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Atari 8-bit family

The Atari 8-bit family is a series of 8-bit home computers manufactured from 1979 to 1992. All are based on the MOS Technology 6502 CPU and were the first home computers designed with custom coprocessor chips, giving them "the most powerful graphic subsystem" of any 8 bit machine. 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. Late models of the machine were being produced as late as 1991.

History

Origins

Design of the 8-bit series of machines started at Atari Inc. as soon as the Atari 2600 games console was released in late 1977. The engineering team from Atari Grass Valley Research Center (originally "Cyan Engineering") felt that the 2600 would have about a three year lifespan before becoming obsolete. They started "blue sky" designs for a new console that would be ready to replace it around 1980, three years after the 2600's introduction. 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 era began in earnest in the form of the Apple II family, Commodore PET and TRS-80—what Byte Magazine would later dub the "1977 Trinity". 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 (named after two attractive Atari secretaries). 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 8 KB cartridge slot, monitor output and a full keyboard, while Candy used a plastic "membrane keyboard" and internal slots for memory (not user upgradable).

At the time, the Federal Communications Commission (FCC) mandated that signal leakage in the television frequency range had to be extremely low. As the Atari machines had TV circuitry inside them, they were subject to this rule and needed to be heavily shielded. Both the 400 and 800 were built around very strong cast aluminum shields forming a partial Faraday cage, with the various components screwed down onto the internal framework. This had the advantage of producing an extremely study computer, although at the disadvantage of being expensive and complex. The FCC ruling also made it difficult to have any sizable holes in the case, which eliminated expansion slots or cards that communicated with the outside word via their own connectors. Instead, Atari designed the Serial Input/Output (SIO) computer bus, a daisy-chainable system that allowed multiple devices to connect to the computer through a single shielded connector. Although the 800 included internal slots, they were reserved for ROM and RAM memory modules.

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 Inc.'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.

The early machines: 400 and 800

The machines were announced in late 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 both released with 8 KB. Originally the 800 shipped with 8 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 Inc. to remove the modules casings, leaving them as "bare" boards. Later, the expansion cover was held down with screws instead of the easier to open plastic latches.

The Atari 400, despite its membrane keyboard and single internal ROM cartridge slot, outsold the more feature rich Atari 800 by some margin. Because of this, developers were generally unwilling to use the 800-only right cartridge slot.

Liz

The 400 and 800 were complex and expensive machines to build, consisting of multiple circuit boards mostly enclosed by massive die-cast aluminum shielding. Additionally, the machine was designed to add RAM only through cards, 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 an upgraded 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 take advantage of these changes. 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. Whereas the previous machines had individual circuit boards mounted inside and outside the internal shield, in the new design a single board supported all of the circuitry and the much thinner shielding was attached to it. Improvements in chip making allowed a number of chips in the original systems to be condensed into 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. An external expansion chassis was also supported.

Like the earlier machines, the Sweet 8/16 was intended to be released in two versions as the 1000 with 16 KB and the 1000X with 64 KB; RAM was still expensive enough to make this distinction worthwhile.

1200XL

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. The expansion connector from the original 1000X design was left off, making the design rely entirely on SIO again. Frustrating this was the fact that the +12V pin in the SIO port was left unconnected; only +5V power was available although some devices made use of the +12V line. An improved video circuit provided more chroma for a more colorful image, but the chroma line was not connected to the monitor port, the only place that could make use of it. 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 functionality 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 discontinued in 1983.

Newer XL machines

By this point in time Atari Inc. was involved in what would soon develop into a full-blown price war when Jack Tramiel of Commodore International was 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 Inc. 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 Inc.; 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 Inc. 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.

Several versions of the new design, the 600XL, 800XL, 1400XL and 1450XLD were announced at the 1983 Summer CES. The machines had Atari BASIC built into the ROM of the computer and a Parallel Bus Interface (PBI) at the back that allowed external expansion. 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 Inc.. 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.

Prototypes which never made it to market include the 1600XL, 1650XLD, and 1850XLD. The 1600XL was to have been a dual processor model capable of running 6502 and 80186 code. This was canceled when James J. Morgan became CEO and wanted Atari to return to its video game roots. The 1650XLD would have been 6502-based in a 1400XLD case. The 1850XLD was to have been based on the Amiga Lorraine (later to become the Commodore Amiga). These models were canceled when Jack Tramiel took over Atari and changed the XL series development into the XE series. (Of course, the new Atari swapped the Amiga system for the ST system. (See Atari ST, Amiga Contract).

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 Inc. 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 Inc.'s consumer division from Warner for an extremely low price.

Tramiel era: XE series and XE Game System

Jack Tramiel's Atari Corporation produced the final machines in the 8-bit series, which were the 65XE and 130XE (XE stood for XL-Expanded). They were announced in 1985, at the same time as the initial models in the Atari ST series, and resembled the Atari ST. 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 128 KB of memory, accessible through bank-selection.

