Simulating the IBM 360/50 mainframe from its microcode

Amazing work by Ken Shirriff

The IBM System/360 was a groundbreaking family of mainframe computers announced on April 7, 1964. System/360 was an extremely risky “bet-the-company” project for IBM, costing over $5 billion, but the System/360 ended up as a huge success, setting the direction of the computer industry for decades. The S/360 architecture was so successful that it is still supported by IBM’s latest mainframes, almost 60 years later. I’m developing a microcode-level simulator1 for the IBM System/360 Model 50 (link to the simulator); this blog post provides background to understand the Model 50 and the simulator.
To build the simulator, I extracted the binary microcode from the listings using a custom OCR tool. I implemented the hundreds of micro-operations, which were tricky to get correct. While most micro-operations are simple operations such as moving a register to the bus, some microinstructions are much more complex, especially for floating-point operations.20 Another complication is that a microinstruction performs many tasks in parallel and it was hard to determine the exact order in which to perform them.

My eventual goal with the simulator is to move it into the physical world. Specifically, I plan to drive the lights on CuriousMarc’s Model 50 control panel to make the panel operate accurately. We also plan to hook up his IBM tape drives and card reader so we can have all the pieces of a Model 50 mainframe working together, except for the processor itself. I plan to port the simulator to C so I can run it in a microcontroller to drive the physical console. An FPGA implementation is another possibility; this would provide the maximum speed, but would be harder to implement.