The Atari 8-bit family is a series of 8-bit home computers manufactured by Atari, starting in 1979. All are based on the MOS Technology 6502 CPU and were the first home computers designed with custom coprocessor chips. Over the following decade several versions of the same basic design were released, including the original Atari 400 and 800 and their successors, the XL and XE series of computers.

Design of the 8-bit series of machines started as soon as the Atari 2600 games console was released in late 1977. The engineering team from Atari's Grass Valley Research Center (who called themselves Cyan Engineering) felt that the 2600 would have about a three year lifespan before becoming obsolete, and started "blue skying" designs for a new console that would be ready to replace it around 1980. 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 would include much better sound hardware. Work on the chips for the new system continued throughout 1978 and primarily focused on much-improved video hardware known as the Color Television Interface Adapter, or CTIA.

During this gestation the home computer revolution "took off" in the form of the Apple II family, Commodore PET and TRS-80. Ray Kassar, the then-new CEO of Atari from Warner Communications, wanted the new chips to be used in a home computer to challenge Apple. In order to adapt the machine to this role, it would need to support character graphics, include some form of expansion for peripherals, and run the then-universal BASIC programming language.

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, and Candy as a game machine (or hybrid game console). Colleen would include slots for RAM and ROM, a second 8k cartridge slot, monitor output and a full keyboard, while Candy used a plastic "membrane keyboard" and internal slots for memory (not user upgradable). Both machines were very sturdy with huge internal aluminum shields, originally 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 had originally intended to port Microsoft BASIC to the machine, as had most other vendors, intending to supply it on an 8 kB ROM cartridge. However the existing 6502 version from Microsoft was 12 kB, and all of Atari's attempts to pare it down to 8 kB 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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  The machines were announced in December 1978 as the 400 and 800, although they weren't widely available until November 1979, much closer to the original design date. The names originally referred to the amount of memory, 4 kB RAM in the 400 and 8 kB 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 8 kB and 16 kB respectively.

Due to the 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 800's shipped with 16 kB, but as memory prices continued to fall Atari eventually supplied the machines fully expanded to 48 kB, using up all the slots. 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 as the 400 outsold the 800 by some margin, developers were generally loath to use the 800-only features like the second cartridge slot.
  The Atari machines consisted of a 6502 as the main processor, a combination of ANTIC and GTIA chips to provide graphics, and the POKEY chip to handle sound and serial input/output. The "support" chips were controlled via a series of registers that could be use-controlled via memory set/get instructions running on the 6502. For example, the GTIA used a series of registers to select colors for the screen; these colors could be changed by inserting the correct values into its registers, which were mapped into "memory" that was visible to the 6502. Some parts of the system also used user-accessible memory as a buffer, notably the ANTIC's display buffer and its "display list"(essentially a small assembler language program that told the ANTIC how to interpret that data and turn it into a display).
  The Color Television Interface Adapter was the graphics chip used in early Atari 400/800 home computers; it was the successor to the TIA chip used in the Atari 2600. The CTIA chip was replaced with the GTIA in later revisions of the 400 and 800 and all other members of the Atari 8-bit family. According to Joe Decuir, George McLeod designed the CTIA (Colleen TIA) in 1977.

GTIA, also designed by George McLeod, received graphics information from ANTIC and also controlled sprites, collision detection, priority control and color-luminance (brightness) control to all objects (including DMA objects from ANTIC). GTIA output them as separate digital luminance and chrominance signals, which were mixed to form an analogue composite video signal.
  The final machines in the 8-bit series were there 65XE and 130XE. They were announced in 1985 at the same time as the initial models in the Atari ST series: the 130ST and 520ST. Originally intended to be called the 900XLF, the 65XE was functionally equivalent to the 800XL minus the PBI connection. The 65XE (European version) and the 130XE had the Enhanced Cartridge Interface (ECI), a semi-compatible variant of the Parallel Bus Interface (PBI). The 130XE shipped with 128KB of memory, accessible through bank-selection.

