CODEWITHSUNDEEP
loopsassemblymasmirvine32
Quaker
Quaker

Reputation: 1553

MASM code with loops not working as expected,

I am currently in the process of learning MASM, and I've faced the following task:

Write a program that will calculate the value of f(n) according to the following recursive formula: f(n) = f(n-1) + 2*f(n-2) -2, f(0) = 0, f(1) = 3

I'm calculating for n = 15 and since I know f(0), f(1) I subtract two from n's value.

TITLE Recursion Solver

INCLUDE Irvine32.inc

.data
n   SDWORD 15       ;
n2  SDWORD 0        ; n-2 (n0)
n1  SDWORD 3        ; n-1 (n1)

.code
main PROC

mov ecx, n      ; initialize loop counter
sub ecx, 2      ; 


again:
    mov eax, n1     ; eax = f(n-1)
    mov ebx, n2     ; ebx = f(n-2)
    add ebx, ebx    ; ebx = 2*f(n-2)
    sub ebx, 2      ; ebx = 2*f(n-2) - 2
    add eax, ebx    ; eax = f(n-1) + 2*f(n-2) -2
    mov ebx, n2     ;
    mov n2, eax     ;
    mov n1, ebx     ;
    loop again

    call WriteInt
    exit

main ENDP

END main

I wrote a simple C++ program to calculate the n'th value according to the formula, but for some reason the assembly program isn't working as expected.

Here's the output of my C++ program:

f(0) = 0
f(1) = 3
f(2) = 1
f(3) = 5
f(4) = 5
f(5) = 13
f(6) = 21
f(7) = 45
f(8) = 85
f(9) = 173
f(10) = 341
f(11) = 685
f(12) = 1365
f(13) = 2733
f(14) = 5461
f(15) = 10925

and this is the integer the assembly outputs: -13859

Upvotes: 0

Views: 209

Answers (1)

gawi
gawi

Reputation: 2952

There is one error that I can see. You mix f(n-1) and f(n-2). The easiest way to fix it is by changing the definition of starting point:

n1 SDWORD 3; n0
n2 SDWORD 0; n1

But of course you can swap all references to these variables in your code.

And the second error that I found:

mov ebx, n1     ;
mov n1, eax     ;
mov n2, ebx     ;

Upvotes: 1

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