Question

The following is a homework problem for an assembly language class. We’re to create Y86 code by reading X86 code generated from C in gdb. The purpose of the function is to sum the elements of a linked list.

As the function currently stands, it works! The correct value is contained in the %eax register when the program terminates. Unfortunately, it is only correct due to a hack. I added a halt instruction right at the end of the function, right before the ret instruction. If I uncomment that, what appears to happen is that when it carries out the ret instruction, the PC is set to 0x0. In other words, it seems to start over at the beginning, when what it should do is return to the point where it was called. It enters an infinite loop.

The code follows. It is standalone, if you have the Y86 simulator installed.

.pos    0
init:    irmovl Stack, %esp  
irmovl    Stack, %ebp  
jmp     Main  

Main:  
    irmovl  ele1, %eax  
    pushl   %eax  
    call    sum_list  
    halt  

sum_list:  
    pushl   %ebp  
    rrmovl  %esp, %ebp  
    irmovl  $16, %edx  
    subl    %edx, %esp  
    irmovl  $0, %edx  
    rmmovl  %edx, -4(%ebp)  
    jmp     L2  

L3:
    mrmovl  8(%ebp), %eax
    mrmovl  (%eax), %eax
    mrmovl  -4(%ebp), %edx
    addl    %eax, %edx
    rmmovl  %edx, -4(%ebp)
    mrmovl  8(%ebp), %eax
    mrmovl  4(%eax), %eax
    rmmovl  %eax, 8(%ebp)

L2:
    irmovl  $0, %ecx
    mrmovl  8(%ebp), %edx
    subl    %ecx, %edx
    jne     L3
    mrmovl  -4(%ebp), %eax
    rrmovl  %esp, %ebp
    popl    %ebp
    halt #THIS DOESN'T BELONG. COMMENT OUT TO SEE BAD BEHAVIOR.
    ret

#linked list
.align  4
    ele1:
            .long   0x00a
            .long   ele2
    ele2:
            .long   0x0b0
            .long   ele3
    ele3:
            .long   0xc00
            .long   0


    .pos    0x300
    Stack:

Thanks for your help!

Answer 1

This may or may not be the cause of the error, but I think you are tearing down the stack frame of sum_list incorrectly. You set it up as follows:

pushl   %ebp  
rrmovl  %esp, %ebp

but then you tear it down like this:

rrmovl  %esp, %ebp
popl    %ebp

Notice that you copy from %esp to %ebp in both cases, which is not correct since the teardown is supposed to undo what the setup does. Instead, try

rrmovl  %ebp, %esp
popl    %ebp

or simply

leave

which does the same thing.