An additional 800XE was available in Europe (mostly Eastern Europe), which was essentially a 65XE. XE stood for XL-Expanded. The reason for repackaging the 130XE into the 800XE was Atari trying to ride on the popularity of the original 800XL in Europe. Unfortunately, 65XE and 800XE machines sold in Eastern Europe had a buggy GTIA chip, specifically those machines made in China in 1991.

Finally, with the resurgence of the gaming industry brought on by Nintendo, Atari brought out the XE Game System (XEGS), released in 1987. The XEGS was sold bundled with a detachable keyboard (first for an Atari computer), a joystick and a light gun (XG-1), and a couple of game cartridges (Bug Hunt and Flight Simulator II). The XEGS was essentially a repackaged 65XE, and was compatible with almost all Atari 8-bit software and hardware as a result. Bad marketing and a lack of newer releases hampered sales.

On January 1, 1992, Atari officially dropped all remaining support of the 8-bit line.
  The 800 was a complex and expensive machine to build, consisting of multiple circuit boards in various locations inside or outside the massive aluminum shield. Additionally the machine was designed to add RAM only through cards, which required expensive connectors and packaging even though it soon shipped fully expanded right from the factory. At the same time the 400 didn't compete technically with some of the newer machines appearing in the early 1980s, which tended to ship with much more RAM and a real keyboard.

Another major change was the introduction of the FCC ratings specifically for digital devices in homes and offices. One of the ratings, known as Class B, mandated that the device's RF emissions were to be low enough not to interfere with other devices, such as radios and TVs. Now computers needed just enough shielding to prevent interference (both ways), not prevent any emissions from leaking out. This requirement enabled lighter, less expensive shielding than the previous 400 and 800 computers.

In 1982 Atari started the Sweet 8' (or "Liz NY") and Sweet 16 projects to address these issues. The result was an upgraded set of machines otherwise similar to the 400 and 800, but much easier to build and less costly to produce. Improvements in chip making allowed a number of chips in the original systems to be condensed into one. For comparison, the original 800 used seven separate circuit boards while the new machines used only one. Atari also ordered a custom version of the 6502, the "C" model, which added a single pin that allowed four support chips to be removed. Sweet 16 also addressed problems with the 800 by adding a new expansion chassis, although it was to be external. Like the earlier machines, the Sweet 8/16 was intended to be released in two versions as the 1000 with 16 kB and the 1000XL with 64 kB; RAM was still expensive enough to make this distinction worthwhile.
  When the machines were actually released there was only one version, the 1200XL, an odd hybrid of features from the Sweet 8/16 projects. Notable features were 64 kB of RAM, built-in self test, redesigned keyboard (featuring four function keys and a HELP key), and redesigned cable port layout. In general terms the 1200XL most closely matched the "high end" Sweet 16 concept.

However the 1200XL also included a number of missing or poorly implemented features. An improved video circuit provided more chroma for a more colorful image, but the line was not connected to the monitor port. The +12V pin in the SIO port was left unconnected; only +5V power was available although some devices made use of the +12V line. Even the re-arrangement of the ports made some joysticks and cartridges difficult or impossible to use. Changes made to the operating system to support the new hardware also resulted in compatibility problems with some older software that did not follow published guidelines. There was no PAL version of the 1200XL.

The 1200XL ended up with functionally similar to the existing 800, but at a hefty price point. For all of these reasons the 1200XL sold poorly. There is an often-repeated story, perhaps apocryphal, that 800 sales shot up after the release of the 1200XL, as existing owners tried to snap them up before they disappeared. Released in late 1982, the machine was quickly discontinued in 1983.
  By this point in time Atari was involved in what would soon develop into a full-blown price war when Jack Tramiel of Commodore International attempting to undercut his old enemy Texas Instruments. TI had undercut Commodore's calculator business only a few years earlier, almost driving him from the market, but this time Tramiel's supply was stronger than TI's, and he could turn the tables. Although Atari had never been a deliberate target of Tramiel's wrath, they, along with the rest of the market, were dragged into "his" price war in order to maintain market share.

