CPSC 355: Tutorial 7

Memory & The Stack

J. J. Horacsek

PhD Student

Outline

• We've been doing most of our work so far with registers...
• What happens when we run out of registers?
• We need to start thinking about how to use memory to overcome the limitations of using registers alone

Memory

Addresses are 64-bit numbers that reference a particular byte in memory:

Memory

Address

Memory

In general, this is a very simplified view of memory

• Clearly we don't have 2^64 bytes = over 16 exabytes of RAM, what happens when we access memory that doesn't exist?
• What about memory that doesn't belong to us? Can we read/write to that memory?
• What about instructions in memory? Should I be able to jump to random locations in memory and execute code there?
• What if I have memory that isn't being used, can I swap it out to disk?

Memory

We have a few instructions to read and write to memory

• str - stores a register in memory
• strh - stores the lower 16-bits of a register in memory, a halfword
• strb - stores the lower 8-bits of a register in memory, a byte

Memory

For example:

Memory

Address

mov x1, wzr
ldr w2, =0xBEEF4DAD
str w2, [x1]

Memory

For bytes:

Memory

Address

mov x1, wzr
ldr w2, =0xBEEF4DAD
strb w2, [x1]

Memory

Reading can be a little trickier due to the sign bit

• ldr - loads a register from memory
• ldrh - loads the lower 16-bits of a register from memory
• ldrb - loads the lower 8-bits of a register from memory
• ldrsw - loads a word into a register with sign extend
• ldrsh - loads the lower 16-bits of register from memory, with sign extend
• ldrsb - loads the lower 8-bits of a register from memory, with sign extend

Memory

For example:

Memory

Address

mov x1, wzr
ldr x2, [x1]

0xFE

0xFF

0xFF

0xFF

0x00

0x00

0x00

0x00

Memory

Similarly, for half words

Memory

Address

mov x1, wzr
ldrh x2, [x1]

0xFE

0xFF

0xFF

0xFF

0x00

0x00

0x00

0x00

Memory

Similarly, for bytes

Memory

Address

mov x1, wzr
ldrb x2, [x1]

0xFE

0xFF

0xFF

0xFF

0x00

0x00

0x00

0x00

Memory

Which parts of memory can I use? If you try to compile any run any of the code snippets from before, they'll crash.

Memory

low

high

Memory

low

high

Before we talk about the stack, let's talk about how a program sits in memory

Memory

Before we talk about the stack, let's talk about how a program sits in memory

Programs are broken into segments

Memory

Before we talk about the stack, let's talk about how a program sits in memory

• Don't use the .data section for assignment 3
• Don't use the heap for assignment 3
• Do use the stack for assignment 3

Advice

Assignment 3

#include <stdio.h>
#define SIZE 50

int main()
{
    int v[SIZE], i=0, j=0, temp=0;

    for(i=0; i < SIZE; i++) {
        v[i] = rand() & 0xFF;
        printf("v[%d]: %d\n", i, v[i]);
    }

    // sort the array using insertion sort
    for(i = 1; i < SIZE; i++) { 
        temp = v[i];
        for(j = i; j > 0 && temp < v[j-1]; j--) {
            v[j] = v[j - 1];
        }
	v[j] = temp;
    }

    printf("\nSorted array: \n");
    for(i = 0; i < SIZE; i++)
        printf("v[%d] = %d\n", i, v[i]);

    return 0;
}

The Stack

low

high

Used to store local variables and arrays whose sizes are known at run time

The Stack

high

The register $sp keeps track of what address is currently at the top of the stack.

The Stack

high

We've actually already been using this

stp  x29, x30, [sp, -16]!

The Stack

high

We've actually already been using this

stp  x29, x30, [sp, -16]!

 $sp

The Stack

Additionally

ldp  x29, x30, [sp], 16

 $sp

Next Day

• Frame Pointers
• Creating variables on the stack