What are loops in assembly language?

Loops in assembly language are used to repeatedly execute a block of code until a certain condition is met. Assembly language doesn't have built-in constructs like high-level languages, so loops are implemented using branch instructions and conditional jumps.

The following assembly language instruction uses a loop:

; Loop 10 times and increment the register EAX each time. loop: inc eax dec ecx jnz loop

This loop will increment the register EAX 10 times. The register ECX is used as a counter to track how many times the loop has executed. The jnz instruction jumps to the label loop if the value of the register ECX is not zero.

The following assembly language instruction uses a conditional loop:

; Loop while the value of the register EAX is less than 10. while_loop: cmp eax, 10 jl while_loop ; Do something.

Here's a step-by-step explanation of how loops are typically implemented in assembly language, using x86 architecture as an example:

Set up loop control variables

Define and initialize loop control variables such as counters and pointers.

section .data count dw 10 ; Counter initialized to 10 section .text global _start _start:

Start the loop

Use a label to mark the beginning of the loop.

loop_start:

Execute loop body

Write the code that needs to be executed inside the loop.

; Your loop body code here

Update loop control variables

Modify loop control variables such as counters or pointers.

dec word [count] ; Decrement the counter

Check loop condition

Use conditional jump instructions to check if the loop should continue or exit.

jnz loop_start ; Jump to loop_start if the zero flag is not set (count is not zero)

Exit the loop

Optionally, include code to handle loop exit or cleanup.

loop_exit: ; Code to execute after the loop
Full Source:

Here's a simple example in x86 assembly that uses a loop to print numbers from 9 to 0:

section .data count dw 10 ; Counter initialized to 10 section .text global _start _start: ; Loop start loop_start: ; Code to print the current value of count (replace this with your actual code) ; mov ax, [count] ; call print_number ; Update loop control variables dec word [count] ; Check loop condition jnz loop_start ; Jump to loop_start if count is not zero ; Loop exit jmp loop_exit loop_exit: ; Exit program or include additional code mov eax, 1 ; System call number for exit xor ebx, ebx ; Exit code 0 int 0x80 ; Invoke system call

In this example, the loop prints the value of count using a hypothetical print_number function and decrements count until it reaches zero. The loop then exits and the program terminates.

Conclusion

Loops are implemented using conditional jumps and branch instructions. Assembly lacks high-level language constructs, so loops involve setting up control variables, executing a loop body, updating variables, and checking conditions for repetition.






Related Topics
  1. Conditional Execution and Branching in Assembly Language
  2. Functions in Assembly Language
  3. Procedures and Subroutines in Assembly Language
  4. Stack and its usage in assembly language
  1. Assembly Branches and Jumps