The timing was particularly bad for Atari; the 1200XL was a flop, and the earlier machines were too expensive to produce to be able to compete at the rapidly falling price points. The solution was to replace the 1200XL with a machine that users would again trust, while at the same time lowering the production costs to the point where they could compete with Commodore.

Starting with the 1200XL design as the basis for a new line, Atari engineers were able to add a number of new IC's to take over the functions of many of those remaining in the 1200XL. While the 1200XL fit onto a single board, the new designs were even smaller, simpler, and as a result much less expensive. To reduce cost even further, manufacturing of a new series of machines was set up in the far east.

These versions, the 600XL, 800XL, 1400XL and 1450XLD were announced at the 1983 Summer CES. These machines had Atari BASIC built into the ROM of the computer and a Parallel Bus Interface (PBI). The machines looked similar to the 1200XL, but were smaller back to front, the 600 being somewhat smaller than the 800 front-to-back (similar to the original Sweet 8 project). The 1400 and 1450 both added a built-in 300 baud modem and a voice synthesizer, and the 1450XLD also included a built-in double-sided floppy disk drive in an enlarged case.

Problems with the new production lines delayed the entry of the machines onto the market. Originally intended to replace the 1200XL in mid-83, the machines did not arrive until late in 1983, and far fewer than anticipated were available during the 1983 Christmas season. Nonetheless, the 800XL was the most popular computer sold by Atari. The 1400XL and the 1450XLD had their delivery dates pushed back, first by the priority given to the 600XL/800XL, and later by the 3600 System. In the end the 1400XL was eventually canceled outright, and the 1450XLD so delayed that it would never ship.

By late 1983 the price war that had started the year before was now reaching a crescendo. Although the 600/800 were well positioned in terms of price and features, their entry into the market was so delayed that Commodore dramatically outsold them over the '83 Christmas season. Combined with the simultaneous effects of the video game crash of 1983, Atari was soon losing millions of dollars a day. Their owners, Warner Communications, became desperate to sell off the division.

Although Commodore emerged intact from the computer price wars, fighting inside Commodore soon led to Jack Tramiel's ousting. Looking to re-enter the market, he soon purchased Atari from Warner for an extremely low price.
  The third custom support chip, named POKEY, was responsible for reading the keyboard, generating sound and serial communications (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 had 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 could be combined for more accurate sound (16-bit). The name POKEY comes from the words "POtentiometer" and "KEYboard", which were two of the I/O devices that POKEY interfaced with.
  ANTIC was a microprocessor which processed display instructions. A complete sequence of instructions was known as a Display List. Each instruction described how a single "line" on the screen 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 calls were performed without any CPU intervention.

