DOS EGA Graphics Programming in Mode 0Dh

Question

I'm doing a bit of retro programming for fun. What I want to create is a DOS game using EGA graphics but I am having a bit of trouble finding a good reference on the web. Everybody who talks about doing DOS programming assumes that the programmer will use mode 13h, and although some pages mention the other graphics modes, I haven't found one yet that discusses their proper use.

Here's what I'm trying to get working right now:

//------------------------------------------------------------------------------
//  DOS graphics test
//
//  Thanks to the following links:
//    http://gamebub.com/cpp_graphics.php
//
//  Written for Digital Mars C compiler to be compiled as a DOS 16 bit binary.
//------------------------------------------------------------------------------

#include <dos.h>
#include <stdio.h>

#define SCREEN_WIDTH  320;
#define SCREEN_HEIGHT 200;

unsigned char far *vram = (unsigned char far *)0xA0000000L;

//------------------------------------------------------------------------------
void set_video_mode(unsigned char mode)
{
    union REGS in, out;
    in.h.ah = 0;
    in.h.al = mode;
    int86(0x10, &in, &out);
}

//------------------------------------------------------------------------------
void plot_pixel(unsigned int x, unsigned int y, unsigned char color)
{
    // this is wrong because it's only 4 bpp not 8
    vram[y * 320 + x] = color;
    //vram[((y<<8)+(y<<6))+x] = color;
}

//------------------------------------------------------------------------------
int main(int argc, char* argv[])
{
    // EGA 320 x 200 x 16
    set_video_mode(0x0d);

    for (unsigned char i = 0; i < 255; i++)
    {
        vram[i] = i;
    }

    //plot_pixel(10, 10, 1);

    getc(stdin);
    return 0;
}

This sample code works great if you change set_video_mode() to take 0x13 instead of 0x0d, but like I said I'm trying to get EGA graphic here, not VGA. :) I realize that in order to do four bits-per-pixel I'm going to either need to assume that plot_pixel writes two pixels at the same time or do a some bit twiddling to make sure I only write the four bits that I actually want.

My problem is that I'm not seeing what I expect as output--in particular, no colors! Everything seems to be mono colored, which is not what I want at all. Is there some kind of different procedure for employing a color palette in this graphics mode than in 13h? Or have I somehow invoked a completely different graphics mode from the one I intended? Guidance would be very much appreciated.

I don't think my compiler args would be relevant, but just in case:

..\dm\bin\dmc test.c -o test -mm

Answer 1

Mode 0x0d is planar based while 0x13 is not (without special configuration as Mode X). You should check more document to see how to work in planar mode.