Inside The Atari 2600

The Atari 2600 was an extremely popular yet very simple game console back in the 1970s. They sold, apparently, over 30 million of them, and, of course, these things broke. We’d get calls from friends and — remember, back then normal people weren’t computer savvy — nine times out of ten, we’d ask them to swap the controllers to show them it was a bad controller, and problem solved. But if you did have to open one up, it was surprising how little there was inside, as [Steve] notes in his recent teardown.

The bulk of the circuit board was switches, the power supply, and a TV modulator if you remember those. The circuit board was a tiny thing with a shrunk-down 6502, a 6532 RIOT chip, and a custom chip called a TIA. If you are familiar with those chips, you might wonder if the TIA had any memory in it. It didn’t. Nearly all the ROM and RAM for the game lived in the cartridge itself. Sure, the RIOT has 128 bytes of memory, but that’s not much.

The TIA generates timing signals using an odd configuration that [Steve] discusses. The limited memory explains some things you may or may not have noticed. Most games have a symmetrical background where the left is a mirror copy of the right. The TIA architecture explains why. You could modify things, but you had to do it between scan lines which left a scant 76 clock cycles to do whatever you needed.

There are a lot of tricks for squeezing the most out of the limited architecture that [Steve] highlights. He also provides links for writing your own games and running them on real hardware or an emulator. Since the box was popular and had a — sort of — 6502 inside, you’d think someone would have turned it into a full computer. Someone did.

17 thoughts on “Inside The Atari 2600

  1. The article mentions using an LFSR instead of a binary counter, and supposed it may be easier to implement: it’s *much* easier. Much much much. It’s just register for each bit + a couple of gates per each tap, rather than multiple gates per bit. Also dramatically faster since there’s no carry chain spanning all bits.

    If you need a counter of less length than 2^n bits (LFSRs lose one state at zero) you pretty much can’t beat an LFSR.

  2. I used to work for a place that wave soldered 2600 cartridges. We got the chips and the boards and stuffed them and soldered them and shipped them out. We all had 2600 game collections that were just the boards with paper tags with the game name scribbled on them.

    Also funny you should mention old games, I was looking through some of my stuff yesterday and and I came across some glass negatives for frogger. I think this was a stand alone game though.. Kind of an interesting piece of the past.

  3. Ok, when you see the unpopulated pads for a 24 pin chip, you have to wonder, what was that about? If I had to guess, maybe a ROM or a static RAM? Undoubtedly this was all traced out ages ago. Quickly, Ginger, to the inter webs!

    1. Ok, here it is ..

      The TLDR: It could hold a 2K or 4K ROM, so you could in theory have a built-in game. (Although apparently switching between internal and cartridge ROMs may have been problematic)

      Fun fact, from the same article: The TIA had stereo audio outputs, and the original heavy 6-er cases had accommodation for internal stereo speakers! But when it went to production they just tied the audio pins together and fed the sum into the RF modulator. Apparently the first 3-4 game carts were programmed to play in stereo!

  4. You think 128 bytes of RAM is laughable? Try the Interton VC4000 / Radofin 1292 family. 37 bytes of RAM. There’s more you can R/W, but it changes stuff on the screen, so you could sacrifice a sprite for a dozen bytes more RAM. And the Signetics 2650 CPU is butt slow.

  5. The French Atari 2600 is very fascinating, too.
    It doesn’t do PAL or NTSC, but SECAM – in a wicked way.

    Essentially, it uses grayscale video ouput of the PAL TIA and uses a separate pong video chip to do colour.
    That’s why it merely has 8 colours.

    The reason behind this, I assume, was that making a SECAM version of the TIA would have been very complicated/costly.
    Not only because of it being more of a niche market, anyway, but also due to technological reasons.
    Unlike PAL or NTSC, SECAM as a system required a simple frame buffer. Which the simplistic TIA didn’t feature, it was “racing the beam”, essentially.

    (Remember, this was the 70s, before SCART/RGB were so popular. The French Nintendo did output RGB, by comparison. It can do that thanks to a PAL-RGB converter chip.)

    What’s even worse, these 8 colours do not match the original colours that would match the hues of the grayscale video.

    Only a few French games were adapted to fix this discrepancy. “Few” in comparison of the huge library, I mean.
    The Space Shuttle game is one of them, I recall.
    Those games were modified in such a way that the colours of the pong chip made sense.

  6. The more I read about the 2600, the more I am amazed that people got it to do anything interesting at all. It’s an interesting study in minimalism and the creative techniques people came up with to overcome extreme limitations.

  7. The Atari 2600, released in 1977, was a home video game console that used interchangeable cartridges to play games. It featured a 6507 microprocessor, which was a variant of the 6502 processor used in the Atari 8-bit computers, running at 1.19 MHz. It had 128 bytes of RAM, and 4K of ROM on the motherboard. The system also had a Television Interface Adaptor (TIA) chip, which handled the video and audio output to the television.

    The Atari 2600 used a unique form of digital-to-analog conversion (DAC) to produce its graphics, which were not as sharp or detailed as those of later consoles. The TIA chip would generate a playfield and a set of movable sprites called “players” and “missiles”, that could be positioned and colored independently. The TIA would then draw these graphics on the screen by rapidly switching the video signal on and off. The games were stored on ROM cartridges that plugged into the front of the console and contained all the necessary code and data for the game to run.

    The Atari 2600 used a one button joystick controller, which was simple but effective. A later addition to the console was the Atari VCS keyboard controller.

    Overall, The Atari 2600 was a groundbreaking console that helped to establish the home video game market and paved the way for future consoles. Its simplicity and affordability made it accessible to a wide audience and resulted in it becoming one of the best-selling consoles of its time.

    1. “Overall, The Atari 2600 was a groundbreaking console that helped to establish the home video game market and paved the way for future consoles. Its simplicity and affordability made it accessible to a wide audience and resulted in it becoming one of the best-selling consoles of its time.”

      Ironically, it also almost destroyed the whole video game business through the video game crash of ’83.

      While it was not the fault of the console itself, it provided the platform for cheap, low-quality games. The whole concept of minimalism that gets glorified these days accelerated the process. While technically interesting, many of them were just ugly and not fun to play. Not at all like what people were used to from playing on the arcade machines.

      Please don’t get me wrong, the VCS was awesome as a pong console on sugar, but sophisticated games were rare or were released in the later life of the 1977 console. Games like Solaris (’86), Space Shuttle (’83), Sky Jinks (’82) or Pitfall II (’84), for example.

      Another thing that’s forgotten is how poor the PAL graphics were by comparison.
      While the NTSC version of, say, Battlezone looked amazing with its colozr gradients, The PAL version was just *blew*. Thanks to sub optimal conversion and a poorly designed PAL TIA.

      But that’s a general problem these days. The focus is always US centric, be it in articles or in YouTube videos. Unfortunately, descriptions don’t have the suffix “.. in the US”.
      It’s either forgotten or silently assumed that the point of view is from the US. ;)

  8. The 2600 is really a good architecture to study. It’s small enough to understand, but with unique design decisions that are interesting on their own.

    The most interesting part of the design is that TIA is a “1D” graphics chip. Its registers literally describe only a single scan line. To show something different, you have to change them. And that’s why it shows a bunch of random vertical lines when you turn it on without a cartridge. It was designed to support exactly two games: Pong and Tank. Everything else was a bonus.

    And a lot of games only update every other scan line, so the “pixels” are two lines high, but now the code has twice as long to determine the next line.

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