The VGA card - basics -

# The VGA card

I do not know a huge amount about the VGA card. I have spent my life writing graphics code for one screen mode, and I have done a small amount in SuperVGA, so this information will only get you started, and you'll have to go and get a big book of you really want to know how to fiddle with it.

## Graphics Mode 13h

This is everyone's favourite graphics mode. It's extremely easy to use. Takes a couple of seconds to learn how it all works. Right here goes

The screen resolution is 320 pixels by 200 pixels. Therefore there are 64000 pixels visible on the screen. Each one of these pixels is capable of showing one of 262144 different colours. However, you may only ever have 256 different colours visible on screen at once. I shall explain all in a sec.

Every pixel on the screen is represented by one byte in the PC's memory. If you change the value of that byte, the colour of the corresponding pixel will change immediately. Writing a pixel to the screen, therefore, is simply a matter of working out which byte you need to change, and changing it.

The bytes are arranged in memory in order. They are arranged in the same order that you read letters from a page. The top-left pixel comes first, followed by the one to its right . . . and the next few . . . then the top-right pixel, then the one below the top-left one. . . and finally the bottom-right pixel.

So the pixels on the first line of the screen are numbered 0 to 319. The pixels on the next line are numbered 320 to 639. etc.

Understand that? Yer it's simple isn't it? OK, so here's a little bit of maths to convert a pair of (x, y) co-ordinates to a byte location:

#### offset = (Y * 320) + X

Of course, if you're programming in real mode, you'll need to know the segment all this is taking place in. The segment is:

#### A000h

Now you have all the information you need to write pixels to the VGA card in mode 13h. Next thing you'll need to know is how to mix your own colours.

## Palette Mixing

Because each pixel is represented by one byte, that gives us a maximum of 256 different colours which can be displayed on screen at once. Exactly which colours these are depends on the palette. You can choose any 256 colours from a selection of 262144. You choose the colours by literally mixing red, green, and blue light.

For each of the 256 colour values, you choose how much red, green, and blue light you want the screen to give out. By mixing these colours in various amounts, you can create almost all the colours the eye can see.

#### Note:

This is not quite the same as mixing paints. Mixing red and blue paint will produce purple, and the same applies for light. However, mixing yellow and blue paint produces green, but mixing red and green light produces yellow. Mixing all three colours of paint produces almost black, but all three colours of light gives white.

#### Getting the colours you want

There are various ways of assigning Red, Green, Blue values to a colour number. There is an interrupt to do it, but interrupts are slow, and there is a much faster way to acheive it. The VGA card has two ports for choosing colours. They are numbered 968 and 969. The way to mix your colours is this:

send a byte to port 968. This is the pixel value you want to assign red, green and blue values to. Next send 3 bytes to port 969. First the red, then the green then the blue value. The range of colour you are allowed is 0 to 63.

#### for example: send 15 to port 968 send 63 to port 969 send 40 to port 969 send 0 to port 969

This will assign orange to colour number 15. So every time you plot a pixel with value 15 to the screen it will be orange.

now, if you do this:

#### send 15 to port 968 send 30 to port 969 send 30 to port 969 send 63 to port 969

All those orange pixels will turn light blue.

And there you have it. Nothing could be simpler.