X86 Fibonacci program

问题

My assignment is to write a program that calculates first seven values of fibonacci number sequence. the formula given is:

Fib(1) = 1, Fib(2) = 1, Fib(n) = Fib(n-1) + Fib(n-2)

I believe that is a function but I do not understand how to incorporate it into code. I need to place the values in EAX register. I am using MASM not that makes any difference. Any hints?

回答1:

I suspect that this is an academic assignment so I'm only going to partially answer the question.

The fibonacci sequence is formally defined for non-negative integers as follows:

F(n) = n                   | n < 2
     = F(n - 1) + F(n - 2) | n >= 2

This gives:

  n | F(n)
  0 |   0
  1 |   1
  2 |   1
  3 |   2
  4 |   3
  5 |   5
  6 |   8
  7 |  13
etc etc...

You can do it with just a few registers, let's identify them:

• R_n (the number of the requested fibonacci number)
• R_f1 (used to calculate fibonacci numbers)
• R_f2 (also used to calculate fibonacci numbers)
• R_x (the register to hold the return value. can overlap with any other register)

R_n is passed as the argument to the function. R_f1 shall start at 0, and R_f2 shall start at 1.

Here's what we do to get the answer, split up by routines:

Begin

1. Initialize R_f1 to 0.
2. Initialize R_f2 to 1.
3. Continue to Loop.

Loop

1. Subtract 2 from R_n.
2. If R_n is less than 0, jump to Finish.
3. Add R_f2 to R_f1, storing the result in R_f1.
4. Add R_f1 to R_f2, storing the result in R_f2.
5. Jump to Loop.

Finish

1. If R_n AND 1 is false (implying that R_n is even) jump to FinishEven.
2. Store R_f1 as the return value.
3. Return.

FinishEven

1. Store R_f2 as the return value.
2. Return.

Tracing through for R_n = 5:

1. R_f1 = 0
2. R_f2 = 1
3. R_n = R_n - 2 // R_n = 3
4. Test R_n < 0 // false
5. R_f1 = R_f1 + R_f2 // R_f1 = 0 + 1 = 1
6. R_f2 = R_f1 + R_f2 // R_f2 = 1 + 1 = 2
7. Unconditional Jump to Loop
8. R_n = R_n - 2 // R_n = 1
9. Test R_n < 0 // false
10. R_f1 = R_f1 + R_f2 // R_f1 = 1 + 2 = 3
11. R_f2 = R_f1 + R_f2 // R_f2 = 3 + 2 = 5
12. Unconditional Jump to Loop
13. R_n = R_n - 2 // R_n = -1
14. Test R_n < 0 // true
15. Jump to Finish
16. Test R_n & 1 // true
17. R_x = R_f2 // 5

Our table shows that F(5) = 5, so this is correct.

回答2:

TITLE  Chapter 4 Exercise 6                (ch04_06.asm)

Comment !
Description: Write a program that uses a loop to calculate the first
seven values in the Fibonacci number sequence { 1,1,2,3,5,8,13 }.
Place each value in the EAX register and display it with a
call DumpRegs statement inside the loop.

Last update: 05/02/2002
!
INCLUDE Irvine32.inc

.code
main PROC
    mov   eax,1
    call  DumpRegs
    mov   ebx,0 ; initial setup
    mov   edx,1
    mov   ecx,6 ; count
L1:
    mov  eax,ebx    ; eax = ebx + edx
    add  eax,edx
    call DumpRegs   ; display eax
    mov  ebx,edx
    mov  edx,eax
    Loop L1

    exit
main ENDP
END main

来源：https://stackoverflow.com/questions/12865984/x86-fibonacci-program