Special registers

[Doomulation]

Messing around with assembly, I seem to be confused about ebp and esp. What are these two registers function? I know they are general purpose, but typically vc++ uses them for its own purpose.
I thought ebp was the stack pointer, but when I push something onto the stack, I see esp changes! Am I wrong?

It also seems it is much faster to read/write to memory rather than the stack. At least due to my tests. Is it supposed to be like that?

 

[Garstyciuks]

I have always thought that esp is the stack pointer...

 

[zilmar]

ESP is stack register
EPB is used for a stack frame .. basicly stores the stack location at the start of the function that way it can always know where a variable is stored in the stack cause it will be in a set position from its starting point.

in function a .. var bcd would be at something like ebp - 4 .. so no matter where the stack pos is .. that variable is always constant

 

[Doomulation]

Well, that's a bother. That means there are only 6 general purpose registers to use...
Unless I use shifting and bit manipulating to fully use each bit of the 32-bit registers.

 

[zilmar]

You are free to use ESP and EBP .. I once played with it with pj64 recompiler, but VC had a spack if ESP was not pointing to valid memory.

You can use EBP, but if it is inlined asm, then it needs to be saved and restored

 

[synch]

Well, you can use EBP freely in a routine, without pushing/poping, if you don't mix C and asm (pure inline asm) in that routine. I haven't profiled the speed results, as I always used it for size optimizations. When using ebp VisualC will generate a warning that you can safely ignore. About ESP, if you're using the stack for something in that routine, it's better to use it only to index the stack.

 

[Doomulation]

You can check omit framepointers in the config to make vc ignore the ebp register unless necessary. But you had better not touch esp unless you know what you're doing.