This is a C program, which has been compiled to assembly using gcc -S. How is string "Hello, world" represented in this program?
This is the C-code:
1. #include <stdio.h>
2.
3. int main(void) {
4.
5. char st[] = "Hello, wolrd";
6. printf("%s\n", st);
7.
8. return 0;
9. }
Heres the assembly code:
1. .intel_syntax noprefix
2. .text
3. .globl main
4.
5. main:
6. push rbp
7. mov rbp, rsp
8. sub rsp, 32
9. mov rax, QWORD PTR fs:40
10 mov QWORD PTR [rbp-8], rax
11. xor eax, eax
12. movabs rax, 8583909746840200520
15. mov QWORD PTR [rbp-32], rax
14. mov DWORD PTR [rbp-24], 1684828783
15. mov BYTE PTR [rbp-20], 0
16. lea rax, [rbp-32]
17. mov rdi, rax
18. call puts
19. mov eax, 0
20. mov rdx, QWORD PTR [rbp-8]
21. xor rdx, QWORD PTR fs:40
22 je .L3
22. call __stack_chk_fail
23. .L3:
24. leave
25. ret
You are using a local buffer in function main, initialized from a string literal. The compiler compiles this initialization as setting the 16 bytes at [rbp-32] with 3 mov instructions. The first one via rax, the second immediate as the value is 32 bits, the third for a single byte.
8583909746840200520 in decimal is 0x77202c6f6c6c6548 in hex, corresponding to the bytes "Hello, W" in little endian order, 1684828783 is 0x646c726f, the bytes "orld". The third mov sets the final '\0' byte. Hence the buffer contains "Hello, World".
This string is then passed to puts for output to stdout.
Note that gcc optimized the call printf("%s\n", "Hello, World"); to puts("Hello, World");! By the way, clang performs the same optimization.