BTW, as I run the instruction, I get permission denied error.
Regards,
Mahmood
On Tuesday, July 9, 2019, 1:23:58 PM GMT+4:30, Mahmood Naderan via gcc-help <gcc-help@xxxxxxxxxxx> wrote:
>You mean x86-64, instead of x86. On x86 "c5 00" means "lds (%eax),%eax".
>
>"c5" itself is not illegal. For example, "c5 00 58 00" means an AVX instruction
>"vaddps (%rax),%xmm15,%xmm8". The disassembler can only determine that it's an
>illegal instruction after it has seen "c5 00 00".
Xi,The code looks like
$ cat machine3.c
void main() {
__asm__(".byte 0xc5, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00");
}
$ gcc -masm=intel -g -c machine3.c
$ objdump -d -M intel -S -z machine3.o
machine3.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <main>:
void main() {
0: 55 push rbp
1: 48 89 e5 mov rbp,rsp
4: c5 00 00 (bad)
7: 00 00 add BYTE PTR [rax],al
9: 00 00 add BYTE PTR [rax],al
b: 00 00 add BYTE PTR [rax],al
d: 00 00 add BYTE PTR [rax],al
f: 00 00 add BYTE PTR [rax],al
11: 00 00 add BYTE PTR [rax],al
__asm__(".byte 0xc5, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00");
}
13: 5d pop rbp
14: c3 ret
Why gcc considers that as a three byte instructions and then tag it as a bad instruction?
Regards,
Mahmood
