What is Stack in Assembly Language?

Stack usage in assembly language involves managing a region of memory called the stack to store data and control information.

The following assembly language code shows how the stack is used to pass parameters to a function:

; Call the function `add_numbers` with the parameters 10 and 20. push 20 push 10 call add_numbers

The push instruction pushes the value of the register EAX onto the stack. The call instruction calls the function add_numbers. When the call instruction is executed, the processor pushes the return address onto the stack.

; Define a local variable called `my_variable`. sub esp, 4 ; Store the value 10 in the local variable `my_variable`. mov [esp], 10 ; ... ; Retrieve the value of the local variable `my_variable`. mov eax, [esp]

The sub esp, 4 instruction subtracts 4 bytes from the stack pointer. This allocates 4 bytes of memory on the stack for the local variable my_variable. The mov [esp], 10 instruction stores the value 10 in the local variable my_variable. The mov eax, [esp] instruction retrieves the value of the local variable my_variable and stores it in the register EAX.

Here's a step-by-step explanation with examples, using x86 assembly as a reference:

Stack Setup

Initialize the stack pointer (esp register) to point to the top of the stack.

section .text global _start _start: mov esp, stack_top ; Initialize stack pointer

Pushing Data onto the Stack

Use the push instruction to add data onto the stack.

section .text global _start _start: mov eax, 42 push eax ; Push the value of eax onto the stack

Popping Data from the Stack

Use the pop instruction to retrieve data from the stack.

section .text global _start _start: mov eax, 0 pop eax ; Pop the top value from the stack into eax

Managing the Stack Pointer

Adjust the stack pointer accordingly after pushing or popping data.

section .text global _start _start: mov esp, stack_top ; Initialize stack pointer ; Push and pop operations push eax pop ebx ; Adjust the stack pointer add esp, 4 ; Adjust for a DWORD (4 bytes) on the stack
Full Source

Here's an example of a program that uses the stack to add two numbers:

section .data value1 dd 5 value2 dd 7 section .text global _start _start: ; Initialize stack pointer mov rdi, stack_top ; Push values onto the stack mov eax, [value1] mov ebx, [value2] sub rsp, 8 mov [rsp], eax mov [rsp + 4], ebx ; Call the add function call add_function ; Result is now on the top of the stack ; Pop the result into rax add rsp, 8 mov rax, [rsp] ; Exit program mov rdi, rax ; Move result to rdi for the exit code mov rax, 60 ; System call number for exit syscall ; Invoke system call add_function: ; Retrieve values from the stack mov eax, [rsp] mov ebx, [rsp + 4] ; Add the values add eax, ebx ; Push the result onto the stack mov [rsp], eax ret section .bss stack_top resb 8 ; Reserve 8 bytes for stack initialization

This example demonstrates using the stack to pass parameters and return values between the main program and a function (add_function). The esp register is used to manage the stack, and push and pop instructions facilitate data movement on and off the stack. Adjust the code inside the function according to your specific requirements.

Conclusion

Stack usage involves managing a designated region of memory, known as the stack, to store data and control information. It includes operations like pushing and popping values onto and off the stack, making it a critical component for managing function calls, local variables, and program flow in low-level programming.






Related Topics
  1. Conditional Execution and Branching in Assembly Language
  2. Loops in Assembly Language
  3. Functions in Assembly Language
  4. Procedures and Subroutines in Assembly Language
  1. Assembly Branches and Jumps