Slide 6.2: A sample program: finding Fibonacci numbers

A Sample Program: Finding Fibonacci Numbers

The following program generates and prints up to N Fibonacci numbers or to EAX unsigned “overflow” (carry), which ever occurs first.

EAX = Fib( ESI ), a 32-bit unsigned integer.
EBX is the other Fibonacci number needed for the addition.
ESI identifies which Fibonacci number is in EAX.

Fibonacci.asm

 TITLE Fibonacci Numbers              (Fibonacci.asm)

 INCLUDE Irvine32.inc

 .data
 prompt1  BYTE   "Find the first N values of the Fibonacci numbers.", 0Dh, 0Ah
          BYTE   0Dh, 0Ah, "Enter the value of N (positive integer):  ", 0
 prompt2  BYTE   0Dh, 0Ah, "Overflow at N = ", 0
 prompt3  BYTE   "  ", 0
 n        DWORD  ?

 .code
 main PROC
       call  Clrscr
       mov   edx, OFFSET prompt1
       call  WriteString
       call  ReadInt
       mov   n, eax
       call  Crlf

       mov   esi, 1                ; esi = 1, 2, 3, 4, 5, 6,  7, ...
                                   ; eax = 1, 1, 2, 3, 5, 8, 13, ...
       mov   edi, 1                ; edi = 1, 2, 3, 4, 5, 1,  2, ...
       mov   ecx, n                ; Computes and prints N numbers.
       mov   eax, 1                ; Initializes Fib(1) = 1.
       mov   ebx, 0                ; Fib(2) = Fib(1) + 0.

 L1:   add   eax, ebx              ; Generates the next number.
                 ; Jumps if carry (overflow).
       cmp   edi, 6                ; Prints 5 values each line.
       jne   L2
       
       mov   edi, 1                ; Advances to the next line.
       call  Crlf                  ; Prints newline.

 L2:   call  WriteDec              ; Prints Fib(esi). 
       mov   edx, OFFSET prompt3   
       call  WriteString           ; Prints spaces.

       xchg  eax, ebx              ; Sets up the sequence.
       inc   esi
       inc   edi
       loop  L1
       jmp   L4                    ; end of loop

 L3:   mov   edx, OFFSET prompt2   ; overflow
       call  WriteString           ; Prints overflow.
       mov   eax, esi
       call  WriteDec              ; Prints N.

 L4:   call  Crlf
       call  Crlf
       exit
 main ENDP
 END main

^†Note that you may not jump out of a loop and jump back into the loop later. It may mess up the ECX register.