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Guide: VGA mode 13h graphics

One of the most widely used PC graphics modes, and certainly the easiest to program, is the VGA mode 13h. This is sized 320x200, can display up to 256 colors at any one time, and will work on any VGA compatible graphics card (it isn't supported by the EGA or CGA boards, but very few people are running djgpp on such old hardware :-)

The first step is to select this mode, which is done by calling the VGA BIOS interrupt 0x10, eg:

   #include <dpmi.h>

   void set_mode_13h()
   {
      __dpmi_regs r;

      r.x.ax = 0x13;
      __dpmi_int(0x10, &r);
   }

The important thing here is the number 0x13, which is put into the AX register before calling the BIOS function. This value specifies want mode you want to set: no prizes for guessing where the name "mode 13h" came from!

Before quitting from your program, you should of course switch back into the normal DOS text mode, which can be done in exactly the same way as above but substituting 3 as the mode number, for example:

   void return_to_text_mode()
   {
      __dpmi_regs r;

      r.x.ax = 3;
      __dpmi_int(0x10, &r);
   }

After setting the video mode, the next step is to draw something onto the screen. The VGA memory is located at physical address 0xA0000, so you will need to use the <sys/farptr.h> or dosmemput() functions to access it: see the DPMI chapter for details of this. At the most basic level, a single pixel can be drawn onto the screen with the code:

   #include <go32.h>
   #include <sys/farptr.h>

   void putpixel_13h(int x, int y, int color)
   {
      _farpokeb(_dos_ds, 0xA0000+y*320+x, color);
   }

Displaying graphics as a sequence of single pixels tends to be very slow, but the same principle can be applied to more complex and useful shapes such as lines, rectangles, and circles. One useful optimization is to only call the _farsetsel(_dos_ds) function once at the start of your routine, and then use the faster _farns*() functions for the rest of the drawing. Using this method, a rectangle fill could be implemented as:

   void rectangle_13h(int x, int y, int w, int h, int color)
   {
      int i, j;

      _farsetsel(_dos_ds);

      for (j=y; j<y+h; j++)
	 for (i=x; i<x+w; i++)
	    _farnspokeb(0xA0000+j*320+i, color);
   }

Another handy technique is to write two or four pixels at the same time with the _farnspokew() or _farnspokel() functions: this can often give as much as a 4x speed improvement over single byte operations!

An alternative approach is to build up your entire picture in a memory buffer before copying it across to the screen. This can most easily be done with the dosmemput() function, for example:

   #include <sys/movedata.h>

   char framebuffer[320*200];
   int i;

   /* clear the framebuffer */
   memset(framebuffer, 0, sizeof(framebuffer));

   /* draw some diagonal lines */
   for (i=0; i<200; i++) {
      framebuffer[i*320+i] = i;
      framebuffer[i*320+i/2] = i;
      framebuffer[i*320+i/3] = i;
   }

   /* copy the buffer across to the screen */
   dosmemput(framebuffer, 320*200, 0xA0000);

So far these functions have just been drawing colors to the screen as numbers ranging from 0 to 255, but for a real program you will obviously need some way of knowing what color each these numbers represents. This is controlled by a hardware component called the palette, which is a table listing the actual color values for each of the 256 values that you can display. When you first select a video mode the first 16 entries in the palette (colors zero to fifteen) will be set to the standard DOS text mode colors (black, blue, green, cyan, red, magenta, brown, light grey, dark grey, pale blue, pale green, pale cyan, pale red, pale magenta, yellow, and white), but the other 240 colors may be set to different values depending on the machine. In order to use anything more than those 16 default colors, you must set the palette to some new values of your own, which is done by writing a palette index to hardware port 0x3C8 followed by three color values to port 0x3C9, eg:

   #include <pc.h>

   void set_color(int color, int red, int green, int blue)
   {
      outportb(0x3C8, color);
      outportb(0x3C9, red);
      outportb(0x3C9, green);
      outportb(0x3C9, blue);
   }

The red, green, and blue color values range from 0 up to 63, so for example calling set_color(10, 0, 0, 0) will change color number 10 to black, while set_color(10, 63, 63, 63) would change it to white, and set_color(10, 63, 40, 0) changes it to a shade of orange.

One nice thing about the palette hardware is that it can be used to alter the colors even after you have drawn something onto the screen, which can be a very handy way of doing fades and some types of animation. For example, color 1 is blue in the default palette, so if you drew lots of 1 pixels onto the screen it would be covered in blue dots. But if you then called set_color(1, 0, 63, 0), all those dots would instantly change to green! Apart from being a very easy way to make some of your graphics flash to a different color, this trick can be used to fade the entire display to or from a blank screen by gradually changing all 256 palette values to make them lighter or darker, and many interesting effects can be achieved by making more subtle alterations to your palette colors.

Before you alter any of the palette values, it is a good idea to synchronize your program with the vertical retrace. This is the period when the electron beam inside your monitor has reached the bottom of the screen and is moving back up ready to display another copy of the picture, and for this brief interval the graphics card gets a chance to rest because there is no need to send any pixel data to the monitor. The retrace happens 70 times a second, and most older cards will only allow you to alter the palette values at this time. You can change the colors whenever you like on many recent cards, and your program will still work if you do it at the wrong time on old hardware, but it might cause some static or "snow" to be visible on the screen, which is ugly and easy to avoid if you take care always to sync with the retrace first. The retrace period can be detected by checking bit 3 of port 0x3DA, eg:

   void vsync()
   {
      /* wait until any previous retrace has ended */
      do {
      } while (inportb(0x3DA) & 8);

      /* wait until a new retrace has just begun */
      do {
      } while (!(inportb(0x3DA) & 8));
   }

You should always call this function before you modify any palette colors, although it is possible to change many colors within a single retrace, so you will only need to call vsync() once even if you are making several calls to set_color() immediately after it.

That's it, folks! This should be everything you need to get your program up and running in the VGA 320x200 mode 13h, and the same techniques can be applied to other VGA modes like the 16 color and tweaked mode-X resolutions. All the djgpp-specific parts of programming in these other VGA modes are the same as for mode 13h, so you can combine the djgpp material from this document with hardware information from the many sources available on the net, and hopefully be able to access any VGA graphics mode that you like.


References:

x2ftp - ftp://x2ftp.oulu.fi/pub/msdos/programming/
The most comprehensive collection of DOS graphics coding material on the net.
PCGPE - ftp://x2ftp.oulu.fi/pub/msdos/programming/gpe/
A good clear introduction to many VGA and general graphics coding techniques, although all the example programs are in Pascal.
Abrash in DDJ - ftp://x2ftp.oulu.fi/pub/msdos/programming/docs/graphpro.lzh
The "legendary" articles written by Michael Abrash and published in DDJ between 91/93, covering mode-X, polygon rasterisation, and many other fascinating topics.

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