assembly - 文字列を反転 - NASM x86 アセンブリ コード

Question

NASM で文字列を元に戻すコードを書き込もうとしていますが、セグ フォールトが発生し続けます。同様の質問をする人がいくつか見つかりましたが、回答は役に立ちませんでした。

コードは次のとおりです。

    %include "asm_io.inc"

    segment .data

    prompt1         db      `Enter String: `,0
    prompt2         db      "Reverse: ",0

    segment .bss

    stringlabel     resd    100

    segment .text

    global reverse
    extern printf
    extern scanf

    reverse:

    call    readstring
    call    strlen
    mov     eax, 5
    mov     ebx, stringlabel        ;save first address of the string
                                    ;eax contains the string length
    add     eax, ebx                ;last address of the string
    sub     eax, 1                  ;account for null character

    whileReverse:

    mov     ecx, [eax]              ;save value of last marker into a temp register
    mov     edx, [ebx]              ;save value of first marker into a temp register
    mov     [ebx], ecx              ;save value in ecx to the memory location pointed by ecx
    mov     [eax], edx              ;save value in edx to the memory location pointed by eax
    inc     ebx                     ;increment first marker, i.e,move closer to the centre
    dec     eax                     ;decrement last marker
    cmp     eax, ebx                ;if first marker is larger then the second marker, then swapping is done
    ja      whileReverse

    whileReverseEnd:

    popa
    leave
    ret


    readstring:

    enter   0,0                     ;setup routine
    pusha                           ;push all registers
    mov     eax, prompt1            ;move user input (String) into the register
    call    print_string            ;print prompt  on terminal


    ;READ A BYTE FROM STDIN
    ;this sub-block of code is to read the user input
    ;I found it on the NASM wikibooks site
    ;I have an idea of what may be going on
    ;But I am not sure why all these registers
    ;are being used

    mov     eax, 3                  ;read
    mov     ebx, 0                  ;read from standard input
    mov     ecx, stringlabel        ;address to pass
    mov     edx, 100                ;input length
                                    ;must save the input string
                                    ;in eax (requirement)
    int     0x80                    ;call the kernal
    call    print_string            ;print user input on terminal
    popa
    ret

    strlen:

    enter   0,0
    pusha

    mov     eax, prompt1            ;save the address of the first memory 'block' of stringlabel in eax
    mov     ebx, eax                ;move both markers to the first memory 'block' address

    whileLen:

    mov     cl, [eax]               ;move value from eax to lower ecx
    cmp     cl, 0                   ;look for null value in the current address
    je      whileLenEnd             ;exit loop if null value is found in the current address
    inc     eax                     ;else, increment eax to move on to the next memory location or 'block'
    jmp     whileLen                ;jump back to while loop and repeat

    whileLenEnd:

    sb      eax, ebx                ;subtract the final positions of the markers to calculate the string length must save the string length in eax {requirement)
    pop     ebx
    pop     ecx
    leave
    ret