On Sun, Mar 04, 2012 at 07:26:58PM +0000, Julian Pidancet wrote:
On Sun, Mar 4, 2012 at 7:04 PM, Kevin O'Connor kevin@koconnor.net wrote:
So, I guess the question is, how important is support for current/legacy x86emu versions?
I've tried to replace .code16gcc with .code16 in src/code16gcc.s to see if gcc would be able to generate code which doesn't use 32bit version of the call/ret instructions. The result was quite disappointing, it generates functions like this:
That definitely wont work. Gcc assumes the return address is 32bits - it will be totally confused if it's not. (Function parameters passed on the stack wont be in the right spot.) Likely other things will break as well.
I am going to propose a patch to xorg-devel in the next few days, but in the meantime, it would be nice to find a solution in SeaBIOS so the code can work with older versions of Xorg.
The only thing I can think of would be to post-process the assembler and replace "retl" instructions with "retw $2" instructions. I'm not sure if it would work and it would be real ugly.
-Kevin
On Sun, Mar 4, 2012 at 7:54 PM, Kevin O'Connor kevin@koconnor.net wrote:
The only thing I can think of would be to post-process the assembler and replace "retl" instructions with "retw $2" instructions. I'm not sure if it would work and it would be real ugly.
As I mentionned, ret is not the only instruction causing problems. I've identified issues with leave, enter, iret, and even some forms of the call instruction, and the list is probaly not complete yet. So it could be even more complicated that we think.
It looks like x86emu was never tested with gcc produced code before. And it looks like handling of the 0x66 instruction prefix has been neglected in a lot of different places in the code.
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Julian Pidancet -
Kevin O'Connor