linux - How to find the length of a string using NASM?

Linux - How to find the length of a string using NASM?

To find the length of a string in NASM (Netwide Assembler) on Linux, you typically iterate through the string until you encounter a null terminator ('\0'). Here's a step-by-step approach to achieve this:

Example NASM Assembly Code

Assume you have a string stored in memory and you want to calculate its length:

section .data msg db 'Hello, NASM!', 0 ; Null-terminated string section .text global _start _start: ; Load address of the string into edi (or any other register) lea edi, [msg] ; Initialize counter for string length xor ecx, ecx .loop: ; Compare current character with null terminator (0) cmp byte [edi + ecx], 0 je .done ; Increment counter inc ecx jmp .loop .done: ; ecx now contains the length of the string (excluding the null terminator) ; Place your code here to use the length (e.g., print it) ; Exit the program mov eax, 1 ; syscall number for exit xor ebx, ebx ; return code 0 int 0x80 ; invoke syscall

Explanation

  1. Declare the String: In the .data section, declare your string (msg) with a null terminator (0) at the end. This ensures that the string is properly terminated, which is essential for the loop termination condition.

  2. Initialize Registers:

    • Use lea edi, [msg] to load the address of the string into the edi register. You can use any other general-purpose register (ebx, esi, etc.) as well.
    • Clear the ecx register (xor ecx, ecx) to use it as a counter for the string length.

  3. Loop to Find String Length:

    • Use a loop (loop:) to iterate through the string:
      • Compare the byte at [edi + ecx] (current character) with 0 (null terminator) using cmp byte [edi + ecx], 0.
      • If they are equal (je .done), exit the loop.
      • Increment the counter (inc ecx) and jump back to .loop.

  4. Length Calculation:

    • After the loop exits, ecx will contain the length of the string (excluding the null terminator).

  5. Further Actions:

    • After calculating the length, you can use it as needed (e.g., print it using syscall for write or other functions).
    • Finally, exit the program using the exit syscall (mov eax, 1).

NASM Specifics

  • Ensure you understand the NASM syntax and how registers are used (lea for loading effective address, cmp for comparison, inc for increment).
  • Adjust the register usage and addressing modes ([edi + ecx]) based on your specific needs and the context of your program.

Running the Assembly Code

To assemble and run the above code on Linux, you typically follow these steps:

nasm -f elf32 your_program.asm -o your_program.o ld -m elf_i386 your_program.o -o your_program ./your_program

Replace your_program.asm with the name of your NASM assembly file. Adjust architecture (elf_i386 for 32-bit) and file formats (elf32) based on your system and requirements.

This approach allows you to effectively find the length of a string in NASM assembly language on Linux by iterating through the string until you encounter the null terminator. Adjustments may be needed based on specific requirements or system configurations.

Examples

  1. NASM assembly program to find string length

    • Description: Writing a NASM assembly program to calculate the length of a null-terminated string.
    • Code:
      section .data msg db "Hello, World!", 0 section .text global _start _start: ; Load address of string into register lea rdi, [msg] ; Initialize counter mov rcx, 0 loop_start: ; Load current character mov al, [rdi] ; Check if end of string ('\0') cmp al, 0 je end_loop ; Increment counter inc rcx ; Move to next character inc rdi ; Repeat loop jmp loop_start end_loop: ; rcx now holds the length of the string ; Use rcx or store the result as needed ; Exit program mov rax, 60 ; syscall number for exit xor rdi, rdi ; exit code 0 syscall

  2. NASM assembly strlen function

    • Description: Implementing a reusable function in NASM assembly to compute the length of a string.
    • Code:
      section .data msg db "Hello, World!", 0 section .text global strlen strlen: ; Input: rdi points to the beginning of the string ; Output: rax contains the length of the string ; Initialize counter mov rcx, 0 loop_start: ; Load current character mov al, [rdi] ; Check if end of string ('\0') cmp al, 0 je end_loop ; Increment counter inc rcx ; Move to next character inc rdi ; Repeat loop jmp loop_start end_loop: ; rax now holds the length of the string ret

