Rotate the characters in place (X86)

Question

I have created a rot13 program using X86,

    .intel_syntax noprefix
    .data

    .text
    .global main

# Program starts here
main:
    # first argument
    mov r14, [rsi + 8]
    # second argument
    mov r15, [rsi + 16]
    
    # move the pointer in r15 to r13
    mov r13, r15
    
    #empty register
    mov r12, 0
    
    #counter
    mov r11, 0
    
    #spot negative
    mov rbx, 0
    
    jmp .num_check
    
.num_loop:
    sub r15, '0'
    
    #r12 have value now
    mov r12, r15
    add r13, 1
    
    #update counter
    add r11, 1
    
.num_check:
    movzx r15, byte ptr [r13]
    
    cmp r15, '-'
    je .neg_sign
    
    cmp r15, 0
    je .single_digit
    
    cmp r11, 1
    je .double_digit
    
    cmp r15, 0
    jne .num_loop
    
.neg_sign:
    mov rbx, r15
    add r13, 1
    jmp .num_check
    
.config_neg:
    neg r12
    mov r15, r12
    mov r13, r14
    jmp .rotate_char
    
.single_digit:
    #spot negative sign
    cmp rbx, '-'
    je .config_neg
    
    mov r15, r12
    mov r13 , r14
    jmp .rotate_char

.double_digit:
    sub r15, '0'
    mov rax, 10
    imul rax, r12
    mov r12, rax
    
    #r15 is now int and not char
    add r12, r15
    
    cmp rbx, '-'
    je .config_neg
    
    mov r15, r12
    mov r13, r14
    jmp .rotate_char
 
.char_loop:
    add r14, r15
    
    #check if is over 'z'
    cmp r14, 'z'
    jg .over_z
    
    #check if is under 'a'
    cmp r14, 'a'
    jl .under_a
    
    mov rdi, r14
    call putchar@PLT
    add r13, 1
    jmp .rotate_char

.under_a:
    add r14, 26
    mov rdi, r14
    call putchar@PLT
    add r13, 1
    jmp .rotate_char

.over_z:
    sub r14, 26
    mov rdi, r14
    call putchar@PLT
    add r13, 1
    jmp .rotate_char
   
.rotate_char:
    movzx r14, byte ptr [r13]
    cmp r14, 0
    jne .char_loop
    mov rdi, '\n'
    call putchar@PLT
    
    ret

The 'makefile' for this program

BIN = rotx

all:
    gcc -g -O0 $(BIN).s -o $(BIN)

The input and output

For this program, I used

movzx r14, byte ptr [r13]

to get a single character and rotate it by adding or substracting , then I used

call putchar@PLT

to output a single rotated character.

Instead of outputting one character at a time, may I know how do I put the rotated character back into the same memory and outputting it at once? (rotating in place)

I know that I could put it back into the same memory by using

mov byte ptr [r13], r14b

but the program doesnt seem to work.

My attempt

    .intel_syntax noprefix
    .data

    .text
    .global main

# Program starts here
main:
    # first argument
    mov r14, [rsi + 8]
    # second argument
    mov r15, [rsi + 16]
    
# Convert char to num
    # move the pointer in r15 to r13
    mov r13, r15
    
    #empty register
    mov r12, 0
    
    #counter
    mov r11, 0
    
    #spot negative
    mov rbx, 0
    
    jmp .num_check
    
.num_loop:
    sub r15, '0'
    
    #r12 have value now
    mov r12, r15
    add r13, 1
    
    #update counter
    add r11, 1
    
.num_check:
    movzx r15, byte ptr [r13]
    
    cmp r15, '-'
    je .neg_sign
    
    cmp r15, 0
    je .single_digit
    
    cmp r11, 1
    je .double_digit
    
    cmp r15, 0
    jne .num_loop
    
.neg_sign:
    mov rbx, r15
    add r13, 1
    jmp .num_check
    
.config_neg:
    neg r12
    mov r15, r12
    mov r13, r14
    jmp .rotate_char
    
.single_digit:
    #spot negative sign
    cmp rbx, '-'
    je .config_neg
    
    mov r15, r12
    mov r13 , r14
    jmp .rotate_char

.double_digit:
    sub r15, '0'
    mov rax, 10
    imul rax, r12
    mov r12, rax
    
    #r15 is now int and not char
    add r12, r15
    
    cmp rbx, '-'
    je .config_neg
    
    mov r15, r12
    mov r13, r14
    jmp .rotate_char
 
.char_loop:
    add r14, r15
    
    #check if is over 'z'
    cmp r14, 'z'
    jg .over_z
    
    #check if is under 'a'
    cmp r14, 'a'
    jl .under_a
    
    mov byte ptr [r13], r14b
    add r13, 1
    jmp .rotate_char

.under_a:
    add r14, 26
    mov byte ptr [r13], r14b
    add r13, 1
    jmp .rotate_char

.over_z:
    sub r14, 26
    mov byte ptr [r13], r14b
    add r13, 1
    jmp .rotate_char
   
.rotate_char:
    movzx r14, byte ptr [r13]
    cmp r14, 0
    jne .char_loop
    mov rdi, r13
    call putchar@PLT
    mov rdi, '\n'
    call putchar@PLT

    ret

output