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 use an 8-bit MOS Technology 6502 CPU and three special chips that help with features like moving images, smooth scrolling, four audio channels, and other functions. Their graphics and sound were more advanced than many other computers of the same time, and video games were an important part of their software collection. The 1980 game Star Raiders, a first-person space combat simulator, became the most popular application for this system.

The Atari 800 was the more expensive model, while the Atari 400 was less costly. The 400 had a keyboard that was spillproof and used a membrane instead of keys, and it came with 8 KB of RAM that could not be upgraded. The 800 had a standard keyboard, a second slot for cartridges, and allowed users to upgrade its RAM to 48 KB. Both models used the same 6502 CPU running at 1.79 MHz (or 1.77 MHz for PAL versions) and the same special chips: 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 these two systems are not compatible.

The original 400 and 800 models were 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 the same year. After Atari was sold and reorganized, Atari Corporation introduced the 65XE (called the 800XE in some European markets) and the 130XE in 1985. These newer 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 software and use peripherals from the 8-bit computers.

Atari 8-bit computers were sold in computer stores and department stores like Sears, often with an animated demonstration to attract buyers. Over 2 million Atari 8-bit computers were sold between late 1979 and mid-1985. Their main global competitor was the Commodore 64, which was released in August 1982. 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 Atari Video Computer System (VCS) was released in late 1977. While working on the VCS in 1976, engineers from Atari Grass Valley Research Center (originally Cyan Engineering) predicted the system would become outdated in three years. They started planning a new console to replace it around 1979.

This new system was a major improvement over the VCS, fixing its main weaknesses while keeping similar ideas. It had better speed, graphics, and sound. Engineers worked on the chips for this system throughout 1978, focusing on a much-improved video coprocessor called CTIA (the VCS used a version called TIA).

In 1977, the home computer era began with the release of the TRS-80, PET, and Apple II, which Byte magazine called the "1977 Trinity." In 1976, Nolan Bushnell sold Atari to Warner Communications for $28 million to fund the VCS. In 1978, Warner hired Ray Kassar as Atari’s CEO. Kassar wanted a chipset for a home computer that could compete with Apple, so it needed character graphics, support for expansion, and the ability to run BASIC programming language.

Atari engineer Jay Miner designed a display system for the Atari 8-bit computer using two chips. The CTIA chip handled moving images and background graphics. To reduce the main computer’s workload, a separate microprocessor called ANTIC managed video registers and buffers. CTIA and ANTIC worked together to create the display, with ANTIC retrieving data from memory and sending it to CTIA for final processing.

The new system was much more advanced than other systems 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 machines, saw Atari’s work during a visit. He realized Commodore’s design would not be competitive, but he could not share this with Commodore due to a non-disclosure agreement.

Atari planned two versions of the new computers: a basic model called "Candy" and a more advanced one called "Colleen" (named after two Atari secretaries). Candy was intended as a game console or hybrid, while Colleen was marketed as a full computer. Colleen had 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.

A goal was to make the computers easy to use. Cartridges were used to simplify access to software, and the system automatically loaded drivers from connected devices. The disk operating system was menu-driven, and the system started with a built-in editor called "Memo Pad" instead of a blank screen.

As development continued, there was debate about whether Candy should have a built-in or external keyboard. By 1978, education became a focus, and Candy was redesigned as a child-friendly computer. Atari planned to use Microsoft BASIC but had to create a new version, Atari BASIC, due to space limitations.

At the time, televisions had only one input (antenna), so computers used RF modulators to convert video signals. Poorly designed modulators caused interference, leading the FCC to set strict standards. To meet these, Atari used metal shields to block radio waves, making the computers sturdy but more expensive to build.

The FCC rules also limited case design, eliminating expansion slots. Instead, Atari used a Serial Input/Output (SIO) bus to connect devices through a single shielded port. Internal slots were reserved for memory and ROM modules.

In December 1978, Atari announced its home computer plans. The Atari 400 and 800 were shown at the Winter CES in 1979 and released in November 1979. Originally named for their 4 KB and 8 KB RAM, both models later included 8 KB RAM due to falling prices. The 800 had more memory and expansion options but sold fewer units than the 400. Both had four joystick ports, though few games used all four. The 400’s membrane keyboard made it more popular than the 800’s full keyboard.

Design

The Atari machines use a 6502 chip as the main processor. They also use ANTIC and GTIA chips to create graphics and the POKEY chip to handle sound and serial input/output. These support chips are controlled through special memory spots called registers. The 6502 processor can change the values in these registers using memory load/store instructions. For example, the GTIA chip uses registers to choose screen colors. These registers are mapped to memory locations that the 6502 can access. Some chips, like ANTIC, use data stored in RAM for tasks such as displaying graphics and managing display lists. GTIA also uses RAM to store information about Player/Missile graphics (sprites).

Atari spent $10 million to develop its three custom chips. These chips allow functions like smooth background scrolling to be handled directly by hardware, which would require software on other systems. Atari used in-store computers with graphics and sound demos to promote its products early on.

ANTIC is a microprocessor that processes a list of instructions called a display list. Each instruction tells the chip to add a row of graphics to the screen. The type of graphics depends on whether the row shows text or a bitmap, the resolution, number of colors, and the height in scan lines. Instructions can also set up interrupts, enable fine scrolling, and specify where to get display data from memory.

