How to convert this for() and while() loops into assembly-CodePudding

After it got converted, store the answer in the sum variable

So I have to convert it into x86 assembly language, and this is what I got so far

.386
.model flat, stdcall
.stack 4096
ExitProcess PROTO, dwExitCode: DWORD

.data
    sum DWORD 0
    array_list DWORD 10,11,13,18,21,23,24,17,45
    array_size = ($ - array_list) / TYPE array_list


.code
main PROC
    mov eax, 0        ; sum
    mov esi, 0        ; index
    mov ecx, array_size

    L1:
                dec ecx
        cmp esi, ecx
        jl L2
        jmp L5
    
    L2:
        cmp esi, ecx
        jl L3
        jmp L4

    L3:
        cmp array_list[esi], array_list[esi%2]
        add eax, array_list[esi]
        jmp L4
    L4:
        add esi, 1
        jmp L1
    L5:
        mov sum, eax
    


    INVOKE ExitProcess, 0
main ENDP
END main

For the array_size, I was trying to make sure that it is 40/4 = 10 I do not understand or know how to the for loop, so the first loop must be wrong what I wrote there. Also, how do you do the if, where it said array_list[index] is even. Do I also need to declare the sample? Cuz it is used for the array_size. I really need help because I did not understand :(

CodePudding user response：

The basic pattern equivalence for a for-loop is as follows:

C for-loop

for ( init; test; incr ) {
    <loop-body>
}

C while-loop

init;
while ( test ) {
    <loop-body>
    incr;
}

That first loop in the assembly is following some different basic form: it has relocated the incr; portion in relation to the <loop-body>.

Surely you can see that in comparison with the following pattern:

init;
while ( test ) {
    incr;
    <loop-body>
}

that <loop-body> will see a different value for current_size each iteration (off by 1 increment, here -1) than in the C code, so it won't run the same as the C code, if that variable is ever consulted in the <loop-body>.

The test condition to continue the outer for in the assembly doesn't reflect the > 0 in the C code.

The % in the assembly code is doing array[index%2] whereas in the C code it is doing array[index]%2, which are clearly different concepts, so that wouldn't run the same (even if it were allowed like that).

Of course, x86 cannot do that compare instruction, because of the dual memory references. x86 requires one operand in register in order for the other to be a memory operand.

But that C code doesn't even require two array references there. Just one and than an even test on its value.

The C code requires an if-then which is missing in the assembly.

Let's convert the following while-loop (which matches the for-loop construct):

init;
while ( test ) {
    <loop-body>
    incr;
}

To assembly's if-goto-label style:

    init;
loop1:
    if ( ! test ) goto endLoop1;
    <loop-body>
    incr;
    goto loop1;
endLoop1: