Games - Atari 8-bit family

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	Games - Atari 8-bit family

Atari built a series of 8-bit home computers based on the MOS Technology 6502 CPU, starting in 1979. Over the next decade several versions of the same basic design would be released. These included the original Atari 400 and 800, and their successors, the XL and XE series of computers. However, the models remained largely identical internally. They were the first home computers designed with custom coprocessor chips. IBM even considered licensing Atari for their own personal computer, but decided to build their own. However, design flaws, internal corporate turmoil and difficult, fast-changing market conditions contributed to the 8-bit Atari computers' eventual demise.

History

Origins

As soon as the Atari 2600 was released the engineering team, calling themselves Cyan, started work on its eventual replacement. They felt that the 2600 would have about a three year lifespan, and tried to limit themselves to those features that could be perfected by that time. What they ended up with was essentially a "corrected" version of the 2600, fixing its more obvious flaws.

The newer design would be faster than the 2600, have better graphics, and include much better sound hardware. Work on the chips for the new system continued throughout 1978, primarily focusing on the much-improved video hardware known as the CTIA (the 2600 used a chip known as the TIA).

However, at this point, the home computer revolution took off in the form of the Apple II family, Commodore PET and TRS-80. Ray Kassar, the new CEO of Atari, wanted the new chips to be used in a home computer to challenge Apple. Atari researched on what would be needed to produce a workable home computer of their own. This included support for character graphics (something the 2600 didn't support), some form of expansion for peripherals, the BASIC programming language, and a keyboard.

Design

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 describe 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 without any CPU intervention.

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 does a DAC (Digital to Analog Conversion) to video signals for display.

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.

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