The ANTIC was primarily responsible for drawing the "background" of the graphics screen, as well as text. ANTIC then passed off the video data through the GTIA, which added color and drew sprites. The combination led to oddities such as the ability to invert all the text on the screen by changing a value in memory. The character set was easily redirected by changing an ANTIC register, allowing the user to create their own character sets with relative ease.
  400 and 800 (1979) original machines in beige cases, 400 had membrane keyboard, 800 had full-travel keys, two cartridge ports, monitor output, expandable memory slots (up to 48 KiB).
  1200XL (1982) new aluminum and smoked plastic cases, 64 KiB of RAM, only two joystick ports. Help key, four function keys. A new OS caused compatibility problems with some older software.  
  600XL and 800XL (1983) replacements for the 400, 800 and 1200XL sans function keys. 600XL had 16 KiB of memory, 800XL had 64 KiB and monitor output. Both had built-in BASIC and an expansion port known as the Parallel Bus Interface (PBI).  
  800XLF 800XL with Atari FREDDIE chip and BASIC rev. C. Released in Europe only.  
  65XE and 130XE (1985) A repackaged 800XLF with new cases and keyboards. The 130XE came with 128 KiB of RAM and a Enhanced Cartridge Interface (ECI) instead of a PBI. The U.S./Canadian version of the 65XE had no ECI or PBI.  
  XEGS (1987) a game machine in a light beige case, with a detachable full-travel but slightly "mushy" keyboard (Atari ST'ish)  
  800XE the final machine in the series. Styling the same as 65XE and 130XE. A 130XE with 64 KiB RAM. Mainly seen in Eastern Europe.  
  1400XL Similar to the 1200XL but with an PBI, FREDDIE chip, built-in modem and speech synthesis chip. Cancelled by Atari.  
  1450XLD basically a 1400XL with built in 5¼? disk drive and expansion bay for a second 5¼? disk drive. Code named Dynasty. Made it to pre-production, but got abandoned by Tramiel.  
  900XLF redesigned 800XLF. Became the 65XE.  
  65XEM 65XE with AMY sound synthesis chip. Cancelled.  
  65XEP "portable" 65XE with 3.5" disk drive, 5" green CRT and battery pack. Never released  
  1090 expansion system, 5 slots in a large case (never released, small numbers leaked out)  
  XF354 3.5" disk drive  
  Atari's peripherals were named after the machines they were intended to be used with, so in general they have names like "410" and "1050". All of them used the proprietary SIO port, which allowed them to be daisy chained together into a single string; a method also used in Commodore's home computers from the VIC-20 onwards. These "intelligent" peripherals were more expensive than the standard IBM PC devices, which did not need the added SIO electronics.
  410 tape drive, 600 bit/s on cassettes
  810 5¼" floppy disk, single-density single-sided, 90 KiB  
  815 dual 5¼" floppy disks, double-density single-sided, 180 KiB (only small numbers produced)  
  820 printer, 40-column dot matrix on adding machine paper  
  822 printer, 40-column thermal on slightly wider paper  
  825 printer, 80-column dot matrix (Centronics 737)  
  830 300-baud modem, using an acoustic coupler, used RS-232 so required an 850 (Novation CAT)  
  835 300-baud modem, direct connect, basic Hayes compatible with SIO interface  
  850 expansion system, included four RS-232 ports and one Centronics parallel port  
  CX-85 Numerical Keypad, external keypad that plugs into the joystick ports.  
  010 tape drive, a smaller replacement for the 410
  1020 color printer, 40-column plotter with 4 pens  
  1025 printer, 80-column dot matrix (Okidata ML-80)  
  1027 printer, 80-column letter quality that printed with a 5-wheels-on-a-drum system kept inked by a top-mounted roller (Mannesmann Tally Riteman LQ)  
  1029 printer, 80-column lower-quality 7-pin dot matrix sold in Europe (Seikosha mechanism)  
  1030 300 baud modem, direct connect  
  1050 5¼" floppy disk, "enhanced density" format single-sided, 130 KiB  
  1064 memory module, 64 KiB memory expansion for 600XL  
  CX77 touch tablet  
  1020 color printer, 40-column plotter with 4 pens  
  XEP80 80-column display module, parallel port
  XC11 tape drive  
  XC12 tape drive (small model like the 1010, sold worldwide). Similar models were released, mainly in Eastern Europe[2]. These included:  
  XCA12 (same case as XC12)  
  CA12 (same case as XC12)  
  XL12 tape drive (an XC12 with minor changes)  
  XC13 - "T2000 ready" version of XC12  
  XF551 5¼" floppy disk, double-density double-sided, 360 KiB  
  XMM801 printer, 80-column dot matrix  
  XDM121 printer, 80-column letter quality daisy wheel  
  XM301 300 baud modem  
  SX212 1200 baud modem (also included RS-232 for use on Atari ST computers)  
  The Atari 8-bit computers came with an operating system built into the ROM. The Atari 400/800 had the following:
  OS Rev. A - 10K ROM (3 chips) early machines.  
  OS Rev. B - 10K ROM (3 chips) bug fixes. Most common for 400/800.  
  The XL/XE Atari 8-bit models all had OS revisions due to added hardware features and changes. But this created compatibility issues with some of the older software. Atari responded with the Translator Disk, a floppy disk which loaded the older 400/800 Rev. B or Rev. A OS into the XL/XE computers.  
  OS Rev. 10 - 16K ROM (2 chips) for 1200XL Rev A  
  OS Rev. 11 - 16K ROM (2 chips) for 1200XL Rev B (bug fixes)  
  OS Rev. 1 - 16K ROM for 600XL  
  OS Rev. 2 - 16K ROM for 800XL  
  OS Rev. 3 - 16K ROM for 800XE/130XE  
  OS Rev. 4 - 32K ROM (16K OS + 8K BASIC + 8K Missile Command) for XEGS  
  The XL/XE models also came with built-in Atari BASIC. Early models came with the notoriously buggy revision B. Later models used revision C.

