The smallest Hello World program

Post by michidk » Sun, Jan 05, 2025, 11:10 AM UTC

bd01 wrote: post #50

michidk wrote: post #48

Earlier quoted context omitted.

Of course - you are completely right, an oversight in wanting to correct my mistake as quickly as possible. With that fixed, is there any reason not to use push here?

Yes, because: push 1 ; 6A 01 (2 bytes) pop rdi ; 5F (1 byte) is longer than a simple: mov edi, eax ; 89 C7 (2 bytes)

  push 1
  pop rax
  mov rdi, rax

  push 1
  pop rax
  push 1
  pop rdi

Post by michidk » Sun, Jan 05, 2025, 1:56 PM UTC

bd01 wrote: post #52

michidk wrote: post #51

Earlier quoted context omitted.

I think your statement might only apply to 32 bit (one of the constraints mentioned early in the blog post was 64 bit). But even if it was 32 bit, then we would't have to copy a 1, since the syscall number for sys_write would be 4 instead of 1. I get the same total size with both variants in 64 bit mode. push 1 pop rax mov rdi, rax Assembling to 48 89 C7 (3 bytes) seems to be same in size as push 1 pop rax push 1 pop…

That's because you're using `mov rdi, rax` again. You keep changing `edi, eax` to `rdi, rax`. Why? The default operand size in 64-bit mode is, for most instructions, still 32 bits. So `mov edi, eax` encodes the same in 32- and 64-bit mode. For `mov rdi, rax` you need an extra REX prefix byte [1], that's the 48 you're seeing above, but you don't need it here. [1] https://wiki.osdev.org/X86-64_Instruction_Encoding#REX_…