Confirmed the MOS 7600/7601 Pong chip is a true microcontroller

I’m reminded of that scene in Battlestar Galactica (2004) where Starbuck cuts open a crashed Cylon raider, climbs in, and finds it’s actually organic rather than robotic — instead of the neatly-organized metal and wires and computers you’d expect to see in a hi-tech starship, it’s full of messy flesh and blood and organs.

In the same way, here they cut open a microcontroller and instead of finding a neatly organized CPU with all the usual recognizable bits and software written in a straightforward ISA, there’s this weird, surprisingly analog amalgamation between hardware and software, between control and data. It’s not obvious how anything even functions because it’s so alien compared to our nice, clean, modular general-purpose CPU. Instead it feels…almost natural, like the hardware and software grew and evolved together as a single unit (which, to be fair, they probably did). It’s beautiful and terrifying at the same time.

At least, that’s how it feels to me as a (sleep-deprived) software engineer. Maybe to a hardware guy, this is just Thursday.

"The way the paddles in particular are drawn is especially ingenious; in fact, the firmware doesn't even need to position them. The paddle controls are simple potentiometers feeding into an R-C circuit (external to the chip, not shown here) that discharges at a rate based on the pot's resistance. This then feeds into an on-chip comparator in the region marked "paddles," delaying drawing the paddle a number of scanlines based on the pot's position until the comparator triggers, thus eliminating the need for a more complex solution like an A-D converter (cheap!) and automatically drawing the paddle at the proper location without any intervention from the firmware."

I love it!

The MOS 7601 was the first CPU I learned how to write code for, circa late 1977 to early 1978. I would scour jumble sales for broken pong games and the other, more sophisticated console variants to cannibalize for parts, with one conversation about haggling with another buyer over who got what parts leading to a job.

There were several machines of the time that had pluggable cartridges, from Binatone, Telarude?/Telalude? (French or Italian company I recall, I forget), Telstra/Telstar, again, I forget, it was 45 years ago, along with many others, where the display circuitry, power and joytsticks were all on the main board, but each cartridge carried its own CPU and ROM, which was the fashion back in the day.

They were an absolute bugger to write code for, no complex ALU to speak of, no register storage, no RAM, you sort of just chased the CPU with a stream of instructions, stored stuff in decay buffers (you want a countdown from 10? charge up that cap for a brief period of time then let it leak) or on the display output circuitry that you could read back if you needed. If your code went wrong, you never knew it.

You were mostly bit-banging analogue circuits to get anything to display, same for the audio, though it could also talk to some AY audio synthesizer.

I recall the 7601 had a reasonably short lifespan in the marketplace for a variety of reasons.

There were two datasheets for it that were the bible, there might have been more but I only ever had access to bad photocopies of those two. One was the usual electrical characteristics and the other was the instruction set with the usual timings, delays and addressing modes.

The addressing modes were stupidly simple by the standards of today. There were EPROM variants (gold pin variant IIRC) available that you wiped with a strong ultraviolet light, but generally you had an off-board EEPROM and you tied two of the pins high to let it know it should use the external EEPROM.

When I tell people my first game I wrote fit in less than 500 (8-bit) bytes, they wonder how that is. The game was never released but Race Electronics and their desire to cash in on this new fad and exploit the younger, more gullible version of me, did give me my start in video game development so I am thankful for that, in a way. The next game I wrote fit in 2KBytes and I had the luxury of using a 6502 CPU and access to actual RAM on a CBM PET.

It's not my blog post, but I can answer questions about the chip.

i had a binatone pong game that sounds like it was based on this chip. It had a bunch of variants of pong you could play based on moving a couple of physical sliders -- squash (bounced off a wall), football (two bars per player), etc.

The crazy thing was that if you turned the sliders just right, you could also play "off menu" games which had combinations of the attributes of the official ganes. I guess this was down to some of the tricks described here. It certainly confused me a lot when I learnt to program: you can't just compose two games together by half-clicking on an option!

I am still genuinely amazed about what was achieved in this era, bearing in mind that these things took years to develop so started much earlier when most people probably had never heard of a transistor.

Despite the infancy of the industry, these people not only designed an enormous electronic circuit but had to work out how to achieve compatability with inputs and outputs as advanced as graphics, how to store data in memory, had to perform a tonne of debugging and had to get this thing etched onto silicon and then produced at scale.

Wow, just wow.

This seems like a lot of transistors for such a simple game....

If they could reduce the number of scanlines on the monitor, they could get many hundreds of clock cycles per pixel, and then do everything in software. 512 instructions seems quite possible for that - compare to 'boot sector games' which run in about one third of the space.

very cool to see the engineering of that time.