Doug Neubauer designed the sound chip in the Atari 400 and 800, and then, in his spare time, wrote the game that sold them. When ANALOG Computing interviewed him in 1986, the magazine told him that many of its readers had bought their Atari “solely because of” Star Raiders. Neubauer had started it in early 1979, before the machines had any games at all, and when he finished the first pass it was 900 bytes too big for its 8K cartridge.
The computers he wrote it for were announced early in 1979 and reached the shops that autumn. Atari had built them around the same 6502 processor as its 2600 console, and then surrounded it with custom chips that did work other machines left to the processor. One of them, ANTIC, runs a program of its own every time the television draws a picture. That idea, and the cost it put on the 6502, runs through everything a programmer did on these machines.
Fast facts
- Maker: Atari, Inc.; from 1984 Atari Corporation.
- Announced: early 1979. BYTE expected limited supplies in August and full availability that autumn.
- Launch prices (US): $549.99 for the 400; $999.99 for the 800 with its program recorder.
- CPU: 6502 at 1.79 MHz.
- Custom chips: ANTIC (display processor), CTIA, later GTIA (colour, player/missile graphics and collisions), POKEY (sound, keyboard, paddles, serial port, timers).
- Memory: 400: 8K, expandable to 16K; 800: up to 48K.
- Colour: 16 hues at eight brightness levels, 128 in all, on the original CTIA.
- Sound: four channels.
- Later models: 1200XL, 600XL and 800XL (1983); 65XE and 130XE (1985); XE Game System (1987).
Four chips and a processor
Atari’s Hardware Manual divides the work between four chips besides the processor. ANTIC fetches the screen data from memory, controls fine scrolling in both directions and can make the processor wait for the start of the next scan line. CTIA turns that data into colour, draws four players and four missiles (Atari’s name for sprites, “originally designed to be used in games for objects such as airplanes and bullets”), decides which object appears in front of which and detects collisions between them. POKEY scans the keyboard, reads up to eight paddles, generates four channels of sound and random numbers, and runs the serial port for cassette and disk drives. A standard 6520 PIA reads the joysticks.
The processor runs at 1.79 MHz, a speed chosen, the manual explains, so that “one machine cycle is equivalent in length to two color clocks”. Every cycle the 6502 spends lines up with a fixed stretch of the picture, which is what lets a program change the display at a precise point on the screen.
POKEY very nearly did less. Neubauer, who ran its chip design, said the 800’s design grew out of the 2600, “conceived primarily by Steve Mayer, Joe Decuir and Jay Miner”, and that the original plan gave POKEY only the keyboard, the paddles and “the same audio as is on the 2600”. His early estimate showed there was room for more, so he added a serial port for the cassette recorder and improved the sound. Disk drives and printers were meant to use a parallel port, but that was dropped for fear of radio interference and of failing the FCC’s tests, so everything went through the serial port. That, Neubauer said, is why Atari disk drives load so slowly: “With hindsight, we should have at least added a couple more serial ports.”
The caution may have paid off. In November 1979 BYTE reported that the FCC had tested six personal computers, the Apple, PET and Radio Shack machines among them, for interference with nearby televisions. “Only the Atari system passed.”
ANTIC runs a program
ANTIC is not configured like a video chip; it is programmed. “It is a true microprocessor,” Linda Schreiber wrote in 1983, “because it has its own instruction set, a program, and data.” The program is the display list, a short list of instructions in memory that ANTIC reads, one line of the display at a time, every frame. Jeffrey Stanton’s Atari Graphics and Arcade Game Design counts “only four basic instruction types”: show a line of bitmap graphics, show a line of characters, show some blank lines, or jump. The last instruction in a list, JVB, “resets the program counter to the top of the display list” and waits for the next frame.
Each line’s instruction also says where its data comes from, so the screen does not have to be one mode or one block of memory. A program can put a large-text title above a high-resolution picture and a text window below it, or scroll a map by changing the address in the list instead of moving any data. The operating system builds a standard list for each BASIC GRAPHICS mode, and the Hardware Manual recommends its layout: 192 lines of picture, 160 colour clocks wide. The finest mode’s pixels are half a colour clock wide, giving 320 across.
