Bring back Blinkenlights!!! 3269

What I did is certainly newer than pre-70's machines, but about 20 years ago, after spending a long time on a crushing project, I decided to build such a gadget. The problem with most of those that I looked at was that you could sort of only see the most significant bit, all the rest were lost in the fuzz.

What I chose to do was take an 8 bit wide slice of the address bus, decode that into 256 leds, let the user position that slice on any nibble boundary, and if it wasn't the most significant eight bits to let the user select the upper bits with toggle switches. Then I added decoding to allow selection of any combination of Read, Write, Input, Output, and I think it was M1 on the 8080, the first byte of an instruction fetch.

With that in a dim room it was possible to actually see the global behavior of programs as they ran. And at the opposite extreme, toggling a few switches would allow zooming in on a particular area. Flip a few switches and I could see the 6 glowing leds that were the first byte of six instructions making up the idle loop while the machine was waiting for input.

I built simpler versions of that for the IBM PC up until the 286 and even early 386.

There was a pretty funny reaction to a call I made to a software vendor, telling the person who answered the phone that I saw their newest version of the software had changed the way they did garbage collection, previously I knew it was doing X but with this version I saw they were now doing Y, and I'd like to make a suggestion about they way they could improve this. There was a long pause, I was asked if I might hold a moment, when someone new picked up the phone he asked me to repeat what I had just said, I did, there was an even longer pause, and finally "How do you know this?" I explained and we became great friends, I still use their stuff and do beta test for them.

But after the 386 PC's stopped presenting all the memory addresses to the ISA bus and the method wouldn't work in that form any more.

I purchased a motherboard with the extra 487 socket which gave access to all the memory cycles, at least those not caught by the internal cache, but never completed that prototype.

I believe it is possible to build similar hardware today. I briefly thought about that as a product but never carried through. The best hardware solution that I've thought of is to create a "dummy SIMM" that captures address cycles and transforms this down into a slow stream of bits to drive the bank of leds. Additional hardware can produce an audio stream, varying pitch, tempo and volume so you could hear the behavior if you wanted to.

The "case mod" people today might be willing to pay the price to have such a toy. But a decade ago it didn't look like "the Egghead Software Christmas impulse buyer" would pay what these would cost to sell in small volumes.

Now, "of any real use", I don't go there, not anymore. But if someone really wants to pursue this, I can tell you all the thought and work I put into this. Maybe you could send me one when you get it working, I'd be curious to watch a modern OS run under this, and I've got a big box of leds in the back room I might give you. (I've thought about it and I don't think software is the answer)