Atari 8-bit family

From Academic Kids

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, but the models remained largely identical internally. They were, for their era, one of the most technically advanced machines on the market, but a combination of factors, largely business related, meant they did not have a major market when the Commodore 64 shipped a few years later and took over the entire market.

Contents

History

Origins

As soon as the Atari 2600 was released the engineering team went back to design its eventual replacement. The newer design would be faster, have better graphics, and include much better sound hardware. Work continued throughout 1978, primarily focussing 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. Atari management saw this as a golden opportunity to re-purpose the machines, and started research 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

All aspects of the machine were considered open to new solutions, and the machine ended up with its own BASIC (ATARI BASIC), a wonderfully simple peripheral system known as SIO (Serial I/O), and a very powerful character/display driver known as ANTIC. Unlike the 2600 where the video was controlled solely by the TIA moving sprites (known as player/missile graphics in Atari lingo) around a colored background, in the new machines the ANTIC did most of the work drawing the screen (including the ATASCII characters), which the GTIA then colored and added sprites. This separation of duties allowed both chips to be as powerful as possible in an era of expensive silicon, and the machine's graphics were the best on the market until the release of the C64 in 1982.

One of the main reasons the 6502 was so widely used in early machines was cost, but the other was a unique feature that made it ideally suited to graphics. During normal processing of instructions, there was a single cycle of every four that it was guaranteed to not be using the bus. During this time the display hardware, in this case the CTIA, would quickly read out the graphics from memory to the screen.

In the Atari design this was not, of itself, a complete solution. Since the ANTIC and 6502 would both be writing to memory, some system needed to be in place to ensure they both did not do so at the same time. Fairly tight timing constraints limited the modes the ANTIC could support without stepping on the 6502's toes. A more convincing solution was offered later in the form of a custom version of the 6502, originally nicknamed SALLY but later officially known as the 6502C, that could be halted when the ANTIC needed memory access.

Another custom support chip, named POKEY, was responsible for reading the keyboard, generating sound and serial communication (The latter in conjunction with the PIA). The same POKEY chip was also a very common solution for sound effects and music in arcade games in the 1980s, producing a distinctive square wave flavor that is popular among chip tune aficionados.

Eventually they identified two "sweet spots" for such machines, the low-end Candy and high-end Colleen. Based on the 6502B, a faster version of the more common A model, both ran at 1.79 MHz (PAL version, European) or 1.77 MHz (NTSC version, US), which made them almost twice as fast as most machines of the era; the Apple II and Commodore PET ran at 1 MHz, the TRS-80 was at 2.03 MHz but was actually about 1/2 of the speed due to its processor, the Zilog Z80's, design.

The primary difference between the two models was expandability, Colleen would include a number of memory slots, monitor output and a full keyboard, while Candy used a plastic "membrane keyboard" and didn't include any memory slots. Both machines were built like tanks with huge internal aluminum shields, a side effect of meeting a FCC specification that was soon to be removed anyway (the first model of the TRS-80 actually never met that FCC spec).

The early machines

Missing image
AtariComputerMemoPad.png
The startup screen of early Atari 8-bit models when no program is loaded

The machines were brought to market in February 1979 as the 400 and 800, although they weren't widely available until late 1979. The names originally referred to the amount of memory, 4kB RAM in the 400 and 8kB 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 8kB and 16kB respectively, making the naming somewhat superfluous. The 800's expansion system allowed it to be fitted with up to 48kB RAM, and it wasn't long before it came fully expanded from the factory anyway. The expansion system wasn't "real" however, and could only be used for memory. The ATARI BASIC interpreter was supplied as a ROM cartridge, which had to be purchased separately. Later a more advanced Microsoft BASIC would also become available in disk form, and then cartridge.

The later machines

The 800 was rather complex and expensive to build and the 400 didn't compete technically with some of the newer machines appearing in the early 1980s, so 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 cheaper 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), while the new machines used only one. Sweet 16 also addressed problems with the 800 by adding a new expansion chassis as well. Like the earlier machines, the Sweet 16 was intended to be released as the 1000 with 16KB and the 1000XL with 64KB.

