The Atari 8-bit computers, officially known as the Atari Home Computer System, were a series of home computers introduced by Atari, Inc. in 1979 with the Atari 400 and Atari 800 models. These computers used an 8-bit MOS Technology 6502 CPU and three custom coprocessors to support features like moving images (sprites), smooth scrolling, four audio channels, and other capabilities. Their graphics and sound were more advanced than most computers of the same time, and video games were a major part of their software collection. The 1980 game Star Raiders, a first-person space combat simulator, was considered the most important application for the system.

The Atari 800 was the more expensive model, while the Atari 400 was designed to be more affordable. The 400 had a spillproof keyboard that responded to pressure and initially came with 8 KB of non-upgradable RAM. The 800 had a standard keyboard, an extra cartridge slot, and allowed users to upgrade RAM to 48 KB. Both models used the same 6502 CPU running at 1.79 MHz (1.77 MHz for PAL versions) and the same coprocessors: ANTIC, POKEY, and CTIA/GTIA. Peripherals connected through the Atari SIO serial bus, and one of the SIO developers later helped create the USB (Universal Serial Bus). The design of the Atari 8-bit computers was later used in the 1982 Atari 5200 game console, but games for the two systems were not compatible.

The Atari 400 and 800 were eventually replaced by other computers with the same core technology but different designs. The 1200XL was released in early 1983 to replace the 800 but was discontinued in June 1983. Its design was later used for the 600XL and the popular 800XL, released later the same year. After Atari was sold and reorganized, Atari Corporation released the 65XE (called the 800XE in some European markets) and 130XE in 1985. These models were lighter, had two joystick ports instead of four, and included Atari BASIC. The 130XE had 128 KB of bank-switched RAM. In 1987, after the Nintendo Entertainment System revived the console market, Atari Corporation released the Atari XEGS, a version of the 65XE packaged as a game console with an optional keyboard. It could run 8-bit computer software and use computer peripherals.

Atari 8-bit computers were sold in computer stores and department stores like Sears, often using animated demonstrations to attract buyers. About two million Atari 8-bit computers were sold between late 1979 and mid-1985. The main global competitor was the Commodore 64, which was released in August 1982 and had similar features. On January 1, 1992, Atari Corporation officially ended all support for the 8-bit line.

History

The design of the "Home Computer System" began at Atari soon after the release of the Atari Video Computer System (VCS) in late 1977. When engineers at Atari Grass Valley Research Center (originally Cyan Engineering) worked on the VCS in 1976, they estimated the system would be outdated in about three years. To prepare for a replacement, they started planning a new console to be ready around 1979.

This new system was a major upgrade from the VCS, fixing its main weaknesses while keeping similar design ideas. It had better speed, graphics, and sound. Engineers worked on the new system’s chips throughout 1978, focusing on a much-improved video coprocessor called the CTIA (the VCS used a chip called the TIA).

In 1976, Nolan Bushnell sold Atari to Warner Communications for $28 million to fund the VCS launch. In 1978, Warner hired Ray Kassar as Atari’s CEO. Kassar wanted the new system’s chipset to compete with Apple, so it needed character graphics, support for adding external devices, and the ability to run BASIC programming language.

Atari engineer Jay Miner designed the display system for the Atari 8-bit computer using two chips. The CTIA chip handled moving images (sprites) and background graphics. To reduce the main computer’s workload, a separate microprocessor called the Alphanumeric Television Interface Controller (ANTIC) managed video settings and memory. CTIA and ANTIC worked together to create the final video output. ANTIC retrieved data from memory and sent it to CTIA, which processed the information for display.

The new system was much more advanced than other computers available at the time. Commodore was also developing a video driver, but Chuck Peddle, who designed the 6502 CPU used in the VCS and new systems, saw Atari’s work during a visit. He realized Commodore’s design would not compete, but he could not share this information due to a confidentiality agreement. Peddle later said, “What Jay did, just kicked everybody’s butt.”

Atari planned two versions of the new computers: a basic model called “Candy” and a more advanced model called “Colleen” (named after two Atari employees). Candy would be marketed as a game console or hybrid, while Colleen would be sold as a computer. Colleen included expansion slots for memory, two 8 KB ROM cartridge slots, and a full keyboard. Candy was initially designed without a keyboard but later included a spill-resistant keyboard for children. Both models had a separate audio port for cassette tapes.

A key goal was to make the computers easy to use. One executive said, “The end user doesn’t care about the machine’s architecture. They only care about what it can do for them.” Cartridges were used to simplify access to software, and memory modules had keyed connectors to prevent incorrect installation. The operating system automatically loaded drivers from devices connected via the serial bus (SIO). The disk system used menus for managing floppy disks, and the OS opened to a built-in editor called “Memo Pad” when no software was loaded.

As development continued, questions arose about Candy’s design, including whether the keyboard should be built-in or external. By 1978, education became a focus for the systems. Colleen’s design was nearly complete by May 1978, but by early 1979, Candy was also planned as a full computer for children.

