Why is my array not changing in x86 Assembly MASM-CodePudding

I'm getting into Assembly and having issues with arrays. Coming from C background, array manipulation is relatively easy as you can do lots of magic with pointers, referencing etc. however this is not the case with Assembly. I'm doing array wrapping and wrote this program, however when I assemble this code and run it, the array does not change (I check it from Watch list). What am I doing wrong?

.386
.model flat,stdcall
.stack 4096

ExitProcess PROTO,dwExitCode:dword

.data
arrayA DWORD 1001d,1002d,1003d,1004d,1005d
len DWORD 5

.code
main proc
    
    mov ebx, OFFSET arrayA
    mov edx,len
    cmp edx,1
    je lengthOne
    
    cmp edx,2
    je lengthTwo

    cmp edx,3
    jge lengthGreater

lengthOne:
    invoke ExitProcess,0

lengthTwo:
    mov eax, [ebx   4]
    mov dx, [ebx]
    mov [ebx   4], dx
    mov [ebx], eax
    invoke ExitProcess,0

lengthGreater:
    mov edx, [ebx   16]                     
    push edx
    mov ecx, 16                                     
start:
    mov eax, [ebx   ecx]
    cmp eax, [ebx]
    je firstIndex

    mov edx, [ebx   ecx - 4]
    mov [arrayA   ecx], edx
    sub ecx, 3
    loop start
    invoke ExitProcess,0

firstIndex:
    pop edx
    mov [arrayA   ecx],edx
    invoke ExitProcess,0

main endp
end main

CodePudding user response：

I debugged your code in visual studio. Your array is actually changing and it is changing nicely. There is only one problem with your code:

start:
    mov eax, [ebx   ecx]
    cmp eax, [ebx]
    je firstIndex

    mov edx, [ebx   ecx - 4]
    mov [arrayA   ecx], edx
    sub ecx, 3
    loop start
    invoke ExitProcess,0

firstIndex:
    pop edx
    mov [arrayA   ecx],edx
    invoke ExitProcess,0

If you look at this part you can see that after loop you are leaving the program. The problem with that is that you are not doing the firstIndex check after modifying the array. To fix it you can just remove the invoke ExitProcess,0 after then loop.

start:
    mov eax, [ebx   ecx]
    cmp eax, [ebx]
    je firstIndex

    mov edx, [ebx   ecx - 4]
    mov [arrayA   ecx], edx
    sub ecx, 3
    loop start

firstIndex:
    pop edx
    mov [arrayA   ecx],edx
    invoke ExitProcess,0

So that once your loop finishes you automatically do the firstIndex

If you run this code you get the following array structure:

Edit:

You can maybe also do something like, but I haven't tested this one so I do not know if it works as this point:

start:

    mov edx, [ebx   ecx - 4]
    mov [arrayA   ecx], edx
    sub ecx, 3
    
    mov eax, [ebx   ecx]
    cmp eax, [ebx]
    je firstIndex

    loop start
    invoke ExitProcess,0

firstIndex:
    pop edx
    mov [arrayA   ecx],edx
    invoke ExitProcess,0