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Atari 8-bit computers

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An Atari 800XL, one of the most popular machines in the series.

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.

The early machines: 400 and 800

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, Candy as a game machine (or hybrid game console). Colleen would include slots for memory and ROM, a second 8k cartridge slot, monitor output and a full keyboard, while Candy used a plastic "membrane keyboard" and internal slots (not user upgradable). Both machines were built like tanks with huge internal aluminum shields, 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 400
Atari 800

The machines were announced in December 1978 as the 400 and 800, although they weren't widely available until November 1979. The names originally referred to the amount of memory, 4K RAM in the 400 and 8K 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 8K and 16K respectively.

Due to new 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 machines shipped with 16k, but as prices continued to fall Atari eventually supplied the machines fully expanded to 48k, using up all the slots.

A few companies made RAM modules for the Atari 800. One company made a module which added an 80-column display system and 16k of RAM so you could remove one of the Atari-supplied cards without losing memory.

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 a replacement was in the works (the so called Sweet-8 or "Liz NY").

XL series

1200XL

File:AtariSelftest.png
The startup screen of later Atari 8-bit models (XL series onwards) when no program is loaded.

The 800 was complex and expensive 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 now 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 emissions from leaking out. This requirement enabled lighter, less expensive shielding than the previous 400 and 800 computers.

In 1982 Atari started the Sweet 16 project to address these issues. The result was an upgraded set of machines otherwise similar to the 400 and 800, but much easier and less costly to produce. Newer fabs allowed a number of chips in the original systems to be condensed into one. For comparison, the original 800 used seven separate circuit boards (many of them small, three of them for RAM and another for ROM), while the new machines used only one. Sweet 16 also addressed problems with the 800 by adding a new expansion chassis as well, although it was to be external. Like the earlier machines, the Sweet 16 was intended to be released in two versions as the 1000 with 16KB and the 1000XL with 64KB. RAM was still expensive enough to make this distinction worthwhile.

When ANTIC needed memory access it halted the CPU. But this required four support chips. Atari later had a custom version of the 6502B to be made with an extra "HALT" pin. Known initially as SALLY, this version later became an official product known as the "6502C" and used in all XL/XE Atari machines.

Atari 1200XL

But when the machines were actually released there was only one version, the 1200XL, an odd hybrid of features from the Sweet 16 project. Notable features were 64kB of RAM (first for an Atari), built-in self test, redesigned keyboard (favored by many users), and redesigned cable port layout. However the 1200XL also included a number of missing or poorly implemented features. For instance it included the expansion chassis connector, but the case did not include a hole for it to be used. A new video chip provided more chroma for a more colorful image, but then someone decided to disconnect that pin in the monitor port, reversing the effect. The +12V pin in the SIO port was also removed, for no obvious reason.

In the end the 1200XL offered little new in comparison to the existing 800, and although it should have been much less expensive, it in fact was sold at the same basic price points. Changes made to the operating system to support the new hardware also had the side effect of making a number of existing programs no longer work. 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.

Newer XL machines

By this point in time Atari was involved in what would soon develop into a full-blown price war. Jack Tramiel of Commodore International watched Texas Instruments enter the home computer market, and decided to push them back out by undercutting their prices. TI had undercut Commodore's calculator business only a few years earlier, 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.

Atari 600XL
Atari 800XL

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 (a reflection of the 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, far fewer than anticipated 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 cancelled 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 was able to take the '83 Christmas season while Atari struggled to get their machines onto the shelves. 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.

Tramiel Era: XE series and XEGS

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.

Atari 65XE

The final machines in the 8-bit series were there 65XE and 130XE. The 65XE was the replacement for the 800XL. Originally called the 900XLF, the 65XE was functionally equivalent to the 800XL minus the PBI connection. The 65XE (Euro version) and the 130XE had a Enhanced Cartridge Interface (ECI), a variant of the Parallel Bus Interface (PBI). The 130XE came with 128kB of memory, accessible through bank-selection (thanks to FREDDIE and a EMMU chip). An additional 800XE was available in Europe (mostly Eastern Europe), it was basically a 130XE with half the memory. 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.