Atari aimed to include Microsoft BASIC as an 8 KB ROM cartridge. However, the existing 6502 version was too large, so Atari hired Shepardson Microsystems to create a new version, resulting in Atari BASIC.

Television sets at the time had only one input (antenna). Computers needed an RF modulator to convert video signals into a format compatible with TVs. Poorly designed modulators caused interference, leading the Federal Communications Commission (FCC) to set strict testing standards.

To meet FCC rules, both new computers used cast aluminum shields to block radio waves, forming a partial Faraday cage. This made the computers sturdy but increased manufacturing costs. FCC rules also limited case openings to prevent RF leakage, eliminating expansion slots. Instead, Atari used the Serial Input/Output (SIO) bus to connect devices through a single shielded port.

In December 1978, Atari announced its entry into the home computer market. The Atari 400 and 800 were shown at the Winter Consumer Electronics Show (CES) in January 1979 and released in November 1979.

The names “400” and “800” originally referred to their 4 KB and 8 KB of RAM, but by release, RAM prices had dropped, so both models had 8 KB. Later models included 16 KB and 48 KB of RAM. The Atari 400 had a membrane keyboard and outsold the 800 by a 2-to-1 ratio. The 800’s second ROM slot was rarely used, and later models had only one slot.

Design

The Atari machines use a 6502 processor as their main brain, along with ANTIC and GTIA chips to create graphics, and the POKEY chip to handle sound and input. These helper chips are controlled through special memory spots that users can change using instructions from the 6502. For example, the GTIA uses these memory spots to choose screen colors. Users can change the colors by entering the correct numbers into these spots, which are connected to the 6502’s memory system. Some helper chips, like ANTIC, use data stored in RAM, such as the display buffer and display list, and GTIA uses data for Player/Missile graphics (sprites).

Atari spent about $10 million to create its three custom chips. These chips allow functions like smooth background scrolling to be done directly in hardware, which would usually require software on other systems. Atari used computers in stores to show graphics and sound demos as part of its early marketing.

ANTIC is a microprocessor that processes a list of instructions called a display list. Each instruction adds one row of graphics to the screen. The type of graphics depends on whether it is text or a bitmap, the resolution, number of colors, and height in scan lines. Each instruction also tells if an interrupt is needed, if fine scrolling is enabled, and where to get display data from memory.

Because each row can be set individually, programmers can create screens with different text or graphics modes. The data for these modes can come from any memory address, not just sequential ones.

ANTIC reads the display list and display data using DMA (Direct Memory Access), then turns it into a pixel stream for the screen. This stream goes to GTIA, which adds colors and includes Player/Missile graphics for the final image on a TV or monitor. Once the display list is set, the screen is created without the main processor’s help.

There are 15 text and bitmap modes. In low-resolution modes, 2 or 4 colors can be used per line. In high-resolution mode, one color is used per line, but the brightness of the foreground and background can be adjusted. High-resolution mode (320×192 pixels) creates NTSC composite artifact colors, which are not available on PAL systems.

For text modes, a register points to the character set data. This data is usually stored in ROM, but if stored in RAM, programmers can create custom characters. The data can be placed on any 1K or 512-byte memory boundary. Other registers can flip characters upside down or toggle inverse video.

ANTIC supports different Playfield modes and widths. The original Atari Operating System provided access to some graphics modes through Atari BASIC’s "GRAPHICS" command and other languages. Later versions of the OS added support for more modes.

ANTIC text modes allow soft, customizable character sets. There are four ways ANTIC renders text: Normal, Descenders, Single color character matrix, and Multiple colors per character matrix.

ANTIC uses the display list and settings to create these modes. Any graphics mode in the CTIA/GTIA color system can be mixed freely by changing instructions in the display list.

The ANTIC screen size is not fixed. The hardware can display a narrow Playfield (128 color clocks/256 hi-res pixels wide), a normal Playfield (160 color clocks/320 hi-res pixels wide), or a wide Playfield (192 color clocks/384 hi-res pixels wide) by changing a register. The default screen height is 192 scan lines, but a custom display list can extend it to 240 scan lines.

The display list allows for horizontal and vertical scrolling with minimal help from the main processor. ANTIC also supports fine scrolling, shifting the screen by single pixels horizontally and single scan lines vertically.

The main processor and video hardware work together using half the NTSC clock frequency. This ensures pixel colors are consistent across the screen, allowing smooth horizontal scrolling without color changes.

The Color Television Interface Adaptor (CTIA) was the original graphics chip in the Atari 400 and 800. It replaced the TIA chip from the 1977 Atari VCS. CTIA was later replaced by the GTIA chip, which added more color options. CTIA/GTIA applies colors to pixels using a 128 or 256-color palette and controls Player/Missile graphics, including collision detection and display priority. It also sends digital signals for color and brightness, which are mixed into an analog video signal.

