x64 (N)ASM: some libc functions

strlen.asm

; nasm -f elf64 strlen.asm -o strlen.o && gcc -c main.c -o main.o && gcc main.o strlen.o && ./a.out

bits 64

global asm_strlen

section .text

asm_strlen:
    ; save rdi
    push rdi

    ; *al = '\0'
    xor al, al
    ; clear rcx
    xor rcx, rcx
    ; *rcx = ~*rcx
    not rcx
    ; while (*al /* '\0' */ != *rdi) ++rdi, --*rcx;
    cld
    repne scasb
    ; *rcx = ~*rcx
    not rcx
    dec rcx

    ; set return value
    mov rax, rcx
    ; restore rdi
    pop rdi

    ret

strncpy.asm

; nasm -f elf64 strncpy.asm -o strncpy.o && gcc -c main.c -o main.o && gcc main.o strncpy.o && ./a.out

bits 64

global asm_strncpy

section .text

asm_strncpy:
    ; save rdi
    push rdi

    ; 1st (rdi)
    ; 2nd (rsi)
    ; switch rsi to rdi
    mov rdi, rsi
    ; copy 3rd (rdx) argument to rcx
    mov rcx, rdx

    ; set *al to 0 ('\0')
    xor al, al
    ; look for a '\0' (*al) in the first *rdx bytes of rdi while (*rcx--)
    ; while ('\0' != *rdi && 0 != *rcw) --*rcx, ++rdi;
    cld
    repne scasb

    mov rcx, rdi
    ; restore rdi
    pop rdi
    ; rcx -= rsi, rcx = the length to copy
    sub rcx, rsi

    push rdi ; to return rdi later

    ; copy from rsi to rdi while
    ; while (0 != *rcx) --*rcx, *rsi++ = *rdi++;
    cld
    rep movsb

    ; append '\0'
    mov byte [rdi], 0

    ; restore rdi
    pop rdi
    ; set return value to initial value of rdi
    mov rax, rdi

    ; return value in rax
    ret