The numbers 65 and 130 were used for the XE series because Atari based their machine-numbers on the amount of RAM they came with, but, unlike the Commodore 64 and 128, Atari decided to use the decimal byte count with a base of 1,000 (sometimes rounded to a more even number) for marketing reasons instead of the traditional base-1,024 used by engineers, as the numbers are larger (64K being 65,536 = 65, instead of 64, and 128K being 131,072, rounded to 130, instead of 128). All subsequent model-numbers used this system, including the 16-bit Atari ST line of computers as well. Interestingly this exact scheme was eventually universally adopted by hard drive manufacturers for the same reason.

Atari XE Game System

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 its detachable keyboard (first for an Atari computer), a joystick and a lightgun (XG-1), and a couple of game cartridges. No PBI or ECI, but a complete computer with the keyboard. Bad marketing and a lack of newer releases hampered sales.

On January 1, 1992, Atari Corp. officially dropped all remaining support of their 8-bit line.

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 48KB).
  • 1200XL (1982) – new aluminum and smoked plastic cases, 64KB 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 16KB of memory, 800XL had 64KB and monitor output. Both had built-in BASIC and an expansion port known as the Parallel Bus Interface (PBI).
  • 800XLF – 800XL with FREDDIE chip and BASIC rev. C. Released in Europe only.
  • 65XE, and 130XE (1985) – A repackaged 900XLF with new cases and keyboards. The U.S., Canadian version of the 65XE had no PBI or expansion port. 130XE came with 128KB of RAM and a Enhanced Cartridge Interface (ECI) instead of a 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 64KB RAM. Mainly seen in Eastern Europe.
  • Prototypes/Vaporware (Never Officially Released)
    • 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 5 1/4" disk drive and expansion bay for a second 5 1/4" 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

Peripherals

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" peripherials were more expensive than the standard IBM PC devices, which did not need the added SIO electronics.

400/800 series

  • 410 tape drive, 600 bit/s on cassettes
  • 810 5¼" floppy disk, single-density single-sided, 90KB
  • 815 dual 5¼" floppy disks, double-density single-sided, 180KB (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.

XL series

  • 1010 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, 130KB
  • 1064 memory module, 64K memory expansion for 600XL
  • 1090 expansion system, 5 slots in a large case (never released, small numbers leaked out)

XE series

  • XEP80 80-column display module, parallel port
  • XC11 tape drive
  • XC12 tape drive (small model like the 1010, sold worldwide)
  • XF551 5¼" floppy disk, double-density double-sided, 360KB
  • 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)

Atari also produced a number of other tape drives for use in eastern Europe where they continued to sell into the late 1980s due to their low cost. Some of these included various "high-speed" modes which made them almost as fast as early disk drives.

In addition to the list above, Atari failed to release a huge selection of machines and peripherals that were otherwise completed. See the externally linked FAQ below for details.

Software

Built-in and disk operating systems

File:AtariComputerMemoPad.png
The startup screen of early Atari 8-bit models when no program is loaded.
File:AtariBasicStartup.png
The startup screen when ATARI BASIC is selected.

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. The only command-line Atari DOS.
  • 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. See Atari DOS under Third-party DOS Programs.

