16

Learning x86 assembly and the frame pointer is blowing my mind a little. I learned from this SO question that EBP makes life awesome for debugging. That's great, but I was curious, "What else is EBP used for?" Looking at the call stack Wikipedia article, EBP is necessary for dynamic stack allocation.

As I've always been under the assumption that dynamic allocation goes on the heap. So, why would I want to use dynamic stack allocation - why isn't the heap good enough? And, how is EBP useful for this?

Community
  • 1
  • 1
CatShoes
  • 3,613
  • 5
  • 29
  • 43

2 Answers2

23

You assume wrongly that ebp is necessary for stack frame allocation. This is not true, esp can be used directly. The use of ebp as a stack frame pointer is in no way necessary nowadays. There are a few points, where is was useful:

  • In 16 bit code, the use of sp (the stack pointer) in addressing was severely limited, relative addressing wasn't directly possible at all in x86, where bp could be used in every addressing mode available.

  • The reasoning for this limited support were possibly the compilers available back then, it's much simpler to generate code with frame pointers then to keep track of the ever changing sp. There are many more processors which had special support for stack frame instructions, like leave or enter, but AFAIK nobody else but Intel has gone so far to cripple the real sp in that process :-)

  • Debugging. It is much easier to unwind a stack when frame pointers are available, but modern debuggers can do even without.

If you want to allocate 100 byte space on the stack, just do sub esp, #100, access the space with move [esp + x] where x is between 0 and 99 and clean up with add esp, #100 and you are done. I would even argue that using stack frame pointers in hand written assembly is like copying the behavior of a thirty years old compiler, when the compilers were really stupid and couldn't do without. It's in no way necessary nor useful if you are writing assembly.

Bo Persson
  • 90,663
  • 31
  • 146
  • 203
Gunther Piez
  • 29,760
  • 6
  • 71
  • 103
  • Please forgive me, I'm a little dense. Are you saying that it is more or less pointless to use EBP for stack frame pointers, and instead to use ESP for that and EBP as a general purpose register? – CatShoes Jun 29 '12 at 15:50
  • Yes, I am saying that using a frame pointer is generally obsolete. It's a thing from the past that still tends to stick around :-) – Gunther Piez Jun 29 '12 at 20:31
  • Great. I appreciate the helpful information. I already chose the other answer, but feel yours is equally useful. Upvoted! – CatShoes Jul 01 '12 at 13:57
9

Allocating small amount of memory from stack is extremely fast compared to heap allocating. When we allocate memory from heap then we must call API or memory manager.

YES you can use EBP register also for any other purpose like other 32bit registers but you must first to store its content and after that restore before you exit your procedure.

You can use PUSH EBP for storing the EBP content to the stack and POP EBP for restoring.

Jens Björnhager
  • 5,632
  • 3
  • 27
  • 47
GJ.
  • 10,810
  • 2
  • 45
  • 62
  • Is the PUSH EBP / POP EBP you mention the same that is generated by the compiler for function calls? – CatShoes Jun 28 '12 at 22:44
  • @CatShoes: Normally compiler use EBP register for access to the dynamic stack allocated variables as stack memory pointer so if your asm procedure is in the middle of program code you must to preserve the content of EBP. – GJ. Jun 28 '12 at 23:06
  • Okay, so this is in addition to the compiler generated usage of EBP. Thanks! – CatShoes Jun 29 '12 at 00:40
  • 6
    Strictly speaking, in non-16-bit modes `EBP/RBP`-relative addressing of the stack contents isn't necessary since `ESP/RSP`-relative addressing is available. Some compilers even have an option to suppress generation of `EBP/RBP`-relative addressing and use `ESP/RSP`-relative instead (e.g. `-fomit-frame-pointer` in gcc). – Alexey Frunze Jun 29 '12 at 06:33
  • 7
    `-fomit-frame-pointer` is default at many architectures nowadays. The use of `ebp` as a standard register is pretty common. Modern debuggers support stack unwinding even without `ebp` as frame pointer, so it isn't required for debugging. – Gunther Piez Jun 29 '12 at 09:16
  • Thanks, I was mistakenly thinking the standard was to use EBP for the stack frame pointer, but I guess the standard has moved away from that now. – CatShoes Jun 29 '12 at 15:52
  • @CatShoes: _If_ a frame pointer is used (for whatever reason) it will be `ebp`. But it isn't needed anymore. – Gunther Piez Jun 29 '12 at 20:32