Because each row can be set individually, programmers can create screens with different text or graphics modes. Data for these modes can be retrieved from any memory location, not just sequential ones.

ANTIC uses DMA (Direct Memory Access) to read the display list and display data. It then translates this into pixel data for the screen. This data is sent to GTIA, which adds colors and combines Player/Missile graphics (sprites) for the final image on a TV or monitor. Once the display list is set up, the screen is generated 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 set per line, but the brightness of the foreground and background can be adjusted. High-resolution bitmap mode (320×192 graphics) creates special colors called 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 programmers can use RAM to create custom characters. The data can be placed on 1K or 512-byte memory boundaries. Other registers can flip characters upside down or enable inverse video.

ANTIC supports various Playfield modes and widths. The original Atari Operating System included with the Atari 800/400 computers provided access to some of these graphics modes. These modes were accessible through Atari BASIC using the "GRAPHICS" command and other programming languages via similar commands. Later versions of the operating system added support for additional modes.

ANTIC text modes allow custom character sets. There are four ways ANTIC can render text: Normal, Descenders, Single color character matrix, and Multiple colors per character matrix.

ANTIC uses a display list and settings to create these modes. Graphics modes can be mixed freely by changing instructions in the display list.

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

The display list allows for horizontal and vertical coarse scrolling with minimal processor input. ANTIC also supports fine scrolling, which shifts the screen by single pixels horizontally and single scan lines vertically.

The system’s CPU clock and video hardware are synchronized to half the NTSC clock frequency. This ensures pixel colors are consistent across the screen, allowing smooth horizontal fine scrolling without color changes.

The Color Television Interface Adaptor (CTIA) was the graphics chip used in the Atari 400 and 800. It replaced the TIA chip from the 1977 Atari VCS and was designed by George McLeod. It was later replaced by the GTIA chip, which added three new graphics modes. GTIA also controls Player/Missile graphics (sprites), collision detection, and color settings.

CTIA/GTIA receives graphics data from ANTIC and applies colors using a 128 or 256-color palette. It sends separate brightness and color signals to create a composite video output. CTIA/GTIA also reads joystick inputs and keyboard speaker data. In later models, audio from the keyboard speaker was mixed with other audio for the TV.

POKEY is a chip used for keyboard input, paddle controls, sound, and serial communication. It includes timers, a random number generator, and four audio channels with independent frequency, noise, and volume controls. Two channels can be combined for higher sound quality. The name "POKEY" comes from "potentiometer" (used in paddles) and "keyboard." POKEY was used in many Atari arcade games, including Centipede and Asteroids Deluxe.

Atari released updated versions of the 400/800 with the same chipset and different designs: the 1200XL, 600XL, and 800XL. Many other projects to develop successors to the 8-bit line were canceled.

Peripherals

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

Atari’s accessories used a special port called the Atari SIO port, which allowed devices to be connected in a line. A main goal of the Atari computer’s design was to make it easy for users to operate, which was helped by the SIO bus. Because all devices used the same type of connector, it was simple for new users to add more equipment to the computer. Atari SIO devices used an early version of a system that allowed devices to work with the computer without needing extra setup. Each device on the bus had a unique number, and could send special instructions to the computer when it started up. The extra parts inside these accessories made them more expensive than similar basic devices used by other computers at the time.

Software

Atari did not share technical details about its computers with the public, except for software developers who promised to keep the information private. This may have been done to encourage more software sales. Early on, cartridge-based software was very rare. In 1980, the magazine InfoWorld joked that Atari owners might use the cartridge slot as an ashtray because it was so unused. The magazine suggested they try Star Raiders, a game that became very popular and helped sell Atari computers, much like how VisiCalc helped sell Apple II computers.

Chris Crawford and others at Atari shared detailed technical information in a book called De Re Atari. In 1982, Atari released two guides: the Atari Home Computer System Hardware Manual and a detailed version of the operating system's code. These resources led to many books and articles about programming the computer's special hardware.

Atari's computers had better graphics than the Apple II, and their marketing focused on home use. Because of this, games made up most of the software available. A 1984 review book used 198 pages for games and only 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 models had two versions of this 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 and XE models had different versions of the operating system, which sometimes caused problems with software compatibility. To solve this, Atari created the Translator Disk, a floppy disk that allowed XL/XE computers to use the older 400 and 800 operating systems.

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

Later XL/XE models included a version of Atari BASIC stored in ROM. This could be turned off at startup by pressing the silver OPTION key. The earliest version had serious errors, but later models used a corrected version.

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

• DOS 1.0
• DOS 2.0S – Improved version of DOS 1.0; became 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, so it was not widely used
• 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 created alternative versions of DOS for Atari computers.

Legacy

In 1992, Atari Corporation stopped supporting all of its 8-bit computers. In 2006, Curt Vendel, who created the Atari Flashback, said that Atari made the 8-bit chipset available for free to the public. People who study and use Atari systems agree that Atari allowed the Atari 800's ROM to be shared through the Xformer 2.5 emulator, which makes the ROM legally available today as free software.

On March 29, 2024, Atari SA and Retro Games Ltd, through the distributor Plaion, released the Atari 400 Mini. The price is £99.99 (€119.99 / $119.99). It is a smaller version of the original Atari 400, designed as a microconsole that plays 25 preloaded games. The device includes an updated Atari CX40 joystick with extra buttons.