CTIA/GTIA reads joystick inputs and keyboard keys, and controls the keyboard speaker in the Atari 400 and 800. Later models combined the keyboard speaker sound with audio output for the TV.

POKEY is a chip that reads the keyboard, paddles, and controls sound, serial communication, and joystick movements. It has four audio channels with separate volume and noise settings. Two channels can be combined for better sound quality. The name "POKEY" comes from "potentiometer" (used in paddles) and "keyboard." POKEY was used in many Atari arcade games, like Centipede and Asteroids.

Atari released three updated versions of the 400/800 with the same hardware but different designs: the 1200XL, 600XL, and 800XL. Many projects to improve the 8-bit line were canceled.

Peripherals

During the time the 8-bit series was available, Atari introduced many different accessories, such as cassette tape drives, 5.25-inch floppy drives, printers, modems, a touch tablet, and an 80-column display module.

These accessories connected to Atari computers through a special port called the Atari SIO port. This port allowed devices to be linked together in a line, which made it easier to use multiple peripherals at once. A major focus of Atari’s computer design was to make the system simple for users to operate. Because all devices used the same type of connector, beginners could easily add new equipment to their computers. The Atari SIO system included an early version of a feature called "plug-n-play," where each peripheral had a unique identifier and could send special instructions to the computer when it started up. However, the extra parts inside these accessories made them more expensive than similar devices used by other computers from the same time period.

Software

Atari did not share technical details about its computers at first, except with software developers who promised to keep the information private. This may have been done to help increase sales of Atari’s own software. Early on, cartridge software was very rare. In 1980, a magazine called InfoWorld joked that Atari owners might think about using the cartridge slot as an ashtray. The magazine encouraged owners to try Atari’s Star Raiders, which became a very popular program that helped people decide to buy the computer.

Chris Crawford and others at Atari published detailed technical information in a book called De Re Atari. In 1982, Atari released the Atari Home Computer System Hardware Manual and a version of its operating system with notes. These resources led to many books and articles about programming the computer’s special hardware.

Because of better graphics than the Apple II and marketing focused on home use, games were the most common type of software for Atari computers. A 1984 review collection used 198 pages for games and 167 pages for all other software combined.

The Atari 8-bit computers have an operating system stored in a special type of memory called ROM. The Atari 400 and 800 had two versions of the operating system:

• OS Rev. A – 10 KB ROM (3 chips) used in early models
• OS Rev. B – 10 KB ROM (3 chips) used in most models

The XL/XE models had different operating system versions, which sometimes caused problems with certain software. Atari provided a solution called the Translator Disk, a floppy disk that allowed the XL/XE computers to use the older 400 and 800 Rev. A or B operating systems.

• OS Rev. 10 – 16 KB ROM (2 chips) used in the 1200XL Rev A
• OS Rev. 11 – 16 KB ROM (2 chips) used in the 1200XL Rev B (fixed some issues)
• OS Rev. 1 – 16 KB ROM used in the 600XL
• OS Rev. 2 – 16 KB ROM used in the 800XL
• OS Rev. 3 – 16 KB ROM used in the 800XE/130XE
• OS Rev. 4 – 32 KB ROM (16 KB OS + 8 KB BASIC + 8 KB Missile Command) used in the XEGS

Later XL/XE models included the Atari BASIC ROM, which could be turned off during startup by pressing the silver OPTION key. The first version of this ROM had serious problems, but later models used a fixed version.

The standard Atari operating system includes basic instructions for using floppy disk drives. A separate program called a disk operating system (DOS) is needed to manage files on the disk. Atari DOS must be loaded from a floppy disk every time the computer starts or restarts. Atari DOS uses menus to guide users.

• DOS 1.0
• DOS 2.0S – Improved over DOS 1.0; became the standard for the 810 disk drive
• DOS 3.0 – Came with the 1050 drive. Used a different disk format that did not work with DOS 2.0, making it less popular
• DOS 2.5 – Replaced DOS 3.0 in later 1050 drives. Functionally the same as DOS 2.0S, but could read and write enhanced density disks
• DOS XE – Designed for the Atari XF551 double-density drive

Other companies also made replacement versions of Atari DOS.

Legacy

In 1992, Atari Corporation officially stopped providing support for all 8-bit computers. In 2006, Curt Vendel, who created the Atari Flashback, stated that Atari made the 8-bit chipset available to the public. It is widely accepted that Atari allowed the distribution of the Atari 800's ROM through the Xformer 2.5 emulator, which makes the ROM legally available today as freeware.

On March 29, 2024, Atari SA and Retro Games Ltd, through distributor Plaion, released the Atari 400 Mini for £99.99 (€119.99 / $119.99). This miniaturized version of the Atari 400 is a scaled-down model of the original console, preloaded with 25 games. It includes an updated Atari CX40 joystick with extra buttons.