An additional 800XE was available in Europe (mostly Eastern Europe), which was essentially a 65XE repackaged in order to ride on the popularity of the original 800XL in Europe. Unfortunately, the 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 Corp. brought out the XE Game System (XEGS), released in 1987. The XE Game System was sold bundled with a detachable keyboard, a joystick and a light gun (XG-1), and a couple of game cartridges (Bug Hunt and Flight Simulator II). The XE Game System 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 corp. officially dropped all remaining support of the 8-bit line.

Design

The Atari machines consist 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. These "support" chips are controlled via a series of registers that can be user-controlled via memory set/get instructions running on the 6502. For example, the GTIA uses a series of registers to select colors for the screen; these colors can be changed by inserting the correct values into its registers, which are mapped into "memory" that is visible to the 6502. Some parts of the system also use some of the machine's RAM as a buffer, notably the ANTIC's display buffer and its Display List (essentially a small program written in the chip's simple machine language that tells ANTIC how to interpret that data and turn it into a display), as well as GTIA's sprite information.

ANTIC

ANTIC is a microprocessor which processes display instructions. A complete sequence of instructions is known as a Display List. Each instruction describes how a single "line" on the screen is to be displayed (specifying one of several character or graphics modes available), where it is displayed, if it contains interrupts, if fine scrolling is enabled or not, and optionally where to load data from memory (text or graphics information). Since each line can be programmed individually, this feature enables the programmer to create displays comprised of mixed graphics and text, as well as different graphics modes on the screen at once. It also enables the machine to quickly "scroll" the screen vertically or horizontally by means of a single memory write. ANTIC read this Display List and the display data using DMA (Direct Memory Access), then translates the result into electrical data for GTIA to process. This process is performed without any CPU intervention.

The ANTIC is primarily responsible for drawing the "background" of the graphics screen, as well as text. ANTIC then passes off the video data through the GTIA, which adds color and drew sprites (which Atari called "players" and "missiles"). The combination leads to oddities such as the ability to invert all the text on the screen by changing a value in memory. The character set is easily redirected by changing an ANTIC register, allowing the user to create their own character sets with relative ease.

CTIA/GTIA

The Colleen Television Interface Adapter (note that "Colleen" was sometimes replaced with "Color") is the graphics chip used in early Atari 400/800 home computers. It is the successor to the TIA chip used in the Atari 2600. According to Joe Decuir, George McLeod designed the CTIA in 1977. 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. GTIA, also designed by George McLeod, adds three new graphics modes that enables the display of more colors on the screen than previously available. The "G" in GTIA stood for "George's.

The CTIA/GTIA receives graphics information from ANTIC and also controls sprites (known at the time as "Player/Missile Graphics"), collision detection, priority control and color-luminance (brightness) control to all objects (including DMA objects from ANTIC). CTIA/GTIA output 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, is responsible for reading the keyboard, generating sound and serial communications (in conjunction with the PIA). It also provides timers, a random number generator (for generating acoustic 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 has 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). The name POKEY comes from the words "POtentiometer" and "KEYboard", which are two of the I/O devices that POKEY interfaces with (the potentiometer is the mechanism used by the paddle). This chip is actually used in several Atari arcade machines of the 80s, including Missile Command and Asteroids Deluxe, among others.

Computer models

  • 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 KB). Later PAL versions had the 6502C processor.
  • 1200XL (1982) – new aluminum and smoked plastic cases, 64 KB of RAM, only two joystick ports. Help key, four function keys. Older software, if it was written improperly, caused compatibility problems with the new OS.
  • 600XL and 800XL (1983) – replacements for the 400, 800 and 1200XL sans function keys. 600XL had 16 KB of memory, PAL versions had a monitor port, 800XL had 64 KB 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 KB 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.
  • XE Game System (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 KB RAM. Mainly seen in Eastern Europe.
  • Prototypes/Vaporware (Never Officially Released)
    • 1400XL – Similar to the 1200XL but with a 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.
    • 1600XL – codenamed Shakti, this was dual-processor system with 6502 and 80186 processors and two built-in 5¼″ floppy disk drives.
    • 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
    • 1090XL expansion system, 5 slots in a large case (never released, small numbers leaked out)
    • 1055 3½" floppy drive
    • XF351 3½" floppy drive
    • XF354 3½" floppy drive

Peripherals

During the lifetime of their 8-bit series, Atari released a large number of peripherals. These included:-

  • Several dedicated cassette tape drives. All were similar, and capable of recording at 600 bit/s on a standard audio cassette. (Unlike some computer systems, it was not possible to use a standard cassette deck with the Atari for this purpose.)
  • Various 5.25-inch Floppy disk drives, including single, enhanced and true double-density models.
  • Several printers of various types; dot matrix, thermal, 4-color plotter and letter-quality daisy wheel.
  • Modems, including one model with an acoustic coupler and other direct-connect models.
  • Other peripherals, including a Centronics/RS-232 expansion system, numeric keypad, memory module, touch tablet and an 80-column display module.