Display list interrupts
Any instruction in the list can also carry an interrupt bit. When ANTIC reaches the end of that line, it interrupts the 6502, which can then change a colour register, a player’s position or the character set before the beam draws the next line. Atari’s own guide for programmers, De Re Atari, calls the display list interrupt “one of the most powerful capabilities built into the ATARI Home Computer” and “one of the least accessible”, since it needs assembly language and exact timing.
Its uses are the tricks Atari games are known for. Changing colour registers partway down the screen puts far more than the usual colours on it: there are nine colour registers, each able to show any of 128 colours, and De Re Atari judges that “a dozen colors is easy, two dozen requires careful planning”. Moving a player between interrupts lets one player appear several times at different heights, a DLI becoming “a pair of scissors with which you can snip the column and reposition sections of it”. A new character set halfway down gives graphics at the top of the screen and ordinary text at the bottom.
What it cost
ANTIC and the 6502 share the same memory. To fetch display data, ANTIC halts the processor for a cycle, “cycle stealing” in the Hardware Manual’s words. Schreiber spelled out the result: “the higher the resolution that is displayed on the screen, the more often the 6502 will be interrupted. If you turn off the ANTIC chip completely, the 6502 will operate at maximum efficiency.” A programmer paid for a detailed screen in processor time, and a program that needed every cycle could blank the display to get them back.
GTIA and 256 colours
The first machines shipped with CTIA. Its replacement, GTIA, was, in De Re Atari’s words, “nothing more than a CTIA with a few more features”: three extra ways of reading ANTIC’s high-resolution data, using four bits per pixel. One mode shows 16 brightness levels of one hue, another 16 hues at one brightness, a third all nine colour registers at once. Neubauer recalled that GTIA was made early but “wasn’t originally put into production”; the Hardware Manual of 1982 already describes fitting a GTIA in place of the CTIA. With 16 brightness levels available, The ABC’s of Atari Computers noted in 1984, a few display list interrupts could put “16 HUES with 16 luminances for a total of 256 colors” on the screen.
XL and XE
In 1983 Atari redesigned the range. Thomas Blackadar’s The Atari 800XL: A Practical Guide tells the story as it looked in 1984: the first new model, the 1200XL, “got a cool reception, because of design flaws and compatibility problems” and was withdrawn after a few months, replaced by the 600XL, with 16K, and the 800XL, with 64K. Both had BASIC built in and extra graphics modes, and the 800XL became “the center of the Atari line”.
Warner Communications sold Atari to Jack Tramiel in July 1984, for $240 million according to Antic. Under Tramiel the range continued as the XE series, previewed in Britain’s Atari User in May 1985 alongside his new ST. The 130XE added a second 64K, which a program switches in 16K at a time through a window at $4000–$7FFF; Ian Chadwick’s Mapping the Atari explains that each bank can be given to the 6502 or to ANTIC separately. The last model, the XE Game System, was an 800XL-compatible computer sold as a games console. Antic reported in 1988 that Atari had sold 100,000 of them in the US over Christmas 1987, every one it had made that year. As of Christmas 1991, AtariUser reported, Atari decided to discontinue it.
Public sources give no documented total for sales of the whole family.
In Britain
Around 1980 Atari’s British distributor was Selmer, a musical instrument maker, and the machines turned up in unexpected places: Woodroffe’s music shops in Birmingham sold Atari 2600s and, later, Atari 800s. A retailer’s advertisement in Computer & Video Games at the start of 1982 put a 16K Atari 400 at £345. The 800XL was £249 from one dealer in May 1984. In July 1985 another dealer advertised it in Page 6 at £79.95.
The machine also started one of Britain’s biggest software houses. Geoff Brown, a teacher, bought an Atari 400, found it “terrible” and swapped it for an 800. American programs he saw in Compute! looked “fantastic compared to the little that was available in the UK”, so he began importing them, starting with Galactic Chase, a Galaxian clone. That business became Centresoft, and then US Gold.
