palin.asm

%include 'functions.asm'


section .data
section .bss
   input resb LEN
   num resb LEN+1        ;store result of itoa function
section .text

itoa:
     push ebx      ; preserve ebx on the stack to be restored after function runs
     push ecx      ; preserve ecx on the stack to be restored after function runs
     push edx      ; preserve edx on the stack to be restored after function runs

     mov ebx,10    ; store 10 to divide the number to get reminder
     xor ecx,ecx   ; clear and store number of digits

     godivide:
            xor  edx,   edx ; clear remainder
        div  ebx        ; divide eax by 10
        push edx        ; put edx on stack
        inc  ecx        ; increment number of digits
        cmp  eax,   0   ; if eax
        jne  godivide   ; is not equal to zero, divide again
        mov  ebx,   0
    nextdigit:
        pop eax     ;pop of digit
        add eax,      30h   ;add '0' to eax
        mov [num+ebx],eax   ;move current digit to buffer
        inc           ebx   ;increment index
        dec ecx     ;move down through digits
        cmp ecx,      0 ;if ecx
        jg  nextdigit   ;is greater than 0, jump nextdigit

    ;mov byte [num+LEN],0h
    mov eax,num                 ;move final result to eax

    pop edx                     ; restore edx from the value we pushed onto the stack at the start
    pop ecx                     ; restore ecx from the value we pushed onto the stack at the start
    pop ebx                     ; restore ebx from the value we pushed onto the stack at the start
ret




reverse:
    push    ebx             ; preserve ebx on the stack to be restored after function runs
    push    ecx             ; preserve ecx on the stack to be restored after function runs
    push    edx             ; preserve edx on the stack to be restored after function runs
    push    esi

    mov     esi, eax        ; move pointer in eax into esi (our number to convert)
    call    slen            ; initialise eax with decimal value 0
    mov     ecx, eax
    dec     ecx             ; initialise ecx with decimal value 0
    mov     eax, 0

.multiplyLoop:
    xor     ebx, ebx        ; resets both lower and uppper bytes of ebx to be 0
    mov     bl, [esi+ecx]   ; move a single byte into ebx register's lower half

    sub     bl, 48          ; convert ebx register's lower half to decimal representation of ascii value
    add     eax, ebx        ; add ebx to our interger value in eax
    mov     ebx, 10         ; move decimal value 10 into ebx
    mul     ebx


    dec     ecx
    cmp     ecx, -1         ; compare ebx register's lower half value against ascii value 48 (char value 0)
    je      .finished       ; jump if less than to label finished
    jmp     .multiplyLoop   ; continue multiply loop

.finished:
    cmp     ecx, 0          ; compare ecx register's value against decimal 0 (our counter register)
    je      .restore        ; jump if equal to 0 (no integer arguments were passed to atoi)
    mov     ebx, 10         ; move decimal value 10 into ebx
    div     ebx             ; divide eax by value in ebx (in this case 10)

.restore:
    pop     esi             ; restore esi from the value we pushed onto the stack at the start
    pop     edx             ; restore edx from the value we pushed onto the stack at the start
    pop     ecx             ; restore ecx from the value we pushed onto the stack at the start
    pop     ebx             ; restore ebx from the value we pushed onto the stack at the start
    ret

global  _start

_start:



    mov eax,SYS_READ
    mov ebx,STDIN
    mov ecx,input          ;read input
    mov edx,LEN
    int 80h

    ;mov byte [input+LEN-1], 0h
    mov eax,input
    call atoi              ;convert string to decimal
    mov edx,eax            ;move result to edx
    mov eax,input          ;prepare arg for reverse
 loop:

    call reverse           ;reverse string number

    cmp eax,edx            ;compare decimal reversed with orginal
    je print               ;if equal jump to print

    inc edx                ;otherwise,incerement original
    mov eax,edx            ;move to eax and convert it to string for reverse function
    call itoa
    jmp loop               ;jump uncontionally

print:
    mov eax,edx
    call iprintLF          ;print decimal result
    call quit              ;exit program