Missing image
AtariSelftest.png
The startup screen of later Atari 8-bit models (XL series onwards) when no program is loaded
Missing image
AtariBasicStartup.png
The startup screen when the built in BASIC is selected

But when the machines were actually released there was only one version, the 1200XL, an odd hybrid of features from the Sweet 16 project. A number of problems in this machine, including a change in its operating system which made many programs written for the 400/800 computers incompatible, made the machine a flop. This was quickly addressed in the hastily-completed 600XL and 800XL, which were largely identical to the original Sweet 16 specifications. The XL series had Atari BASIC built into the ROM of the computer.

However the 1200XL was released at potentially the exact wrong time. By the time the new machines replaced it the Commodore 64 had already become the market leader, and Atari was unable to address this. This also triggered the demise of the 1450XLD, which boasted a built-in 3" diskdrive but was never released.

The final machines in the series were there 130XE and 65XE. These were really cut-down versions of the 600XL and 800XL in much cheaper cases, a result of Jack Tramiel's efforts to wring every dollar out of the platform before finally killing it. The X in XE stood for XL-Compatible.

A theory for why the number 65 was used for the first machine in the XE series is because Atari wanted their machine-numbers to correspond with the amount of RAM they came with, but as Commodore already used the numbers 64 and 128, Atari decided to add one to 64 and chose 65. All subsequent model-numbers were multiples of 65 instead of 64. This numbering-scheme was also used in the Atari ST line of computers as well.

An additional 800XE was available in Europe (mostly Eastern Europe), it was basically a 130XE with half the memory. Almost as an afterthought, there was also the XE Game System (XEGS), released in 1987. Launched at the very end of the family's market life, the XEGS was sold bundled with its detachable keyboard, a joystick and a lightgun, and a couple of game cartridges.

Computer models

  • 400 and 800 (1979) – original machines in beige cases, 400 had membrane keyboard, 800 had full-travel keys
  • 1200XL (1982) – new aluminum and smoked plastic cases, 64KB of RAM, new but buggy OS and BASIC, which made it a market flop
  • 1450XLD – basically a 1200XL with built in 5 1/4" diskdrive and expansion bay for a second 5 1/4" disk drive (never released, small numbers (5) leaked out)
  • 1400XL – Identical to the 1450XLD except without the disk drive and the expansion bay for a second disk drive.
  • 800XL and 600XL (1982) – replacements for the failed 1200XL, basically fixed the OS and BASIC bugs and added an expansion port; the 16KB 600XL was later dropped
  • 130XE and 65XE (1985) – 800XLs repackaged with highly inexpensive cases and keyboards with 128KB of RAM in the 130
  • XEGS (1987) – a game machine in a light beige case, with a detachable full-travel but slightly "mushy" keyboard (Atari ST'ish)

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. This resulted in far less "cable spaghetti" on the desk, but it also meant plugging in "standard" components like printers and modems was practically impossible without the purchase of separate conversion units (in Atari's case, the 850 interface expansion system).

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

XL series

  • 1010 tape drive, a smaller replacement for the 410
  • 1020 plotter, 20, 40 or 80-column with 4-colors on paper similar to the 822
  • 1025 printer, 80-column dot matrix (Okidata ML-80)
  • 1027 printer, 80-column letter quality daisy wheel (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
  • 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.

Operating systems

The Atari 8-bit computers came with an operating system built into the ROM. The first Atari 400/800s had the Rev. A OS, and later 400/800s had it upgraded to Rev. B. The later Atari 8-bit models all had an adittional OS revision.

Several third-party replacement OSes were also available. The most common one was Q-MEG.

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.

(Only a very few comparable DOSes were stored elsewhere than floppy disks; among these exceptions were the British BBC Micro's optional Disc Filing System, DFS, offered as a kit with a disk controller chip, a ROM chip, and a handful of logic chips, to be installed inside the computer; and Commodore's CBM DOS, located in a ROM chip in each disk drive.)

The authors of Atari DOS, Optimized System Software (OSS), also produced two improved versions - OS/A+ version 2 (highly compatible with Atari's version) and version 4 (with very advanced features for its time, such as long filenames). OS/A+ used a CP/M-like command line.

Available programming languages

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.

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. 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.
    • 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.
  • 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, Jeff 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.

See also

External links

General

Software

Software-Driven Graphics Modes

Games

History

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