How use "asm" in C++ ?

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Hi, I have some problem when I'm using 'asm' in C++.
I need to find max value in array, I wrote the code but it doesn't compare.

What I have tried:

void func1(int x[],int lenght){ // Find MAX 
    
    int max, z = x[0]; //Make first elem is MAX
    
    __asm__ volatile 
        (   
            "movl %1, %%eax;"
            "movl %%eax, %0;"
            : "=r" ( max )        
            : "r" ( z )         
            : "%eax" 
        );

    
    for(int i = 1 ; i < lenght; i++) //From second element to end
    {
        __asm__ volatile
        (   
            "movl %1, %%ebx;" // Move x[i] to EBX
            "cmpl %%ebx,%%eax;" 
            "jl DONE;"// If EAX less EBX JUMP to 'DONE'
            "DONE: movl %%ebx,%%eax;" // MOVE EBX to EAX
            "movl %%eax , %0" // MOVE EAX to 'max'
            :"=r"(max)
            :"r"(x[i])
        );
    }

    cout << "MAX" << endl;
    cout << max << endl;
}

Posted 22-Mar-20 10:56am

Nabiev Anush

Updated 3-Apr-20 5:39am

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Comments

Rick York 22-Mar-20 23:15pm

This is how length is spelled.

I don't know assembler language because I have never found it necessary. I once wrote an on-the-fly machine code generator though. I did it by figuring out what machine code was emitted by writing code snippets in C and then examining the compiler's assembler and machine code listings. I recommend doing that for this code. It is quite simple so write it in C and then look at what the compiler does with the code in its assembler listings. That's how I would proceed anyway.

Nabiev Anush 24-Mar-20 6:05am

Thank you for advice !

Stefan_Lang 26-Mar-20 3:48am

That's actually very good advice: maybe it will give people an idea just how pointless it is nowadays to provide the assembler code by yourself ;-)

2 solutions

Solution 1

Totally compiler dependent. Your case seems like the GCC. In MSVC it will be more like:

__asm {
   mov al, 2
   mov dx, 0xD007
   out dx, al
};

Posted 25-Mar-20 12:23pm

steveb

Updated 25-Mar-20 12:24pm

v2

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Nabiev Anush · Accepted Answer · 2020-04-03T05:40:00

I use another way how to find max value in array.
I used MS Visual Studio.

C++

include 

define LEN 6

using namespace std;

int task1();

int main() {
    cout << "Task 1: " << task1() << endl;
    return 0;
}

int task1() {
    int mas[LEN] = { 1, 2, 7, 4, 5, 6 };

    __asm {
        xor eax, eax
        mov ecx, LEN

        cycle_mark :
            mov edx, [mas + 4 * ecx]
            cmp eax, edx
                jl max_mark
                jg check_mark

        max_mark :
            mov eax, edx

        check_mark :
            dec ecx
            cmp ecx, 0
                jne cycle_mark
    }
}