ANTIC - Software-driven Modes

Software-driven Modes

Due to the ANTIC/GTIA's flexibility, it is possible with clever programming to create a number of software-driven "pseudo-modes" beyond those directly supported in hardware. These modes use combinations of mixed display lists, scrolling, page flipping, and display list interrupts to achieve displays with alternate resolution and increased color that are not possible using the hardware alone. Some possibilities include pseudo-256-color displays, 80 character wide displays, and vertical interlace.

One difficulty with some of these modes is that the grating on PAL and NTSC televisions is very different, as is the update speed, so often the modes that display well on European systems display awfully on US systems, or vice versa. For the same reason the pseudo-modes may not display well – or display rather too well – on Atari 8-bit computer emulators. Also, modern LCD TVs used as monitors may not be able to display pseudo-modes correctly.

The display list interrupt facility can be used to display far more than the usual number of sprites on the screen, with more colours shown on screen than the available color registers in the ANTIC/GTIA. This is done by writing machine language routines to change the values of hardware registers at different vertical positions on the screen. The machine language routines modifying hardware registers allow the programmer to move the sprites around and change their colours 'on the fly'. The same technique can be employed to display far more colours than seemingly allowed in any particular graphics mode, which can be mixed and altered at will.

Because the screen memory can be accessed by two pointers and relocated anywhere in available memory, it is extremely easy to implement hardware scrolling and page flipping to enable easy game design and programming.

There are many examples of demo programs available on the internet, displaying these and other features. See also Software-driven graphics modes for the Atari 8-bit computers