memcpy by hand in Assembly

Load a byte, store a byte, walk both pointers.

; memcpy by hand: load a byte, store a byte, walk both pointers.
        global _start

        section .data
src:    db "relay", 10
SLEN:   equ $ - src

        section .bss
dst:    resb SLEN

        section .text
_start:
        xor rcx, rcx
.copy:
        mov al, [src + rcx]
        mov [dst + rcx], al
        inc rcx
        cmp rcx, SLEN
        jl .copy

        mov rax, 1
        mov rdi, 1
        mov rsi, dst            ; print the copy, not the original
        mov rdx, SLEN
        syscall

        mov rax, 60
        xor rdi, rdi
        syscall

How it works

1. mov al, [src + rcx] then mov [dst + rcx], al.
2. The copy lands in .bss space reserved with resb.
3. Printing the copy proves the bytes moved.

Keywords and builtins used here

• al
• cmp
• db
• equ
• global
• inc
• jl
• mov
• rax
• rcx
• rdi
• rdx
• resb
• rsi
• section
• syscall
• xor

The run, in numbers

Lines

29

Characters to type

420

Tokens

78

Three-star pace

55 tpm

At the three-star pace of 55 tokens a minute, this run takes about 85 seconds.

Type this snippet

Step 4 of 4 in Data, step 10 of 31 in Language basics.

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