V1 trouble with gcc 3.2.3 SOLUTION!!!

[Eric W. Biederman]

Dave Ashley <linuxbios at xdr.com> writes:

> Aside from the = vs == bug which probably wasn't too damaging,
> the problem was caused because the newer compiler doesn't use
> as many registers in its code, specifically the EBX register.
> So at the start of the do_vgabios() function it doesn't bother
> to push EBX on the stack to preserve it. However code higher
> up that calls this function had stored something in EBX, which
> the call to the vgabios happily trashes.
> 
> The fix is to push ebx within the real_mode_switch_call_vga function:
> 	/* save the stack */
>  "push %ebx\n"
> 	"mov %esp, __stack\n"
> 
> ...
> 
>        "    mov  %ax, %ss          \n"
>        "    mov  __stack, %esp\n"
>  "pop %ebx\n"
>        );
> 
> It won't hurt to push other registers like %esi for example. In fact
> why not push all the registers just to be safe...
> 
> This is a pretty subtle bug, people should understand what was
> wrong and how the fix works.

Well it is a little less subtle than it might be.  In the C calling
conventions frequently some of the registers are designated callee save.
Which means if you are going to use that register you must save it.
%ebx, %ebp, %esi, and %esi are the callee save registers on x86.

With inline assembly you can avoid some of that by telling the compiler
which registers you will be using and it will handle the nuances
of the calling conventions.  That is possibly the better approach.

Eric