The standard Atari OS only contained very low-level routines for accessing floppy disk drives. An extra layer, a disk operating system, was added to assist in organizing file system-level disk access. Enter Atari DOS, which, like most home computer DOSes of the era, had to be booted from floppy disk at every power-on or reset. Unlike most others, Atari DOS was entirely menu driven.
  DOS 1.0 - Initial DOS for Atari.  
  DOS 2.0S, 2.0D - Improved over DOS 1.0, became the standard for the 810 disk drive. 2.0D was for never-released 815 drive.  
  DOS 3.0 - Came with 1050 drive. Used a different disk format from previous DOSes, and was incompatible with DOS 2.0, making it very unpopular.  
  DOS 2.5 - Replaced DOS 3.0 in later 1050s. Functionally identical to DOS 2.0S, but able to read and write Enhanced Density disks.  
  DOS 4.0 - Designed for 1450XLD, cancelled, rights given back to the author.  
  DOS XE - Designed for the XF551 drive.  
  Several third-party replacement DOSes were also available.  
  In 1992, Jeff D. Potter created a GIF decoder and image viewer for the Atari called APACView. APAC, or Any Point, Any Color, was a software-driven method of displaying an image using all 256 of the Atari's possible colors. By taking 80×192 mode lines that displayed 16 hues, and those that displayed 16 shades, and either interlacing rows of them, quickly alternating between rows of them, or both, a screen displaying 80×96 or 80×192 pixels in 256 colors could be perceived.
  Later, Potter created another GIF decoder, and later a JPEG decoder was created, which broke an image into the three red, green and blue channels. 16 shades of each, at 80×192 pixels, would be displayed in an interlaced and flickering fashion. The human eye's persistence of vision would allow the viewer to see 4096 colors (12 bpp) at 80×192, with slight 'rolling' artifacts in solid red, green or blue fields in the image. This was called ColrView mode.  
  In 1994, Clay Halliwell created a modem terminal program for the Atari (FlickerTerm80) which uses 40×24 text mode, combined with two character sets with an identical 4×8 font one with the pixels on the left half of the 8×8 grid, the other on the right. By altering where in memory ANTIC looks for graphics, and which font to display, an 80×24 character screen can be displayed. It uses less memory (about 2 KiB) and can be more quickly manipulated, compared to rendering 80×24 characters using a 320×192 bitmap mode (which would require about 8 KiB).  
  In 1998, Bill Kendrick created a puzzle video game for the Atari (Gem Drop) which utilized a similar effect, but by using two alternating character sets (fonts) in colored text. (Each character is 4×8 pixels, each pixel being one of 4 colors.) No color palette changes occurred, and ANTIC's Display List wasn't altered only a vertical blank interrupt was used to change the character set. This allowed for approximately 13 colors on the screen. Solid color fields that were based on two actual colors (e.g., dark red created by flickering between red and black) had less artifacting because they could be drawn in a checkerboard fashion. This mode was called SuperIRG. (Normal 4×8 multi-colored text on the Atari is called IRG.)  
  In 1996, Atari demo coders HARD Software from Hungary created HARD Interlacing Picture (HIP), which can display 160×192 pixels in 30 shades of grey. It interlaces two modes 80×192 with 16 shades of grey, 80×192 with 9 paletted colors and utilizes a bug in the GTIA chip that causes one of the modes to be shifted ½ pixel, allowing for a perceived 160 pixels across.  
  Later, other demo coders created RIP graphics mode, which is similar to HIP, but can display 160×192 pixels in color.