Incorrect output to Fibonacci series in MASM

2
votes

I've been working on this code for an assignment, can't seem to get the output right. Can anyone please help me out?

NOTE: The program is compiled in MASM

  1. I'm only allowed to use reg-mem and reg-reg architecture commands.
  2. Only use MOV, ADD, DEC, JMP, or Jcc instructions.
  3. Only use the four main registers, i.e., EAX, EBX, ECX, and EDX, along with ESI register and their sub registers for arithmetical/logical operation.
  4. Other then the string memory variables no other memory variable is allowed.

Following is the code:

INCLUDE Irvine32.inc
.data

string1 byte "Enter number to generate Fibonacci series: ",0
string2 byte "Fibonacci is ",0

.code
main PROC
call    DumpRegs;
mov     edx,offset string1;
call    writestring;
call    ReadInt;
mov     ecx,eax;
mov     eax,1;
call    DumpRegs;
dec     ecx;
mov     esi,eax;
JMP     Jumpzero;
mov edx, offset string2;    
call writeint           ;   Display the contents of eax register on the output device

Jumpzero:
add     eax,esi;
call    DumpRegs;
inc     esi;
dec     ecx
jnz     Jumpzero
exit
MAIN ENDP   
END main
3
assemblyx86masmirvine32
What's the problem? What's the output? What output do you expect? – m0skit0
If I want a fibonacci series of 4 then the output is 1,1,2,4. While the output should be 1,1,2,3. The problem is probably somewhere in the Jumpzero loop but I can't fix it. – user1797972
@user1797972 Why, instead of dec cx jnz Jumpzero, aren't you using loop Jumpzero? – Powerslave
Fibonacci sequence may be considered as starting at 0: 0,1,1,2,3,5,8, ... . Find out if the first number should be zero or one for this assignment. – rcgldr

3 Answers

0
votes
; ...
call ReadInt
mov ecx,eax

mov eax,1
mov edx,eax
call writeint    ; Assuming EDX is incremented by writeint
_generate:
    call writeint
    add eax,edx
    mov edx,eax
loop _generate
; ...
0
votes

Oh dear, people are teaching based on that Irvine stuff? That explains a lot.

I think you need to think about it like this:

you need to remember 3 numbers:

  • current number

  • previous number

  • sum of current and previous

then move current to previous, sum to current and continue.

So if you set things up as ebx=current value, edx=previous value, ecx= number of values to output

then you can use a central loop like:

fib_loop:
    mov eax, ebx
    add eax, edx
    mov edx, ebx
    mov ebx, eax
    call writeint
    dec ecx
jnz fib_loop

NOTE: I'm taking the behavior of the 'writeint' proc from your comment that it displays eax on the output device only, and assuming it doesn't mess with any of the other registers.

0
votes

Only two numbers need to be kept track of for Fibonacci. If XCHG were allowed the logic would be:

    swap(A, B)
    A += B

If not, then a temp variable is needed for the swap:

    D = B
    B = A
    A = D
    A += B

The initial values for A and B depend on what you want the first output to be. For example, start with A = 1 (fib(-1)), B = -1 (fib(-2)), then the sequence 0,1,1,2,3,5, ... will be produced. Or start with A = 0 (fib(0)), B = 1 (fib(-1)), to produce the sequence 1,1,2,3,5, ...