  3. NASM assembly program to find string length without using loop

    • Description: Calculating the length of a string in NASM assembly without explicitly using a loop structure.
    • Code:
      section .data msg db "Hello, World!", 0 section .text global _start _start: ; Load address of string into register lea rdi, [msg] ; Calculate length using repne scasb instruction xor rcx, rcx ; Zero out rcx not rcx ; Set rcx to -1 (all 1s) xor al, al ; Set al to 0 (null terminator) repne scasb ; Scan string until null terminator ; rcx now holds the negative length of the string, convert to positive dec rcx ; rax = rcx (length of the string) ; Exit program mov rax, 60 ; syscall number for exit xor rdi, rdi ; exit code 0 syscall

  4. NASM assembly program to find length of string with SSE

    • Description: Using SSE instructions in NASM assembly to efficiently compute the length of a string.
    • Code:
      section .data msg db "Hello, World!", 0 section .text global _start _start: ; Load address of string into xmm0 mov rdi, msg movups xmm0, [rdi] ; Find null terminator in xmm0 pxor xmm1, xmm1 pcmpeqb xmm1, xmm0 pmovmskb eax, xmm1 ; Count leading zeroes (bit scan reverse) bsrl rcx, eax xor rax, rax cmp rcx, 16 jz no_null ; Calculate length of string mov rax, rcx sub rax, 16 add rax, 1 shr rax, 1 no_null: ; rax now holds the length of the string ; Exit program mov rax, 60 ; syscall number for exit xor rdi, rdi ; exit code 0 syscall

  5. NASM assembly program to find length of string in C

    • Description: Integrating NASM assembly code to find the length of a string within a C program.
    • Code (NASM portion):
      section .data msg db "Hello, World!", 0 section .text global strlen strlen: ; Input: rdi points to the beginning of the string ; Output: rax contains the length of the string ; Initialize counter mov rcx, 0 loop_start: ; Load current character mov al, [rdi] ; Check if end of string ('\0') cmp al, 0 je end_loop ; Increment counter inc rcx ; Move to next character inc rdi ; Repeat loop jmp loop_start end_loop: ; rax now holds the length of the string ret
      Code (C portion):
      #include <stdio.h> extern int strlen(char *); int main() { char *msg = "Hello, World!"; int len = strlen(msg); printf("Length of string: %d\n", len); return 0; }

  6. NASM assembly program to find string length using recursive function

    • Description: Implementing a recursive approach in NASM assembly to determine the length of a string.
    • Code:
      section .data msg db "Hello, World!", 0 section .text global strlen_rec strlen_rec: ; Input: rdi points to the beginning of the string ; Output: rax contains the length of the string ; Base case: if current character is '\0', return 0 cmp byte [rdi], 0 je end_strlen ; Recursive call: increment rdi to next character inc rdi call strlen_rec ; Add 1 for current character inc rax ret end_strlen: ; rax now holds the length of the string ret

  7. NASM assembly program to find length of string with AVX

    • Description: Utilizing AVX instructions in NASM assembly to compute the length of a string.
    • Code:
      section .data msg db "Hello, World!", 0 section .text global _start _start: ; Load address of string into ymm0 mov rdi, msg vmovups ymm0, [rdi] ; Find null terminator in ymm0 vpxor ymm1, ymm1, ymm1 vpcmpeqb ymm1, ymm0, ymm1 vmovmskps eax, ymm1 ; Count leading zeroes (bit scan reverse) bsrq rcx, rax xor rax, rax cmp rcx, 32 jz no_null ; Calculate length of string mov rax, rcx sub rax, 32 add rax, 1 shr rax, 1 no_null: ; rax now holds the length of the string ; Exit program mov rax, 60 ; syscall number for exit xor rdi, rdi ; exit code 0 syscall

  8. NASM assembly program to find string length without loop or recursion

    • Description: Calculating the length of a string in NASM assembly without using explicit loop or recursive structures.
    • Code:
      section .data msg db "Hello, World!", 0 section .text global _start _start: ; Load address of string into register lea rdi, [msg] ; Calculate length using repne scasb instruction xor rcx, rcx ; Zero out rcx not rcx ; Set rcx to -1 (all 1s) xor al, al ; Set al to 0 (null terminator) repne scasb ; Scan string until null terminator ; rcx now holds the negative length of the string, convert to positive dec rcx ; rax = rcx (length of the string) ; Exit program mov rax, 60 ; syscall number for exit xor rdi, rdi ; exit code 0 syscall

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