Question

Change the following C++ functions to Assembly x86:

int problem1_ ( )
{
int numberArray [3] = {1, -2, 15 };
int result = 0, index = 0;
int numElements = 3;
while (index < numElements)
{
if ( index >= 1 && numberArray[index] > 3 )
{
result += numberArray[index] ;
}
else
{
result -= 3;
}
index++;
}
return result;
}

int problem2_ ( )
{
int a, modulo
int answer = 0, b = 3;
for ( a = 5; a > 0; a-- )
{
if ( ( a % 4 == 0 ) || ( a % 4 == 3) )
{
answer += b ;
}
else
{
answer *= 2;
}
}
return answer;
}

Answer 1

Solution:-

When it comes to code conversion c++ to assembly it is very complicated and time consuming.

problem1_():

        push    rbp

        mov     rbp, rsp

        mov     DWORD PTR [rbp-24], 1

        mov     DWORD PTR [rbp-20], -2

        mov     DWORD PTR [rbp-16], 15

        mov     DWORD PTR [rbp-4], 0

        mov     DWORD PTR [rbp-8], 0

        mov     DWORD PTR [rbp-12], 3

.L5:

        mov     eax, DWORD PTR [rbp-8]

        cmp     eax, DWORD PTR [rbp-12]

        jge     .L2

        cmp     DWORD PTR [rbp-8], 0

        jle     .L3

        mov     eax, DWORD PTR [rbp-8]

        cdqe

        mov     eax, DWORD PTR [rbp-24+rax*4]

        cmp     eax, 3

        jle     .L3

        mov     eax, DWORD PTR [rbp-8]

        cdqe

        mov     eax, DWORD PTR [rbp-24+rax*4]

        add     DWORD PTR [rbp-4], eax

        jmp     .L4

.L3:

        sub     DWORD PTR [rbp-4], 3

.L4:

        add     DWORD PTR [rbp-8], 1

        jmp     .L5

.L2:

        mov     eax, DWORD PTR [rbp-4]

        pop     rbp

        ret

problem2_():

        push    rbp

        mov     rbp, rsp

        mov     DWORD PTR [rbp-8], 0

        mov     DWORD PTR [rbp-12], 3

        mov     DWORD PTR [rbp-4], 5

.L12:

        cmp     DWORD PTR [rbp-4], 0

        jle     .L8

        mov     eax, DWORD PTR [rbp-4]

        and     eax, 3

        test    eax, eax

        je      .L9

        mov     eax, DWORD PTR [rbp-4]

        cdq

        shr     edx, 30

        add     eax, edx

        and     eax, 3

        sub     eax, edx

        cmp     eax, 3

        jne     .L10

.L9:

        mov     eax, DWORD PTR [rbp-12]

        add     DWORD PTR [rbp-8], eax

        jmp     .L11

.L10:

        sal     DWORD PTR [rbp-8]

.L11:

        sub     DWORD PTR [rbp-4], 1

        jmp     .L12

.L8:

        mov     eax, DWORD PTR [rbp-8]

        pop     rbp

        ret

Note:- One simple approach that you can use is to write down the c++ code and in the cmd you can call the following command, this command will convert the cpp code to assembly code.

gcc -S programName.cpp