How to output centered text input by a user in x86 assembly?

Question

How would I display the string that the user entered?

And I need to display it at the center of a 80x25 text screen with the following formula:

column = (80 - string length) / 2;
row = 25/2;

.MODEL    TINY 
.386 
STACK 256     
DATASEG
  msg    db 'Enter string:$' 
  sbuf label word        ;адрес буфера для функции 0Ah
  max    db 0            ;максимальное количество символов
  len    db 0            ;количество фактически введенных
  string db 254 DUP('#') ;буфер для строки (адрес строки)
  ;string db 255,0, 256 dup (' ') 
  new db 0ah, 0dh, '$' 

CODESEG         
start:     
  mov ax,@data 
  mov ds,ax 
  mov ah,09h 
  mov dx,offset msg 
  int 21h 
  mov ah, 0Ah 
  mov dx,offset string 
  int 21h 

  xor ax,ax 
  mov al,offset string+1 
  inc dx 
  add dx, ax 
  mov si,dx 
  mov ah,09h 
  mov dx,offset new       ;output new line symbol 
  int 21h 

  std                     ;устанавливаем флаг направления дляпрохода обратно 
print_next_char: 
  lodsb                   ;читаем символ из DS:SI и уменьшаем SI на 1 
  cmp si,offset string+1  ;конец строки? (то есть начало) 
  jb endprog              ; если да, то переход к концу программы 

  cmp al,0dh 
  je skip 
  cmp al,023h 
  je skip 

  movzx bx,al 
  dec bx 

  inc byte[bx+len] 
skip: 
  mov dl, al              ;загружаем прочитанный из строки символ в DL 
  mov ah,02h              ;DOS-функция вывода символа 
  ; int 21h               ;вызов DOS 
  jmp print_next_char ;возвращаемся к следующему символу 
endprog: 
  mov dx,offset new 
  mov ah,09h 
  int 21h 

  mov DX, OFFSET string   ; ds:dx points to string
  mov AH, 09h
  int 21h

  mov AX, 4C00h           ; Return to DOS with ERRORLEVEL=0
  int 21h
end start

My task is to develop an executable (EXE) program, which initially displays (function 09h, DOS 21h interrupt) an invitation to enter a string of characters from the keyboard.

After entering (function 0Ah, DOS 21h interrupt) line, the program should display its string centered on the screen (function 13h, BIOS 10h interrupt). The coordinate of the screen position where the output has to be put is calculated as follows:

column = (80 - dlina_stroki) / 2;
row = 25/2;

To enter the string you can define the following buffer:

; ...
.data
; ...
  sbuf label word       ; buffer address 0Ah function
  max db 0              ; the maximum number of characters
  len db 0              ; the number is actually imposed
  string db 254 dup (0) ; buffer line (row address)
; ...

Then, in the program of each of the referenced sbufs of the buffer, it can be used as separate variables.

Sep Roland Sep Roland · Accepted Answer · 2016-05-15T19:29:42

xor ax,ax 
mov al,offset string+1 
inc dx 
add dx, ax 
mov si,dx

Here your intent is to calculate the address of the last character of the user input, but then you need to drop the offset tag and write mov al, [string+1] or else write mov al, len.

cmp al,0dh 
je skip 
cmp al,023h 
je skip

Compairing for a carriage return 0Dh is useless here since there won't be any found going to the left as you do.
Why do you compare to 23h which is a #?

movzx bx,al 
dec bx 
inc byte[bx+len]

You're using the ASCII character in AL as an offset in memory. I don't see how that's going to be of any use for your task.

How to output centered text input by a user in x86 assembly?

1 Answers