Available programming languages

  • Action! – A product of Optimized Systems Software. A high performance language that compiled to machine code, with good support for Atari's hardware. While it was popular with hobbyists, it never attained widespread acceptance, particularly since it was limited to the Atari 8-bit platform.
  • Assembly language
    • Atari Assembler Editor – A 6502 assembler editor and compiler was released by Atari in a ROM cartridge.
    • Atari Macro Assembler (AMAC) – A macro assembler released by Atari Program Exchange (APX) in disk form. Copy protected.
    • MAC/65 – 6502 macro assembler/editor developed by Optimized Systems Software. Released in disk and cartridge forms.
    • Synassembler – Assembler from Synapse Software. Written by Steve Hales. Not compatible with XL/XE computers without patching.
    • Macro Assembler/Text Editor (MAE) – Assembler from Eastern House Software. Written by John Harris.
  • BASIC dialects.
    • Atari BASIC – This was the original BASIC for the Atari 8-bit family. Came as a ROM cartridge with the Atari 400/800 models, but was built into the computer's ROM in later models. In the XL/XE models, BASIC could be disabled by holding down the OPTION key while booting.
    • Atari Microsoft BASIC – A version of BASIC for the Atari 8-bit family released by Atari that was more compatible with Microsoft BASIC. Microsoft BASIC II was the same except it was in a ROM cartridge plus extension disk.
    • Turbo Basic XL – An improved version of Atari BASIC released by a third party that is not only faster, but offers many more commands. There is also a compiler that makes Turbo-BASIC XL programs even faster by compiling them into machine-language binaries.
    • BASIC A Plus — An extended BASIC from Optimized Systems Software
    • BASIC XL — An improved BASIC from Optimized Systems Software [1]
    • BASIC XE — An enhanced version of BASIC XL from Optimized Systems Software [2]
  • C
    • Deep Blue C – A C compiler. Written by John Palevich, APX.

After Atari's 8-bit machines entered the realm of retrocomputing in the late 1990s, cross platform development tools such as XASM, TASM, and cc65, most commonly run on PCs, have been much used by enthusiasts to do programming intended for the machines.

Applications

See Category:Atari 8-bit family software.

Games

See Category:Atari 8-bit family games.

Graphics capabilities

Standard modes

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.

  • 40x24 text modes
    • 1 color of text, with each character's 8x8 pixels the same size as those in 320x192 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 40x24 text mode, because of the unusual height.
    • Colored text, where every two bits represents a colored pixel (characters were 4x8 pixels that were the same size as those in 160x192 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 2x8 pixels that were the same size as those in 80x192 graphics mode and had the same color limitations). This mode was not directly available through GRAPHICS but required setting GTIA flags in text mode.
  • 20x24 text mode
    • 1 color of text, with each character's 8x8 pixels the same size as those in 160x192 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.
  • 20x12 text mode
    • (Same as 20x24 text mode, but with larger pixels and fewer rows of text)
  • 40x24 graphics mode -- 4 colors (2bpp)
  • 80x48 graphics modes -- Either 2 colors (1bpp), or 4 colors (2bpp)
  • 160x96 graphics modes -- Either 2 colors, or 4 colors
  • 160x192 graphics modes -- Either 2 colors, or 4 colors
  • 320x192 graphics mode -- 2 colors (1bpp). The pixels were a shade of the playfield color, and could not be different hue.
  • 80x192 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

Software-driven modes

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 display an image using all 256 of the Atari's possible colors. By taking 80x192 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 80x96 or 80x192 pixels in 256 colors could be perceived. (This mode may have been invented by someone earlier.)

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 80x192 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 (12bpp) at 80x192, with slight 'rolling' artifacts in solid red, green or blue fields in the image. This was called ColorView mode.

In 1994, Clay Halliwell created a modem terminal program for the Atari (FlickerTerm80) which uses 40x24 text mode, combined with two character sets with an identical 4x8 font -- one with the pixels on the left half of the 8x8 grid, the other on the right. By altering where in memory ANTIC looks for graphics, and which font to display, an 80x24 character screen can be displayed. It uses less memory (about 2KB) and can be more quickly manipulated, compared to rendering 80x24 characters using a 320x192 bitmap mode (which would require about 8KB).

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 4x8 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 4x8 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 160x192 pixels in 30 shades of grey. It interlaces two modes -- 80x192 with 16 shades of grey, 80x192 with 9 paletted colors -- and utilizes a bug in the GTIA chip that causes one of the modes to be shifted 1/2 pixel, allowing for a perceived 160 pixels across.

Later, other demo coders created RIP graphics mode, which is similar to HIP, but can display 160x192 pixels in color.

Sources

Template:Mnb The Atari 800 Personal Computer System, by the Atari Museum, accessed August 28th, 2005

See also

General

Technical information

Software, games

Software-driven graphics modes

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