For loop in x86 assembly and optimising code?

Question

I am currently learning assembly programming as part of one of my university modules. I have a program written in C++ with inline x86 assembly which takes a string of 6 characte

Answer 1

I would suggest to look into assembly code which is generated by compiler. You can change and optimize it later.

How do you get assembler output from C/C++ source in gcc?

Answer 2

I can't help with optimization or the cryptography but i can show you a way to go about making a loop, if you look at the loop in this function:

void f()
{
        int a, b ;

        for(a = 10, b = 1; a != 0; --a)
        {
            b = b << 2 ;            
        }       
}

The loop is essentially:

        for(/*initialize*/; /*condition*/; /*modify*/)
        {
            // run code
        }

So the function in assembly would look something along these lines:

_f:
        push ebp
        mov ebp, esp

        sub esp, 8                 ; int a,b

initialize:                        ; for
        mov dword ptr [ebp-4], 10  ; a = 10,
        mov dword ptr [ebp-8], 1   ; b = 1

        mov eax, [ebp-4]
condition:      
        test eax, eax              ; tests if a == 0
        je exit

runCode:
        mov eax, [ebp-8]
        shl eax, 2                 ; b = b << 2
        mov dword ptr [ebp-8], eax

modify:
        mov eax, [ebp-4]
        sub eax, 1                 ; --a
        mov dword ptr [ebp-4], eax
        jmp condition

exit:
        mov esp, ebp
        pop ebp
        ret

Plus I show in the source how you make local variables;

• subtract the space from the stack pointer.
• and access them through the base pointer.

I tried to make the source as generic intel x86 assembly syntax as i could so my apologies if anything needs changing for your specific environment i was more aiming to give a general idea about how to construct a loop in assembly then giving you something you can copy, paste and run.