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x64 Assembly: strcpy Procedure

Copy ASCII string using x86-64 (64-bit) assembly.

This code compiled and tested with Visual Studio 2022.

This procedure, strcpy_asm_x64, copies a C-string from the source to the destination and returns a pointer to the end of the destination so that another string can be concatenated.

The default behavior of strcpy is to return the destination buffer but it is much more useful to return the end of the text in the destination buffer.

TITLE 'extern "C" char * strcpy_asm_x64(char *destination, const char *source);'

PUBLIC	strcpy_asm_x64

strcpy_asm_x64 PROC

	mov  r8b, BYTE PTR [rdx] ; rdx = source
	test r8b, r8b
	mov  rax, rcx ; rax = destination
	je   SHORT label2


	mov  BYTE PTR [rax], r8b
	mov  r8b, BYTE PTR [rdx+1]
	inc  rax
	inc  rdx
	test r8b, r8b
	jne  SHORT label1


	mov  BYTE PTR [rax], 0

	; could set rax to rcx if wanting to return the destination pointer instead of a pointer to the end of destination

	ret  0
strcpy_asm_x64 ENDP

This example C++ code uses this function.

#include <iostream>
using namespace std;

extern "C" char* strcpy_asm_x64(char* destination, const char* source);

int main()
	char buffer[256];

	int length = strcpy_asm_x64(strcpy_asm_x64(strcpy_asm_x64(buffer, "HELLO "), "WORLD"), "!!!") - buffer;

	cout << buffer << endl;

	return 0;

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