On Mon, Mar 5, 2012 at 3:33 PM, Kevin O'Connor kevin@koconnor.net wrote:
Looking at the current generated code, it would seem that the only problematic instructions actually created by gcc in the current SeaVGABIOS code are "leavel" and "retl". (I don't see "enterl" generated, there are currently no function pointers, and there wont be any 32bit far calls/returns.) I wonder if we could post-process the assembler and replace "retl" with "retw $2" and "leavel" with "movl %ebp, %esp ; popl %ebp". Do you see any issues with that?
Well, it is not a very elegant solution, but it seems to be the best plan we have so far.
I can see two problems:
1) If you look at the patch I tried to submit to xorg-devel. Other instructions are concerned, in particular some forms of call (opcode 0xFF). Which means that if we decide to write a postprocess tool, we'll have to check that the ROM doesn't use those instructions.
2) Replacing instructions in the binary is simple, as long as the new instruction is the same size as the replaced instruction.
66 c3 retl (2 bytes) c2 02 00 ret $0x2 (3 bytes)
66 c9 leavel (2 bytes) 66 89 ec mov %ebp,%esp (3 bytes) 66 5d pop %ebp (1 bytes)
Replacing instructions and handling displacement is probably going to be a huge pain.