\n", regs->bx);
int mode = regs->bx & ~MF_VBEFLAGS;
int flags = regs->bx & MF_VBEFLAGS;
diff --git a/vgasrc/vgabios.h b/vgasrc/vgabios.h
index fd796f2..985bbbb 100644
@@ -100,6 +100,7 @@ extern u8 vgafont16alt;
extern struct video_save_pointer_s video_save_pointer_table;
extern struct video_param_s video_param_table;
extern int VgaBDF;
diff --git a/vgasrc/vgainit.c b/vgasrc/vgainit.c
index 8d12261..4616883 100644
@@ -86,6 +86,21 @@ allocate_extra_stack(void)
+ if (!CONFIG_VGA_ALLOCATE_EXTRA_STACK)
+ extern void entry_10(void);
+ if (GET_IVT(0x10).offset == (u32)entry_10)
+ // Disable the extra stack if a VBE mode set call is made. This
+ // works around a bug in Windows Vista when the stack is in the
+ // e-segment.
+ dprintf(1, "Disabling SeaVGABIOS extra stack.\n");
+ SET_IVT(0x10, SEGOFF(get_global_seg(), (u32)entry_10));
* Timer hook