assembly rot13()

rot13.S

//as -o rot13.o rot13.S && ld -o rot13 rot13.o
.section .data
string:
    .asciz "Hello World.\n"
.section .text
.global _start
_start:
    leaq string, %rdi
    call rot13

    xorq %rdi, %rdi
    xorq %rax, %rax
    movb $60, %al
    movb $0, %dil
    syscall

rot13:
    push %rbp
    movq %rsp, %rbp
    movq %rdi, -8(%rbp)

    movq -8(%rbp), %rbx

    .L01:
    cmpb $0, (%rbx)
    je .L02

    cmpb $'A', (%rbx)
    jb .L03
    cmpb $'z', (%rbx)
    ja .L03

    cmpb $'a', (%rbx)
    jae .LOWERCASE

    .UPPERCASE:
    addb $0x0d, (%rbx)
    cmpb $'Z', (%rbx)
    jbe .L03
    subb $0x1a, (%rbx)
    jmp .L03

    .LOWERCASE:
    addb $0x0d, (%rbx)
    cmpb $'z', (%rbx)
    jbe .L03
    subb $0x1a, (%rbx)

    .L03:
    addq $1, %rbx
    jmp .L01

    .L02:
    movq $0, %rax
    leave
    ret