Task: Displaying the Reversed String

Navigate to tasks/print-rev-string/support/.

In the file print_rev_string.asm, add the reverse_string() function so that you have a listing similar to the one below:

[...]
section .text
extern printf
extern puts
global print_reverse_string

reverse_string:
    push ebp
    mov ebp, esp
    push ebx                ; preserve ebx as required by cdecl

    mov eax, [ebp + 8]
    mov ecx, [ebp + 12]
    add eax, ecx
    dec eax
    mov edx, [ebp + 16]

copy_one_byte:
    mov bl, [eax]
    mov [edx], bl
    dec eax
    inc edx
    loopnz copy_one_byte

    inc edx
    mov byte [edx], 0

    pop ebx                 ; restore ebx
    leave
    ret

print_reverse_string:
    push ebp
    mov ebp, esp
[...]

IMPORTANT: When copying the reverse_string() function into your program, remember that the function starts at the reverse_string() label and ends at the print_reverse_string label. The copy_one_byte label is part of the reverse_string() function.

The reverse_string() function reverses a string and has the following signature: void reverse_string(const char *src, size_t len, char *dst);. This means that the first len characters of the src string are reversed into the dst string.

Reverse the mystring string into a new string and display that new string.

NOTE: To define a new string, we recommend using the following construction in the data section:

store_string times 64 db 0

This creates a string of 64 zero bytes, enough to store the reverse of the string. The equivalent C function call is reverse_string(mystring, ecx, store_string);. We assume that the length of the string is calculated and stored in the ecx register.

You cannot directly use the value of ecx in its current form. After the printf() function call for displaying the length, the value of ecx is not preserved. To retain it, you have two options:

  1. Store the value of the ecx register on the stack beforehand (using push ecx before the printf call) and then restore it after the printf call (using pop ecx).
  2. Store the value of the ecx register in a global variable, which you define in the .data section.

You cannot use another register because there is a high chance that even that register will be modified by the printf call to display the length of the string.

To test the implementation, enter the tests/ directory and run:

make check

In case of a correct solution, you will get an output such as:

./run_all_tests.sh
test_reverse_simple              ........................ passed ...  33
test_reverse_special             ........................ passed ...  33
test_reverse_long                ........................ passed ...  34

Total:                                                           100/100

If you’re having trouble solving this exercise, go through this reading material.