/**************************************************************************
* pixel.c *
* written by David Brackeen *
* http://www.brackeen.com/home/vga/ *
* *
* This is a 16-bit program. *
* Tab stops are set to 2. *
* Remember to compile in the LARGE memory model! *
* To compile in Borland C: bcc -ml pixel.c *
* *
* This program will only work on DOS- or Windows-based systems with a *
* VGA, SuperVGA or compatible video adapter. *
* *
* Please feel free to copy this source code. *
* *
* DESCRIPTION: This program demostrates how much faster writing directly *
* to video memory is. *
**************************************************************************/
#include <stdio.h>
#include <stdlib.h>
#include <dos.h>
#define VIDEO_INT 0x10 /* the BIOS video interrupt. */
#define WRITE_DOT 0x0C /* BIOS func to plot a pixel. */
#define SET_MODE 0x00 /* BIOS func to set the video mode. */
#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 */
typedef unsigned char byte;
typedef unsigned short word;
byte *VGA=(byte *)0xA0000000L; /* this points to video memory. */
word *my_clock=(word *)0x0000046C; /* this points to the 18.2hz system
clock. */
/**************************************************************************
* set_mode *
* Sets the video mode. *
**************************************************************************/
void set_mode(byte mode)
{
union REGS regs;
regs.h.ah = SET_MODE;
regs.h.al = mode;
int86(VIDEO_INT, ®s, ®s);
}
/**************************************************************************
* plot_pixel_slow *
* Plot a pixel by using BIOS function 0x0C (Write Dot). *
**************************************************************************/
void plot_pixel_slow(int x,int y,byte color)
{
union REGS regs;
regs.h.ah = WRITE_DOT;
regs.h.al = color;
regs.x.cx = x;
regs.x.dx = y;
int86(VIDEO_INT, ®s, ®s);
}
/**************************************************************************
* plot_pixel_fast *
* Plot a pixel by directly writing to video memory. *
**************************************************************************/
void plot_pixel_fast(int x,int y,byte color)
{
VGA[y*SCREEN_WIDTH+x]=color;
}
/**************************************************************************
* Main *
* Plots 50000 pixels two different ways: using the BIOS and by *
* directly writing to video memory. *
**************************************************************************/
void main()
{
int x,y,color;
float t1,t2;
word i,start;
srand(*my_clock); /* seed the number generator. */
set_mode(VGA_256_COLOR_MODE); /* set the video mode. */
start=*my_clock; /* record the starting time. */
for(i=0;i<50000L;i++) /* randomly plot 50000 pixels. */
{
x=rand()%SCREEN_WIDTH;
y=rand()%SCREEN_HEIGHT;
color=rand()%NUM_COLORS;
plot_pixel_slow(x,y,color);
}
t1=(*my_clock-start)/18.2; /* calculate how long it took. */
set_mode(VGA_256_COLOR_MODE); /* set the video mode again in order
to clear the screen. */
start=*my_clock; /* record the starting time. */
for(i=0;i<50000L;i++) /* randomly plot 50000 pixels. */
{
x=rand()%SCREEN_WIDTH;
y=rand()%SCREEN_HEIGHT;
color=rand()%NUM_COLORS;
plot_pixel_fast(x,y,color);
}
t2=(*my_clock-start)/18.2; /* calculate how long it took. */
set_mode(TEXT_MODE); /* set the video mode back to
text mode. */
/* output the results... */
printf("Slow pixel plotting took %f seconds.\n",t1);
printf("Fast pixel plotting took %f seconds.\n",t2);
if (t2 != 0) printf("Fast pixel plotting was %f times faster.\n",t1/t2);
return;
}
