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Hardware boffin working on IBM S/360 Model 50 simulation

Hardware guru Ken Shirriff is working on a simulator for the IBM S/360 Model 50 mainframe, launched in April 1964. His program runs the original machine’s microcode so it can control and be controlled by an original front panel.

Circuitry-wizard Ken has featured on The Reg quite a few times, from reverse-engineering a Sinclair calculator to rendering Mandelbrots on an IBM mainframe. His latest exploit is a microcode-level simulator for a mid-range System/360, one of IBM’s most significant machines ever.

This reporter wrote about the importance of the S/360 a decade ago. It’s the computer that introduced the idea of software compatibility, as well as the computer on which virtualisation was invented. It not only predates microprocessors, but also the minicomputers that influenced the first microcomputer OSes such as CP/M. 1960s big iron is seriously weird and different – all modern computers, from a Raspberry Pi to a rack full of x86-64 servers, are identical twins compared to these things. Still, even today their descendants manage your bank account and your airline bookings.

If you want to experiment with IBM S/360 operating systems and software – several of the old ones are freeware now – there is already a FOSS emulator called Hercules. It runs on most things, and there’s even a choice of FOSS or commercial GUIs for it. IBM tends to frown upon commercial efforts, although some have survived.

IBM sold a whole range of S/360s, from a Model 30, with about 8K of memory and which with great effort can fit into a single truck, up to a huge Model 91. The big innovation of S/360 is that all models could run the same software. OSes talked to the microcode, which talked to the very different underlying architectures.

The microcode is what Shirriff’s simulator runs. Hercules is compatible enough that you can attach a real terminal to it, but Shirriff is emulating the lower-level 85-to-90-bit architecture (it depends how you count) of the Model 50, so that he can run IBM’s original microcode… and control it using the original front panel, which has a formidable array of blinkenlights.

For now, the controls too are simulated in JavaScript, but the plan is ultimately to attach a real IBM operator’s console to the emulator. Specifically, the console below. ®

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