is there a difference between al and dl? [duplicate] - macos

Question

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How do AX, AH, AL map onto EAX?
(6 answers)
Closed 2 years ago.
First, it is my first assembly. And I use it with "NASM" and "64bit ASM" and "Intel" code and Mac OS.
When I try to write strdup function, it didn't work with al.
In using my strdup function, the string was broken.
So, my code is here.
global _strdup
extern _malloc
section .text
_strdup:
xor rcx, rcx
jmp strlen
strlen:
cmp [rdi + rcx], byte 0
je malloc
inc rcx
jmp strlen
malloc:
inc rcx
push rdi
mov rdi, rcx
call _malloc
pop rdi
cmp rax, 0
je error
xor rcx, rcx
jmp copy
copy:
mov al, byte [rdi + rcx]
mov byte [rax + rcx], al
cmp al, byte 0
je return
jmp increment
increment:
inc rcx
jmp copy
error:
mov rax, 0
ret
return:
ret
But, when I write same code with dl not al, it works!
global _strdup
extern _malloc
section .text
_strdup:
xor rcx, rcx
jmp strlen
strlen:
cmp [rdi + rcx], byte 0
je malloc
inc rcx
jmp strlen
malloc:
inc rcx
push rdi
mov rdi, rcx
call _malloc
pop rdi
cmp rax, 0
je error
xor rcx, rcx
jmp copy
copy:
mov dl, byte [rdi + rcx]
mov byte [rax + rcx], dl
cmp dl, byte 0
je return
jmp increment
increment:
inc rcx
jmp copy
error:
mov rax, 0
ret
return:
ret
I don't know why dl works and the difference between al and dl.
Anyone know about it? Thanks.

Answer 1

al is the smallest part of rax while dl is the smallest part of rdx. Changing al will change the value of rax as well.
Why your code is not working with al?
copy:
mov al, byte [rdi + rcx] ; you move "byte [rdi + rcx]" in "al"
; thus changing the value of "rax" as well.
; the result of the following statement becomes unexpected
mov byte [rax + rcx], al
But when you do it with dl, rax is not affected and hence your program works as expected.
Try the following code and run it through debugger to see the effect of changing the value of al on `rax:
section .text
global main
main:
nop
mov rax, 1000
mov al, byte 10
nop
Result:
After mov rax, 1000, value of rax: 1000
00000000 00000000 00000000 00000000 00000000 00000000 00000011 11101000
After mov al, 10, value of rax: 778
00000000 00000000 00000000 00000000 00000000 00000000 00000011 00001010
^^^^^^^^
So, changing al will affect rax.