Maybe machine

When I took 6.004, Computation Structures, we built the “Maybe” machine, a pedagogical microcomputer made from discrete TTL. It had no CPU. It had no ALU. It had a shift register. The microcode could load the shift register from memory, test the low bit, shift the register, and store the register to memory. That was it. It was universal (Turing complete), but it was primitive. If I recall correctly, Steve Ward, Chris Terman, and Dan Nussbaum designed it.

We each built our own Maybe machine on a “nerd kit” — a large briefcase/small suitcase with a power supply, some solderless breadboards, some switches and LEDs, and a bunch of TTL chips. We had to wire up the chips ourselves. I was maybe a bit anal about wiring it up. I color coded the wires, made neat right angle bends, and ran the wires in parallel in the troughs between the breadboards. But when it came to testing the gates, this made it easier to find the wires that had ended up in the wrong place.

When we were building it, we had a test PROM with microcode that would just exercise the logic gates. It had no functional program in it. If the gates were wired correctly, certain patterns would appear on the LEDs.

At last I had it all wired up. Each gate responded appropriately to the test PROM. I double checked to make sure I ran each and every test. I was ready to try the real microcode now. I put my EEPROM under the UV light to erase it, and then programmed it with the microcode. I verified that the PROM contained the correct bits. I popped the PROM into the socket, and powered up the machine. Nothing. The machine did not display the expected pattern of LEDS. It didn’t display anything. Crap.

I erased the EEPROM and reprogrammed it as a test PROM again. I ran all the tests to find which one was failing. All passed. I ran them again. Everything checked out. I started to think maybe a loose wire? I ran the tests again and wiggled the wires. The tests still passed.

Confused, I erased the EEPROM and reprogrammed it with the microcode. I put it back in the Maybe machine and powered it up. Nothing. I was stumped. I had no idea what was wrong. I tried wiggling the wires. I tried wiggling the chips. Nothing.

I reprogrammed the test PROM. Each reprogramming of the test PROM took at least 15 minutes as I waited for the UV light to erase the old program and then wrote in the new one. I verified one last time that all the tests ran correctly and went to consult the TA.

The TA had heard it all before. Some freshman can’t follow instructions, wires it up wrong, and is sloppy about running the tests. He was going to demonstrate the right way to thoroughly test the machine. He had a checklist of each and every test. He was going to exhaustively go through the checklist and tally each one as he went. He was going to show me how to do it right.

He was a little surprised that he didn’t find the problem right away. As each test passed, be became more and more puzzled. Finally, he got to the end of his checklist and every test had passed. So he programmed the microcode into the EEPROM and powered up the machine. Nothing.

Ok, it was a challege. The machine made him look foolish in front of this lower classman. He was going to find the problem. He reprogrammed the test PROM and ran the tests again. As he ran each test, he’d wiggle the wiring and the chips involved in the functional unit being tested. Everything was in order. But when he put in the microcode ROM, everything was dead.

We went through a couple of cycles of this, but it was getting to be dinner time so we decided to call it a night. I could only think that I was going to have to rewire the whole thing from scratch. I was dreading it. I headed back to my dorm room.

But...

It was odd that when you tried to run the microcode, although you didn’t get the expected pattern of LEDs, they did briefly flicker when you powered up the machine. The instruction after loading the LEDs was a conditional branch to itself. The LEDs would be loaded and then stay on until the conditional became true when the user pressed a button. It was almost as if the conditional was taken immediately rather than waiting for the button press. Maybe the conditional was backwards? If the conditional were backwards, the LEDs would flicker because they’d be loaded and the mahine would immediately branch.

When I got to my dorm room, I opened the nerd kit and checked. Sure enough, the conditional was wired to the inverted output of a flip-flop. Every conditional branch would be taken the wrong way. The non-inverted output was the next hole over. The test PROM checked that the wire was connected, but didn’t check that it was connected to the positive sense of the conditional bit.

The next day, I went back to the lab with the fix. I reprogrammed the EEPROM with the microcode and powered up the machine. The LEDs lit up in the expected pattern and the machine ran as expected.