64 bit - 64 bit assembly, when to use smaller size registers

Question

I understand in x86_64 assembly there is for example the (64 bit) rax register, but it can also be accessed as a 32 bit register, eax, 16 bit, ax, and 8 bit, al. In what situation would I not just use the full 64 bits, and why, what advantage would there be?

As an example, with this simple hello world program:

section .data
msg: db "Hello World!", 0x0a, 0x00
len: equ $-msg

section .text
global start

start:
mov rax, 0x2000004      ; System call write = 4
mov rdi, 1              ; Write to standard out = 1
mov rsi, msg            ; The address of hello_world string
mov rdx, len            ; The size to write
syscall                 ; Invoke the kernel
mov rax, 0x2000001      ; System call number for exit = 1
mov rdi, 0              ; Exit success = 0
syscall                 ; Invoke the kernel

rdi and rdx, at least, only need 8 bits and not 64, right? But if I change them to dil and dl, respectively (their lower 8-bit equivalents), the program assembles and links but doesn't output anything.

However, it still works if I use eax, edi and edx, so should I use those rather than the full 64-bits? Why or why not?

Answer 1

You are asking several questions here.

If you just load the low 8 bits of a register, the rest of the register will keep its previous value. That can explain why your system call got the wrong parameters.

One reason for using 32 bits when that is all you need is that many instructions using EAX or EBX are one byte shorter than those using RAX or RBX. It might also mean that constants loaded into the register are shorter.

The instruction set has evolved over a long time and has quite a few quirks!