[SeaBIOS] [PATCH 1/2] Don't pass return address to transition(32, 16, 16big) on stack.
H. Peter Anvin
hpa at zytor.com
Wed Dec 8 03:51:07 CET 2010
On 12/07/2010 06:46 PM, Kevin O'Connor wrote:
>> In Syslinux I have this formalized so that the sequence:
>> pushl $func32
>> callw _pm_call
> This is similar to what SeaBIOS used to do - it had: "pushl $func32;
> jmp transition32" and "pushl $func16; jmp transition16".
> The problem with this is that I can't use "popl" to get the
> destination address in transition16 because a popl in 16bit mode only
> looks at %sp and not %esp. So, if %esp==0x90000 and I do "pushl
> $func16; transition16", then when transition16 does a "retl" (or
> "popl") then it ends up pulling the address at 0x0000 instead of
Right, the code needs to compute the 32-bit flat version and look at it.
You don't want to use popl at all. In my code I just use the stored
reference on the stack as the target address of the call once we're well
within the 32-bit code, at the very end it is dropped by a simple "ret 4".
Anyway, sounds like there might be a fuller version and I'm only seeing
part of it.
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel. I don't speak on their behalf.
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