Fibonacci Program in Assembly Language

Here’s how to write a Fibonacci sequence program in Assembly language from scratch for the x86-64 processor. Also includes instructions on how to install the Flat Assembler.


First, let’s install the Flat Assembler. You can download the program from here:

Doesn’t need Windows setup/installation. Simply download the zip file and extract it to your preferred location.

Personally I extracted mine here: C:\Users\sanjib\bin\fasmw17330

Location of the Flat Assembler program on my machine

Environment Variables

We need to set 2 environment variables:

  • Set the INCLUDE variable to the INCLUDE folder in your fasm folder (where you extracted the Flat Assembler program): this will allow you to include additional assembly files in your assembly program. Mine was: C:\Users\sanjib\bin\fasmw17330\INCLUDE
  • Add to the Path variable your fasm folder (where you extracted the Flat Assembler program): this will allow you to call the fasm.exe program from anywhere in your machine. I added C:\Users\sanjib\bin\fasmw17330
The INCLUDE environment variable on my machine
The Path environment variable on my machine

Download the file

Download the file from here:

We need to download this file at put it in the INCLUDES folder of your fasm directory.

We need this file as we are going to include it in our Fibonacci program. This program allows user input and output to the console.

How my INCLUDE folder looks like

Fibonacci Program in Assembly Language

This is how the Fibonacci program looks like in Assembly language. Please read the inline comments which explains all the lines of code. Also watch the video that shows from the beginning to end on how to install the Flat Assembler and write the program from scratch.

I mention x and y below as if they were variables. But these are just imaginary variables to explain Fibonacci algorithm.

We are using 4 registers as follows:

  • eax: to store the x value
  • ebx: to store the y value
  • ecx: as the counter
  • edx: as the tmp value during the swap
format PE console       ; tells the assembler that this is a console program
entry start             ; marks the start of the program, see the start: label below

include ''    ; includes the file necessary for Windows operating system
include ''  ; includes the file to call read_hex and _print_eax instructions

start:                  ; the entry-point of the program
    call    read_hex    ; reads user input and stores it in the eax register
    mov     ecx, eax    ; move content eax to ecx, will use ecx as the counter n

    mov     eax, 0      ; now set eax to 0 (initial value of let's say x)
    mov     ebx, 1      ; set ebx to 1 (initial value of let's say y)

    ; the 3 lines below are the fibonacci algorithm: sets x to y, and y to x+y
    mov     edx, eax    ; store value of eax in edx (used as a tmp area)
    mov     eax, ebx    ; move content of ebx to eax (sets x)
    add     ebx, edx    ; sum ebx with value of edx (what used to be eax, sets y to x+y)

    jc      fib_carry   ; register 32-bits, not big enough to calculate over 30 hex

    dec     ecx         ; decrease counter
    jz      exit        ; jump to exit: if ecx reaches 0

    jmp     fib         ; jump back to fib: label, creates the loop

    mov     eax, -1     ; carry over occurred, mark it with value ffffffff

    call    print_eax   ; print answer

    push    0           ; exit console program
    call    [ExitProcess]

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