How to make a loop in x86 assembly language?

user_267288 0
Viewed 133750

I have written the code so far as:

.code

main
 Clrscr

  mov dh,10            ;row 10

  mov dl,20            ;column 20

  call Gotoxy          ;locate cursor

  PromptForIntegers  

    WriteString       ;display string
    ReadInt           ;input integer
  ArraySum

    WriteString       ;display string
    WriteInt          ;display integer

DisplaySum  ENDP

END main

How do I get it to repeat the same steps three times using a loop, clearing the screen after each loop iteration?

x86 assembly loops
6 Answers
mov cx,3

loopstart:
   do stuff
   dec cx          ;Note:  decrementing cx and jumping on result is
   jnz loopstart   ;much faster on Intel (and possibly AMD as I haven't
                   ;tested in maybe 12 years) rather than using loop loopstart
user_2586630

Yet another method is using the LOOP instruction:

mov  cx, 3

myloop:
    ; Your loop content

    loop myloop

The loop instruction automatically decrements cx, and only jumps if cx != 0. There are also LOOPE, and LOOPNE variants, if you want to do some additional check for your loop to break out early.

If you want to modify cx during your loop, make sure to push it onto the stack before the loop content, and pop it off after:

mov  cx, 3

myloop:
    push cx
    ; Your loop content
    pop  cx

    loop myloop
user_1792160

Use the CX register to count the loops

mov cx, 3
startloop:
   cmp cx, 0
   jz endofloop
   push cx
loopy:
   Call ClrScr
   pop cx
   dec cx
   jmp startloop
endofloop:
   ; Loop ended
   ; Do what ever you have to do here

This simply loops around 3 times calling ClrScr, pushing the CX register onto the stack, comparing to 0, jumping if ZeroFlag is set then jump to endofloop. Notice how the contents of CX is pushed/popped on/off the stack to maintain the flow of the loop.

user_2063670

You need to use conditional jmp commands. This isn't the same syntax as you're using; looks like MASM, but using GAS here's an example from some code I wrote to calculate gcd:

gcd_alg:
    subl    %ecx, %eax      /* a = a - c */
    cmpl    $0, %eax        /* if a == 0 */
    je      gcd_done        /* jump to end */
    cmpl    %ecx, %eax      /* if a < c */
    jl      gcd_preswap     /* swap and start over */
    jmp     gcd_alg         /* keep subtracting */

Basically, I compare two registers with the cmpl instruction (compare long). If it is less the JL (jump less) instruction jumps to the preswap location, otherwise it jumps back to the same label.

As for clearing the screen, that depends on the system you're using.

Admin1
Related