In terms of using the stack, why do we need a base pointer and a stack pointer

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In terms of x86 assembly code anyways. I've been reading about function calls, but still can't fully grasp the need for a base / frame pointer (EBP) along with a stack pointer (ESP).

When we call a function, the current value of EBP will be placed on the stack and then EBP gets the current ESP value.

Place holders for the return value, function arguments and local variables of the function will then be placed on the stack, and the stack pointer ESP value will decrease (or increase) to point to after the last placeholder placed on the stack.

Now we have the EBP pointing to the beginning of the current stack frame, and ESP pointing to the end of the stack frame.

The EBP will be used to access the arguments and local variables of the function due to constant offsets from the EBP. That is fine. What I don't understand is, why can't the ESP just be used to access these variables also by using its offsets. The EBP points to the beginning of the stack frame , and the ESP points to the end of the stack frame. What's the difference?

The ESP shouldn't change from once there has been a placeholder for all the local variables etc. or should it?

abi x86 assembly stack c
2 Answers

Technically, it is possible (but sometimes hard) to track how many local and temporary variables are stored on the stack, so that accessing function input, and local variables can be done without EBP.

Consider the following "C" code ;

int func(int arg) {
   int result ;
   double x[arg+5] ;
   // Do something with x, calculate result
   return result ;
} ;

The numbers of items that are stored on the stack is now variables (arg+5 items of double). Calculating the location of 'arg' from the stack require run time calculation, which can have significant negative impact on performance.

With extra register (EBP), the location of arg is always at fixed location (EBP-2). Executing a 'return' is always simple - move BP to SP, and return, etc.

Bottom line, the decision to commit the EBP register to a single function (instead of using it as a general register) is a trade off between performance, simplicity, code size and other factors. Practical experience has shown the benefit outweigh the cost.

user_120984050

Side note about debugger/runtime tools:

Using of EBP make it easier for debugger (and other runtime tools) to 'walk the stack'. Tools can examine the stack at run-time, and without knowing anything about the current program stack (e.g., how many items have been pushed into eac frame), they can travel the stack all the way to the "main".

Without EBP pointing to the 'next' frame, run-time tools (including debugger) will face the very hard (impossible ?) task of knowing how to move from the ESP to specific local variables.

user_120984050
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