Cordoomceps – replacing an Amiga's brain with Doom

Summary

There's a lovely device called a pistorm, an adapter board that glues a Raspberry Pi GPIO bus to a Motorola 68000 bus. The intended use case is that you plug it into a 68000 device and then run an emulator that reads instructions from hardware (ROM or RAM) and emulates them. You're still limited by the ~7MHz bus that the hardware is running at, but you can run the instructions as fast as you want.These days you're supposed to run a custom built OS on the Pi that just does 68000 emulation, but initially it ran Linux on the Pi and a userland 68000 emulator process. And, well, that got me thinking. The emulator takes 68000 instructions, emulates them, and then talks to the hardware to implement the effects of those instructions. What if we, well, just don't? What if we just run all of our code in Linux on an ARM core and then talk to the Amiga hardware?We're going to ignore x86 here, because it's weird - but most hardware that wants software to be able to communicate with it maps itself into the same address space that RAM is in. You can write to a byte of RAM, or you can write to a piece of hardware that's effectively pretending to be RAM[1]. The Amiga wasn't unusual in this respect in the 80s, and to talk to the graphics hardware you speak to a special address range that gets sent to that hardware instead of to RAM. The CPU knows nothing about this. It just indicates it wants to write to an address, and then sends the data.So, if we are the CPU, we can just indicate that we want to write to an address, and provide the data. And those addresses can correspond to the hardware. So, we can write to the RAM that belongs to the Amiga, and we can write to the hardware that isn't RAM but pretends to be. And that means we can run whatever we want on the Pi and then access Amiga hardware.And, obviously, the thing we want to run is Doom, because that's what everyone runs in fucked up hardware situations.Doom was Amiga kryptonite. Its entire graphical model was based on memory d...