CODEWITHSUNDEEP
loopsassemblyx86
user267288
user267288

Reputation: 183

How to make a loop in x86 assembly language?

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?

Upvotes: 18

Views: 148028

Answers (6)

rafeef
rafeef

Reputation: 21

.model small
.stack 100h
.code 
Main proc
Mov cx , 30 ; //that number control the loop 30 means the loop will 
;excite 30 time 
Ioopfront:
Mov ah , 1
Int 21h 
Loop loopfront;

this cod will take 30 character

Upvotes: 2

t0mm13b
t0mm13b

Reputation: 34592

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.

Upvotes: 4

Arthur Kalliokoski
Arthur Kalliokoski

Reputation: 1647

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

Upvotes: 19

Pranav Singh
Pranav Singh

Reputation: 20081

I was looking for same answer & found this info from wiki useful: Loop Instructions

The loop instruction decrements ECX and jumps to the address specified by arg unless decrementing ECX caused its value to become zero. For example:

 mov ecx, 5
 start_loop:
 ; the code here would be executed 5 times
 loop start_loop

loop does not set any flags.

loopx arg

These loop instructions decrement ECX and jump to the address specified by arg if their condition is satisfied (that is, a specific flag is set), unless decrementing ECX caused its value to become zero.

  • loope loop if equal

  • loopne loop if not equal

  • loopnz loop if not zero

  • loopz loop if zero

Source: X86 Assembly, Control Flow

Upvotes: 0

Jeff B
Jeff B

Reputation: 30099

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

Upvotes: 14

user257111
user257111

Reputation:

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.

Upvotes: 0

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