Question

I’m trying to solve a homework assignment – I managed to produce a working piece of code but it’s producing the wrong answer. I tried debugging with gdb but I still can’t see what’s wrong with my code.

.data
        a : .long 6                                                                                                                                                             
        r : .long 0
        out : .string "result %d\n"
.text
.global main                                                                                                                                                                    
fib:
        pushl %ebp
        movl %esp, %ebp
        movl 8(%ebp), %eax
        cmpl $0, %eax #fib(0)=0
        je endf
        cmpl $1, %eax #fib(1)=1
        je endf
        decl %eax #eax=n-1
        movl %eax, %edx #edx=n-1
        pushl %edx  #save n-1
        pushl %eax #set arg
        call fib #re in ecx                                                                                                                                                     
        popl %eax #get n-1
        decl %eax #eax=n-2
        pushl %ecx #save result for n-1                                                                                                                                         
        pushl %eax #set arg
        call fib #res in ecx                                                                                                                                                    
        popl %eax # eax=fib(n-1)                                                                                                                                                
        addl %eax, %ecx #fib(n)=fib(n-1)+fib(n+2)                                                                                                                               
        movl %ebp,%esp #Exit                                                                                                                                                    
        popl %ebp
        ret
endf:
        movl %eax, %ecx#fib(0) or fib(1) to %ebx                                                                                                                                
        movl %ebp,%esp                                                                                                                                                          
        popl %ebp
        ret

main:
        pushl a  #stack [a]                                                                                                                                                     
        call fib #result in %ecx                                                                                                                                                
        pushl %ecx                                                                                                                                                              
        pushl $out                                                                                                                                                              
        call printf 

Answer 1

Note that you are not removing any arguments you pass to fib anywhere, so your stack becomes unbalanced. See the fixed version in operation.

Also, typical calling conventions return values in eax. Using ecx for no good reason is confusing. See a simplified version that adheres to conventions better.