How is assembly IO performed for text based stuff?

Question

I'm looking to understand how to do basic input/output to write a text based game in x86 assembly, simply for the sake of learning of the instruction set and the internals.

I don't want to use stdlib.h or stdio.hin my assembly code unless it involves something complicated like printf, which I'd then call from assembly.

I'd like to learn how to emulate enums and structs if possible. AFAIK writing functions and sending them params is just a case of pushing/popping specific registers on and off the stack and/or manipulating esp using multiples of 4.

How would I do this in x86 using intel syntax?

Update

Sorry, I forgot to specify the target - I'm using Linux.

Example Code - Function prototype implementation omitted for sake of brevity

#include <stdio.h>
#include <stdlib.h>

typedef enum __weapon_type__ {

    weapon_type_sword = 1, 
    weapon_type_spear = 2, 
    weapon_type_knife = 3

} weapon_type;

typedef struct __weapon__ {

    unsigned int damage;
    char*        name;
    weapon_type  type;  

} weapon;

weapon* weapon_create( int damage, char* name, weapon_type type );

void putline( const char* msg );

int main( int argc, char** argv )
{
    unsigned int weapon_selection, weapon_damage;
    weapon_type weptype;
    weapon* player_weapon = NULL;
    char* weapon_name = NULL;

    putline( "Choose your weapon type:\t" );
    putline( "(1) Sword" );
    putline( "(2) Spear" );
    putline( "(3) Knife" );

    while ( weapon_selection > 3 || weapon_selection < 1 )
    {
        scanf( "%u", &weapon_selection );

        switch( weapon_selection )
        {
            case 1:
                weptype = weapon_type_sword;
                break;
            case 2: 
                weptype = weapon_type_spear;
                break;
            case 3:
                weptype = weapon_type_knife;
                break;
            default:
                putline( "ERROR! Please select options 1 - 3\t" );
                break;
        }
    }

    /*do the same thing for weapon_damage and weapon_name, etc.
      Then ask for player name, type of character, blah blah blah.
    */

    player_weapon = weapon_create( weapon_damage, weapon_name, weptype );

    return 0;
}

I thought of that, the only problem is it wouldn't necessarily cover the IO aspects of the code (unless I'm wrong here) in terms of the implementation; i.e., wouldn't it just be making extern calls to the c library? — zeboidlund, Sep 29 '12 at 05:12
IO is platform dependent - ultimately code will need to call OS function to do so - will be different depending on Linux/DOS/Windows/other target. Basically it looks like you need to learn OS functions to output text and how to call OS functions from Asm - specifying exact target will let someone answer the question. — Alexei Levenkov, Sep 29 '12 at 05:15

score 1 · Accepted Answer · edited May 23 '17 at 12:27

On Linux applications, all I/O is done thru syscalls (which from the application point of view, are elementary operations, usually implemented thru a mode-switching machine instruction like SYSENTER, SYSCALL, INT ...). I suggest reading the linux assembly howto.

How is assembly IO performed for text based stuff?

1 Answers1