Gerard Visser writes: > I was wondering if you could help me with a problem. I can't seem to find an > answer anywhere. > When I try to alter data in instruction space in a c-programme, like: > call 0 > pop bx > mov byte ptr [bx+7], 3 This writes to memory at %ebx. But %ebx points to write-protected memory. If you want to write-enable that memory, you have to ask your operating system to make it possible. That may or may not work: code segments are usually write protected and shared between processes. So, this is nothing to do with gcc, but with whatever OS you are using. > shl ax, 2 > > I get a "segmentation fault" because of the third instruction. > Could you tell me how to avoid this, or where I could find literature that > explains these kinds of questions. > I'm trying to do this, because "shl ax, cl" is not pairable and much slower > than a shift with an immediate constant second operand. Nothing like as slow as altering code segments, I suspect. This typically has a large overhead. Andrew.
