What is "real"? How do you define "real"? If you're talking about what you can feel, what you can smell, taste, and see... then "real" is simply electrical signals interpreted by your brain.
— Morpheus, The Matrix
If the processor is the brain of the computer, could it also be part of some kind of virtual reality? Simulated memory, software-defined peripherals, artificially generated interrupts.
My first computer was a 286 with 1 MB of RAM and a 50 MB HDD (if I remember correctly). So I decided to pick up a 286 processor and try to simulate the rest of the computer around it. Or at least make it to boot up and run some simple assembly code.
Two years ago, I ordered two (that's how many came in a package) Harris 80C286-12 processors. My memories are a bit hazy, but I believe the C in its name is important because these are the types that are less sensitive to clock accuracy (the 12 at the end means it likes to run at 12 MHz), and can even be stepped manually.
At first, I wasn't too successful with it, and the project ended up in a drawer. Then this year, I picked it up again and tried to figure out where things went wrong.
Wiring up
The processor fits into a PLCC-68 socket. The pins of the socket are not suitable for plugging in jumper wires directly, so the socket was mounted onto an adapter PCB with jumper-compatible headers. The pinout of both the chip and the socket is included in the datasheet, but the adapter PCB complicates things a bit, so I created a small conversion table to make my life easier.
The table also helped identify the various inputs and outputs, which would later be useful when connecting to the Raspberry Pi. As you can see, no fewer than 57 pins are required, which is more than the Pi can provide. The MCP23S17 IO expander came to the rescue. While it wouldn't allow us to drive the processor at the breakneck speed of the supported 12 MHz, fortunately, that's not our goal.
The chip contains 16 IO pins, so we'll need four of them. Although each pin can individually be configured as input or output, I tried to group them logically. The expander has side A and side B, each with 8 pins, and the final result looked like this:
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