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