Re: Mode 13h

["Mike Main" <anime6@earthlink.net>]

The best way I found to plot pixels is to use an unsigned char far pointer to the memory address 0xA0000000L, which is the begginning memory address for the video memory in mode 13h.  So to plot a pixel all you need to do is after you put the video card in mode 13h:

 

/*Assign this somewhere as a global declaration*/

unsigned char far *video_buffer=0xA0000000L;

 

/*Call this function to plot a pixel*/

void Write_Pixel(int x, int y, int nColor)

{

    video_buffer[(y<<8)+(y<<6)+x]=(unsigned char)nColor;

}

 

The video memory must have colors recorded in unsigned characters, because it's the only thing the video card understands.  It's like machine code.  And then the reason for the weird looking "(y<<8)+(y<<6)+x" for the coordinates is because mode 13h is a chained memory mode.  So it's just a straight line.  So in order to access an x,y coordinate, you need to multiply the y coordinate by 320 (the horizontal resolution) and then add the x.  Just think about it, you'll figure it out.  Also the reason for the binary shift operators is because it is faster than actual multiplication.  2^8 + 2^6 is 320, so shifting the binary of y this way is the same as multiplying by 320.  I hope this helps.