Atari Graphics

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Understanding Atari Graphics

The Atari personal computers were equipped with utilizing graphics because of a special chip known as POKEY. Later versions required the GTIA chip to obtain Graphics in a higher resolution. The original systems only came shipped with the CTIA chip which limited the user to Graphics modes 0 through 8. However with the GTIA chip, the Atari can enter Graphics modes 9 through 16. Graphics modes 1-16 contained a text window that could enter text by the user. Adding 16 to any GRAPHICS statement, such as GRAPHICS 1+16 would remove the text window in that mode. This provided the user with a full screen Graphics mode.

Graphics Mode 0

Graphics mode 0 is the default Graphics mode for the Atari computers. So when you turn on your system this is the first screen you will get. It contains a width of 40 characters and a height of 24 characters. This mode only accepts asci (pronounced "askee") characters. This is essentially any of the characters you can type on your computer including the control characters, and the escape characters. This mode was popular back in the day for text adventures (such as from Infocom) and word processors. It also contains no text window.

Graphics Mode 1

Graphics mode 1 is popular for creating larger text displays that work because of the ANTIC chip in the computer. The text is twice as wide as those in Graphics mode 0. We will see an example here shortly. Also notice that is has it’s own text window. The text window is actually Graphics 0 since the Atari is using a display list to point to that part of memory.
These can create some nice graphics examples and games since they produce a nice contrast on a black foreground. Possibly later we will see some animations to show the power of this mode. The resolution has a display of 20 characters wide and 20 characters in height.

Graphics Mode 1 example

Now we will now observe an example of text displayed in Graphics 1. Notice that there are 3 different colors. Graphics mode 1 can utilize these special characters by accessing the display lists built into the Atari computer.

Code Revealed

These characters are displayed on the screen in multiple ways. First you can utilize the code below to plot these on the screen according to the individual colors.
Keep in mind that to increase the color each time you must add a 33 to the number to get orange (COLOR 65), 64 to the number to get blue (COLOR 193), and 128 to the number to get red (COLOR 225). An example is seen here.

Atari Final Example

Secondly you can use PRINT #6;CHR$(value) to see them displayed on the screen. This time we are using the POSITION command instead of PLOT to get them placed on the screen in order.

Please note: it is necessary to use POSITION x, y before the PRINT #6 so they can be positioned correctly.