Atari's peripherals used the proprietary SIO port, which allowed them to be daisy chained together into a single string; a method also used later 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.

Software

Operating system

Built-in

The Atari 8-bit computers came with an operating system built into the ROM. The Atari 400/800 had the following:

  • OS Rev. A - 10 KB ROM (3 chips) early machines.
  • OS Rev. B - 10 KB 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 - 16 KB ROM (2 chips) for 1200XL Rev A
  • OS Rev. 11 - 16 KB ROM (2 chips) for 1200XL Rev B (bug fixes)
  • OS Rev. 1 - 16 KB ROM for 600XL
  • OS Rev. 2 - 16 KB ROM for 800XL
  • OS Rev. 3 - 16 KB ROM for 800XE/130XE
  • OS Rev. 4 - 32 KB ROM (16 KB OS + 8 KB BASIC + 8 KB 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.

Disk Operating System

The standard Atari OS only contained very low-level routines for accessing floppy disk drives. An extra layer, a disk operating system, was required to assist in organizing file system-level disk access. This was known as Atari DOS, and like most home computer DOSes of the era, had to be booted from floppy disk at every power-on or reset. Unlike most DOSs, 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 the 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 with 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, sometimes quite advanced, such as SpartaDOS X.

Other software

Amongst the many pieces of software released for the 8-bit Atari computers, a large number of programming languages were implemented, including:-

More recently, cross platform development tools (most commonly run on PCs), have become popular for retrocomputing software development.

Graphics capabilities

While the ANTIC and GTIA chips allowed a variety of graphics modes to be combined, and different playfield widths to be used, the Atari's Operating System provided a basic set of graphics modes. In most cases, these were exposed to Atari BASIC via the "GRAPHICS" command, and to some other languages, via similar system calls.

  • 40×24 text modes
    • 1 color of text, with each character's 8×8 pixels the same size as those in 320×192 graphics mode, with the same hue restriction. Characters with the high-bit on were represented in inverse-video.
    • "Lowercase with descenders" mode, which was not available through GRAPHICS, only as part of custom display lists. In this mode characters were 10 pixels high and occupied either the upper or lower 8 pixels of that height. This was not strictly speaking a 40×24 text mode, because of the unusual height.
    • Colored text, where every two bits represents a colored pixel (characters were 4×8 pixels that were the same size as those in 160×192 graphics mode). Characters with the high-bit on were displayed using a 5th color palette registered where the 4th would normally be used.
    • Colored text, where every four bits represents a colored pixel (characters were 2×8 pixels that were the same size as those in 80×192 graphics mode and had the same color limitations). This mode was not directly available through GRAPHICS but required setting GTIA flags in text mode.
  • 20×24 text mode
    • 1 color of text, with each character's 8×8 pixels the same size as those in 160×192 graphics mode. Characters with various bits enabled or disabled (which would normally appear as 'control-characters', lower-case characters, or inverse-video) were displayed with different colored pixels.
  • 20×12 text mode
    • (Same as 20×24 text mode, but with larger pixels and fewer rows of text)
  • 40×24 graphics mode — 4 colors (2 bpp)
  • 80×48 graphics modes — Either 2 colors (1 bpp), or 4 colors (2 bpp)
  • 160×96 graphics modes — Either 2 colors, or 4 colors
  • 160×192 graphics modes — Either 2 colors, or 4 colors
  • 320×192 graphics mode — 2 colors (1 bpp). The pixels were a shade of the playfield color, and could not be different hue. (In practice on NTSC models, this actually resulted in 4 possible colors on a standard TV screen: black, white, blue, and brown, the last two colors being caused by using pixels in only even or only odd columns — thus the blue and brown regions were effectively in 160×192 resolution).
  • 80×192 graphics modes (GTIA chip only)
    • 9 colors from the color palette registers
    • All 15 Atari hues, but only of one brightness (plus black)
    • All 16 Atari shades, but only of one hue

It should be noted that these were only the modes the OS setup by default. As described above, the ANTIC chip used a display list and other settings to create these modes. The actual hardware could by programmed to display up to 384 pixels wide by putting the hardware in wide or overscan mode and up to 240 pixels tall by creating a custom display list.

Software-driven modes

Due to the 8-bit Ataris' flexibility, it was possible (with clever programming) to create a number of software-driven pseudo-"modes" beyond those directly supported in hardware. These included pseudo-256-color 80x192 modes and 80x24 character displays.

See also

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References

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