CODEWITHSUNDEEP
linuxassemblyx86-64callstack
Absolutelyrun
Absolutelyrun

Reputation: 43

how the stack frame works？

i am reading the csapp and some codes (x86-64) confuse me.

the book says "pushq %rbp" equals :

    subq $8,%rsp 
    movq %rbp,(%rsp)

The c code is :

  long P(long x,long y)
  {
      long u = Q(y);
      long v = Q(x);

      return u + v;
  }

And the a part of the assembly code the book gives is :

    pushq %rbp 
    pushq %rbx
    subq $8,%rsp

The 'subq' confuse me.

Why it is like this?

Upvotes: 1

Views: 522

Answers (1)

Michał Walenciak
Michał Walenciak

Reputation: 4369

Stack is a block of memory which grows down. There is a point in memory indicated by rsp/esp register which is a stack top. All memory above it is occupied by things placed on stack and all memory below it is free.

If you want to put something on stack you need to decrease rsp register (that is what sub instruction does) by number of bytes you need and rsp will point now to the newly reserver area you needed.

Lets look at this simple example:

rsp points to address 100. As said - whole memory above address 100 is used, and memory below 100 is free. So if you need 4 bytes you decrease rsp by 4 so it points to 96. As you have just decreased rsp you are aware that memory cells 96, 97, 98 and 99 are yours and you can use them. When you need more bytes on stack, then you again can decrease rsp to get it more.

There are two ways puting things on stack. 1. you can decrease rsp as shown above. 2. you can use push instruction which does exactly the same but in one step: push rax will decrease rsp by 8 bytes (size of rax register) and then will save its value in reserved area.

Sometimes also rbp register is being used to operate on stack. If you need a bigger area on stack, for example for local variables, you reserve required amount on stack and then you save current rsp value into rbp. So rbp is a kind of bookmark remembering where your area is. Then you can push more things on stack, without loosing information where the allocated area was.

Before leaving function all things placed on stack need to be taken from it. It is done by pop instruction which is opposite to push - takes value from stack and moves it to register and then increases rsp. Or you can just increase rsp if you do not need to restore register values.

Upvotes: 2

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