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?
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