Task: Try GTIA Graphics Modes in BASIC
Time: 30-60 mins
When the Atari 400 and 800 first shipped they included the Color Television Interface Adaptor or CTIA chip that creates the signal that goes to the television for display. The CTIA is controlled by the ANTIC microprocessor that includes an instruction set and a program called the display list that dictates what graphics modes will be used for each line on the screen. The display list and the information that will be drawn on the screen are written to memory and then the ANTIC sends that to the CTIA for display. This is all described on page 1-1 of the wonderful book De Re Atari.
A second chip called the Graphic Television Interface Adaptor or GTIA had the same functionality of the CTIA but with several additional graphics modes. It wasn’t quite ready for the initial shipments of Atari 400 and 800 computers but was introduced in the early 1980s such that nearly all the 8-bit computers you come across today have the more advanced GTIA chip. I recall reading about the GTIA in one of the computer magazines back in the day and wondering whether my Atari had the newer chip. I tested this by typing in some BASIC programs that called on the GTIA’s extra graphics modes. Indeed, my Atari had the GTIA and I was impressed with the additional capabilities. The goal of this project is to try some BASIC programs that use the extra GTIA modes.
Here is a text file with the Brass GTIA demo from The Creative Atari book by Small et al. (1983). Simply copy this text to your clipboard and then paste it into the Altirra emulator running BASIC using the View tab and then paste text option at the bottom of that menu.
To load BASIC in Altirra select the File menu and then choose Attach Special Cartridge and then BASIC at the bottom of that list. Alternatively you could load one of the many BASIC ATR files that are out there.
I chose this demo because it is short enough to type in on your original hardware. You can find other demos in the GTIA chapter of The Creative Atari book. There are easy enough to cut and paste. Be sure to add a blank line between each code line to make the paste work in Altirra.
The GTIA effects are impressive because they have mode shades of one color allowing for more effective 3-D effects like the Brass demo above. These extra graphics modes are fun to play with and were used in some games. Here is an Atari Age forum post that discusses this. Here is an article in Compute! from 1982 talking about the new GTIA chips. Here is a GTIA article from 1983 in ANTIC magazine. This last article includes a nice demo you can try.