Star Raiders
Star Raiders was Atari’s own cartridge. Neubauer described it as “a 3-D version of the Star Trek game played on the mainframe computers of that time”, taking from it ship damage, sector scanners and charts, and ranks such as “commander” in place of a score. He was frank about the code: “The routines in Star Raiders are total hacks! It was the first game to use 3-D algorithms, and the ones I came up with were terrible. They worked, but were slow. That’s why the game slows down when there’s an explosion.” Each explosion had about 64 pieces, all moving in three dimensions. There was no room left over for secret messages, and he was paid no royalties.
Jay Miner and the Amiga
Jay Miner directed the design of ANTIC and CTIA after designing the 2600’s TIA. He left Atari when it would not fund a 68000 computer, and in 1982 he joined Hi-Toro, a start-up set up by Larry Kaplan and Dave Morse, as its chief engineer when Kaplan left. Hi-Toro became Amiga. The Amiga’s copper is another co-processor that works through a list of instructions every frame, timed to the beam.
Sources
- BYTE, February 1979, BYTE News: the announcement, the 400’s touch keyboard, 16 colours at eight luminance levels, the prices and the expected availability.
- BYTE, September 1979: ComputerLand advertisement for the Atari 800. BYTE, November 1979: Atari advertisement (prices, 128 colours, four sound channels, memory), and BYTE News on the FCC’s interference tests.
- Atari, Atari 400/800 Hardware Manual (1982), sections I and II: the chips and their functions, the 1.79 MHz clock and colour clocks, cycle stealing, players and missiles, nine colours at once, the recommended display size and pixel width, the GTIA.
- Atari, De Re Atari (Atari Program Exchange, 1982), section 5, “Display List Interrupts”, pp. 5-1 and 5-8; appendix E, “GTIA”.
- Linda M. Schreiber, Advanced Programming Techniques for the Atari (TAB Books, 1983), chapter 2, p. 9.
- Jeffrey Stanton, Atari Graphics and Arcade Game Design (Arrays Inc., 1984), chapter 3, pp. 37 and 41.
- The ABC’s of Atari Computers (1984), entry “HUE”, p. 112: 128 colours, and 256 with GTIA mode 9 and display list interrupts.
- Lee H. Pappas, “An interview with Doug Neubauer”, ANALOG Computing issue 47, October 1986, pp. 89–90: POKEY’s design, the serial port and the abandoned parallel port, the GTIA, the architects of the 800, Star Raiders and its code.
- Thomas Blackadar, The Atari 800XL: A Practical Guide (Sybex, 1984), pp. 6–8: the XL series, the 1200XL, 600XL and 800XL.
- Ian Chadwick, Mapping the Atari, revised edition (Compute! Books, 1985), appendix 16: memory management on the 130XE.
- Nat Friedland, “The Ever-Changing Atari Marketplace”, Antic, May 1988: Warner’s sale to Tramiel in July 1984; 100,000 XE Game Systems sold in the US.
- Atari User (UK), vol. 1 no. 1, May 1985: preview of the XE and ST ranges.
- AtariUser issue 15, July 1992, “The Game’s the Thing”: the XE Game System discontinued as of Christmas 1991.
- Computer & Video Games issue 3, January 1982: dealer advertisement, Atari 400 with 16K at £345. Page 6 issue 9, May 1984, and issue 16, July 1985: dealer advertisements for the 800XL at £249 and £79.95.
- Chris Wilkins and Roger M. Kean, The Story of US Gold (Fusion Retro Books, 2015), pp. 13–15: Selmer, Woodroffe’s, Geoff Brown and Galactic Chase.
- Chris Wilkins and Roger M. Kean, The Story of the Commodore Amiga in Pixels (Fusion Retro Books, 2017), pp. 8–11: Miner’s work on ANTIC and CTIA, his departure, and Hi-Toro.