assembly reverse a string

问题

The string is read until 1 is pressed, and the 1 will be on the last position of the string. I don't know why my output is off, for example the input is: asd1 and the output is: $1111. Anyway, here is my code

data segment
msg db 0dh,0ah,"Your string: $"
rev db 0dh,0ah,"Reverted: $"
s1 db 20 dup('$')
s2 db 20 dup('$')
data ends

code segment
assume cs:code,ds:data

start:
mov ax,data
mov ds,ax

lea dx,msg
mov ah,09h
int 21h

lea si,s1
lea di,s2
mov bx,0
l1: mov ah,01h
int 21h
mov [si],al
inc bx
inc si
cmp al,31h
jnz l1


mov cx,bx
mov di,bx

dec1:   dec si
loop dec1

mov cx,bx
l2: mov al,[si]
mov [di],al
dec di
inc si
loop l2 

lea dx,rev
mov ah,09h
int 21h

mov cx,bx
l3: mov ah,02h
mov dl,[di]
int 21h
inc di
loop l3


mov ah,4ch
int 21h


code ends
end start

Edit:This is what my code looks now and if I input asd1, then I get 1dserted

回答1:

EDIT: After Ped7g's comment, I reworked the code. This new one doesn't use the stack to reverse the string and the string is not read as a whole string, but it is read char by char until "Enter" is pressed. Below is the new code.

assume cs:code, ds:data

data segment
    message db 0Dh, 0Ah, "String: $"
    reverse db 0Dh, 0Ah, "Result: $"
    string db 255 dup(0)
    result db 255 dup('$')
data ends

code segment
start:
    mov ax, data
    mov ds, ax

    ; Print "String: "
    mov ah, 09h
    lea dx, message
    int 21h

    ; Set SI where we read the string
    lea si, string

    read:
        ; Read a single character from the keyboard
        mov ah, 01h
        int 21h

        ; Save it in the memory
        mov [si], al
        inc si

        ; Check if Enter is pressed (if not, then repeat reading)
        cmp al, 0Dh
        jnz read

    ; Calculate the length of the string read
    mov ax, si
    lea bx, string
    sub ax, bx

    ; Set DI at the last char of result
    lea di, result
    add di, ax

    ; Decrement one byte to position DI on the last char
    ; of the string (the Carriage Return)
    dec di

    ; Decrement one byte because we don't want to consider
    ; the Carriage Return as a part of our reversed string
    dec di

    ; Set SI at the first char of string
    lea si, string

    reverse_string:
        ; Copy from the beginning of the initial string
        ; to the end of the reversed string
        mov al, [si]
        mov [di], al

        ; Step
        inc si
        dec di

        ; Verify if we have reached the end of the initial string
        ; (if the "current" char is Carriage Return)
        cmp byte ptr [si], 0Dh
        jnz reverse_string

    ; Print "Result: "
    mov ah, 09h
    lea dx, reverse
    int 21h 

    write:
        ; Write the whole reversed string on standard output
        mov ah, 09h
        lea dx, result
        int 21h

    mov ah, 4Ch
    int 21h
code ends

end start

Old answer:

You can try to use the LIFO property of stack. Below is an example of code that reverses a string using it. The algorithm puts every character from the beginning of the input string, and then pops out to the result (in the reverse order).

assume cs:code, ds:data

data segment

    msg db 0Dh, 0Ah, "String: $"

    rev db 0Dh, 0Ah, "Result: $"

    buffer label byte
    str_maxlen db 255
    str_length db 0
    str_string db 255 dup(0)

    result db 255 dup('$')

data ends

code segment

start:
    mov ax,data
    mov ds,ax

    mov ah, 09h
    lea dx, msg
    int 21h         ; print "Your string"

    mov ah, 0Ah
    lea dx, buffer
    int 21h         ; read your string

    cmp str_length, 0
    je skip         ; check if the input is null

    mov ch, 0
    mov cl, str_length
    lea si, str_string
    put_on_stack:
        push [si]   ; copy on the stack (from string)
        inc si
        loop put_on_stack

    mov ch, 0
    mov cl, str_length
    lea di, result
    get_from_stack:
        pop [di]    ; copy back to memory (in result)
        inc di
        loop get_from_stack

    mov byte ptr [di], '$'

    skip:
    mov ah, 09h
    lea dx, rev
    int 21h         ; print "Result: "

    mov ah, 09h
    lea dx, result
    int 21h         ; print the result

    mov ah,4Ch
    int 21h

code ends

end start

来源：https://stackoverflow.com/questions/38824670/assembly-reverse-a-string

标签

assembly

tasm