Coding games for ELKS

Several text (ASCII art) based games are available for ELKS

• ELKSmoria port of Moria 1983
• ELKSAdvent port of Advent
• ttytetris
• ttypong

There are 3 ways you can program graphical games on ELKS

1. Using x-nano

There are several VGA demos based on X-nano shipped with ELKS. There are at least two games available:

• nxtetris
• landmine

More code samples are available in: /elkscmd/nano-X/demos

2. Using direct access to the VGA memory

The following code switches to VGA mode 0x13, draws two lines, waits 3 seconds and then it switches back to text mode. It can be compiled with ../cross/bin/ia16-elf-gcc ./vgatest.c -o vgatest -melks-libc -mcmodel=small. Start it with ./vgatest. It uses the ia16-elf-gcc compiler used to compile the entire ELKS project.

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

#define VGA_256_COLOR_MODE  0x13      /* use to set 256-color mode. */
#define TEXT_MODE           0x03      /* use to set 80x25 text mode. */

#define SCREEN_WIDTH        320       /* width in pixels of mode 0x13 */
#define SCREEN_HEIGHT       200       /* height in pixels of mode 0x13 */
#define NUM_COLORS          256       /* number of colors in mode 0x13 */

#define sgn(x) ((x<0)?-1:((x>0)?1:0)) /* macro to return the sign of a
                                         number */
typedef unsigned char  byte;
typedef unsigned short word;

byte __far *VGA=(byte __far *)0xA0000000L;        /* this points to video VGA memory. */

/**************************************************************************
 *  set_mode                                                              *
 *     Sets the video mode.                                               *
 **************************************************************************/

void set_mode(byte mode)
{
   // SI, DI, BP, ES and probably DS are to be saved
   // cli and sti are used to make a proper BIOS call from ELKS
   __asm__(
  "push %%si;"
  "push %%di;"
  "push %%bp;"
  "push %%es;"
  "cli;"
  "mov %%ah,0;" 
  "mov %%al,%0;" 
  "int $0x10;"
  "sti;"
  "pop %%es;"
  "pop %%bp;"
  "pop %%di;"
  "pop %%si;"
     : /* no outputs */
     : "r" (mode)
     : ); //list of modified registers
}

/**************************************************************************
 *  plot_pixel                                                            *
 *    Plot a pixel by directly writing to video memory, with no           *
 *    multiplication.                                                     *
 **************************************************************************/

void plot_pixel(int x,int y,byte color)
{
  /*  y*320 = y*256 + y*64 = y*2^8 + y*2^6   */
  VGA[(y<<8)+(y<<6)+x]=color;
}

int main()
{
  set_mode(VGA_256_COLOR_MODE);       /* set the video mode to 256 colors 320 x 200 */
								 
  for (int i=0;i<60;i++)
  	plot_pixel(100+i,100,5);

  for (int i=0;i<60;i++)
        plot_pixel(100,100+i,0xA);

  sleep(3);
  
  set_mode(TEXT_MODE);                /* set the video mode back to text mode. */

  return 0;
}

You can adapt code from David Brackeen's VGA tutorial and draw whatever you need on the screen.

3. Using Basic

Currently graphics routines are available only on the PC-98 platform, but not on a standard IBM PC compatible.

Discussion

More on programming games for ELKS is here: https://github.com/ghaerr/elks/issues/871