Hardware guru Ken Shirriff works on a simulator for the IBM S/360 Model 50 mainframe computer, launched in April 1964. His program runs the original machine’s microcode so it can control and be controlled by a panel original front.
Circuitry-wizard Ken has been featured on The Reg many times, from reverse engineering a Sinclair calculator to rendering Mandelbrots on an IBM mainframe.
His latest feat is a microcode-level simulator for a midrange System/360, one of IBM’s most important machines to date.
This journalist wrote about the importance of S/360 ten years ago. It was the computer that introduced the idea of software compatibility, as well as the computer on which virtualization was invented. It not only predates microprocessors, but also minicomputers that influenced early microcomputer operating systems such as CP/M. The big iron of the 1960s is really strange and different – all modern computers, from a Raspberry Pi to a rack full of x86-64 servers, are identical twins when compared to these things. Yet, even today, their descendants manage your bank account and your airline reservations.
If you want to experiment with IBM S/360 operating systems and software – many of the older ones are now free – there is already a FOSS emulator called Hercules. It works on most things, and there’s even a choice of FOSS Where commercial GUI for this. IBM tends to frown on commercial efforts, although some have survived.
IBM sold a whole range of S/360s, starting with a Model 30, with about 8K of memory and which with a lot effort can fit in a single truck, up to a huge Model 91. The smaller Model 20 scale still requires considerable effort to fit into a single truck.
The great innovation of S/360 is that all models could run the same software. Operating systems talked to microcode, which talked to very different underlying architectures.
The microcode is what the Shirriff Simulator runs. Hercules is compatible enough for you to to attach a real terminal, but Shirriff emulates the lower level 85-90 bit architecture (depends on how you count) of the Model 50, so it can run IBM’s stock microcode… and control it using the original front panel, which has a terrific array of flashing lights.
For now, the commands are also simulated in JavaScript, but the plan is eventually to attach a real IBM operator console to the emulator. Specifically, the console below. ®
