ANTIC
Cyan’s main area of work during this period was the creation of two advanced LSI chips known as ANTIC and GTIA. Together, they formed the heart of Atari’s graphics. ANTIC was really a microprocessor which processed display instructions. A complete sequence of instructions was known as a Display List. Each instruction described how each line was to be displayed (character or graphics), where it was displayed, if it contained interrupts, if fine scrolling was enabled or not, or where to load data from memory (character sets or graphics information). ANTIC read this display list using DMA (Direct Memory Access), then translated this display list into electrical data for GTIA to process. All of this was without any CPU intervention.
CTIA
The Color Television Interface Adapter is the graphics chip used in early Atari 400/800 home computers. The chip is the successor to the TIA chip used in the Atari 2600. The chip was later replaced with the GTIA in later revisions of the 400 and 800, and in all other members of the Atari 8-bit family. In BASIC, type POKE 623,64 [RETURN] and if the screen blackens, you have the GTIA chip. If it stays blue, you have the old CTIA chip.
According to Joe Decuir, George McLeod designed the CTIA (Colleen TIA) in 1977. McLeod also designed the GTIA.
GTIA
GTIA receives graphics information from ANTIC and also controls sprites, collision detection, priority control and color-luminance (brightness) control to all objects including DMA objects from ANTIC. GTIA takes all this information and outputs them as separate digital luminance and chrominance signals, which are mixed to form an analogue composite video signal.
POKEY
The third custom support chip, named POKEY, was responsible for reading the keyboard, generating sound and serial communications (the latter in conjunction with the PIA). It also provided timers, a random number generator (for sound noise as well as random numbers), and maskable interrupts. POKEY has four semi-independent audio channels, each with its own frequency, noise and volume control. Each 8-bit channel had its own audio control register which selected the noise content and volume. For higher sound resolution (quality), two of the audio channels can be combined for more accurate sound (16-bit).
Atari had originally intended to port Microsoft BASIC to the machine, as had most other vendors, intending to supply it on an 8k ROM cartridge. However the existing 6502 version from Microsoft was 12k, and all of Atari’s attempts to pare it down failed. Eventually they farmed out the work to a local consulting firm, who recommended writing their own version from scratch, which was eventually delivered as ATARI BASIC.
The early machines: 400 and 800
Management identified two sweet spots for the new computers, a low-end version known as Candy, and a higher-end machine known as Colleen (rumored to be named after attractive Atari staff). The primary difference between the two models was marketing; Atari marketed Colleen as a computer, Candy as a game machine (or hybrid game console). Colleen would include slots for memory and ROM, a second 8k cartridge slot, monitor output and a full keyboard, while Candy used a plastic “membrane keyboard” and internal slots (not user upgradable). Both machines were built like tanks with huge internal aluminum shields, to meet FCC rules for TV signals emitted in RF space (Part 15 Type I). Apple machines, without a RF modulator, didn’t need to meet those requirements (the first model of the TRS-80 actually never met that FCC spec).
Atari 400
Atari 800The machines were announced in December 1978 as the 400 and 800, although they weren’t widely available until November 1979. The names originally referred to the amount of memory, 4K RAM in the 400 and 8K in the 800. However by the time they were released the prices on RAM had started to fall, so the machines were instead released with 8K and 16K respectively.
Due to new FCC restrictions, the 400/800 couldn’t allow slots like those found on the Apple II computers. Instead, they created a proprietary, expensive serial-based interface called SIO (Serial Input/Output). All external devices were connected using this interface (cassette drive, disk drive, interface box) adding to the cost of ownership. On the 800, the internal slots were reserved for ROM and memory modules. Originally the machines shipped with 16k, but as prices continued to fall Atari eventually supplied the machines fully expanded to 48k, using up all the slots.
A few companies made RAM modules for the Atari 800. One company made a module which added an 80-column display system and 16k of RAM so you could remove one of the Atari-supplied cards without losing memory.
Overheating problems with the memory modules eventually led Atari to remove the casings on the memory modules, leaving them as “bare” boards. Later, the expansion cover was held down with screws instead of latches.
The Atari 800 sold respectably, but not nearly as well as the Apple machines. The crippling of the 400 only confused buyers and a replacement was in the works (the so called Sweet-8 or “Liz NY”).
