Tuto Hello World

Hello World

Whenever you use Code::Block, Eclipse, QtCreator or any other source code editor, you need your main function.

int main(u16 hard)
{
    return 0;
}

As any C program this function is your starting point and is automatically called on start up. Note that with SGDK, your main function receive a single parameter named hard which allow to know if you are on a hard reset (hard = 1 mean that we just turned the system ON) or on a soft reset (hard = 0 mean that reset button was pressed).

Before calling main method, SGDK initializes several things for you:

• clear RAM and initialize variables
• setup the VDP with a default configuration (64x32 plane size, enable Vint, setup table addresses... see VDP registers doc)
• clear video RAM
• load 4 default palettes: grey, red, green and blue
• load a default font for your text print need
• init input handling
• reset sound system

So, for basic stuff and testing, everything is ready to use :)

Now if you want to use VDP, input, sprite or whatever function from SGDK, you first need to include the genesis.h header. It will let you access to the whole SGDK functions !

Let's see that by replicating the infamous Hello World example :)

Printing on screen is a VDP task so you need to call VDP_drawText(<your_text>, <x>, <y>). Don't forget to use the doxygen documentation (in doc folder) when you are looking for help on a specific function, SGDK organizes its functions per unit, for instance SYS_xx methods are declared in sys.h unit, JOY_xx in joy.h unit... Ok so if you correctly read the VDP_drawText(<your_text>, <x>, <y>) documentation you know that x and y parameters are in tile unit with 1 tile = 8 pixels.

#include <genesis.h>

int main(u16 hard)
{
    VDP_drawText("Hello World!", 10, 13);

    return 0;
}

This code is correct but not really Genesis friendly: do you think we can return (and stop) a program ? Video games repeatedly update the TV screen, and it's up to you to handle things before or while a refresh.

So a more Genny Hello World is more like this:

#include <genesis.h>

int main(u16 hard)
{
    VDP_drawText("Hello Genny World!", 10, 13);

    while(TRUE)
    {
        // read input
        // move sprite
        // update score
        // draw current screen (logo, start screen, settings, game, gameover, credits...)
    }

    return 0;
}

This code is correct, even if return 0; will never be reached. The code inside the while loop will execute in full speed...great! but do you think the Genesis will handle so much ? The TV screen is updated 50 (PAL) or 60 (NTSC) times per second...you have to be sync with this. A useful way is to wait for screen update, or vertical synchronization.

SGDK is here to help you with the VDP_waitVSync function !

So, this time, you could write your first Genesis fully compliant demo:

#include <genesis.h>

int main(u16 hard)
{
    VDP_drawText("Hello Genny World!", 10, 13);

    while(TRUE)
    {
        // read input
        // move sprite
        // update score
        // draw current screen (logo, start screen, settings, game, gameover, credits...)

        // wait for screen refresh
        VDP_waitVSync();
    }

    return 0;
}

Congratulations ! You're now ready to make a little more :)

Download: Sample Hello World project