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Introducing PDPjs for the DEC PDP-11

PDPjs, a new DEC PDP-11 emulator, is the newest addition to the PCjs family of emulators, joining PCx86, PCx80, and C1Pjs.

While PDPjs may eventually support a range of DEC PDP machines, my current focus is on the PDP-11, starting with the PDP-11/70. From there, I’ll work backwards to support other PDP-11 models, such as the PDP-11/45, until I reach the beginning of the PDP-11 line: the PDP-11/20.

I’m starting with the top-of-the-line PDP-11/70 largely because the core of the emulator is being adapted from the JavaScript PDP-11/70 Emulator (v1.3) written by Paul Nankervis, who has generously given permission to use his code in PCjs. Since his emulator is a fully functional 11/70, it made sense to start there and work backwards, disabling features according to the model.

The code has already undergone a lot of refactoring. Opcodes are now decoded by function tables rather than a single switch statement, and every opcode is implemented with a discrete function. Other refactoring includes flag management, interrupt management, and device management.

Most of the work remaining is in device management. Like other PCjs emulators, PDPjs has a Bus component, bus.js, that allows separate device components to register I/O handlers for specific UNIBUS addresses. During the initial port, I moved all of Paul’s original device management code into one “catch-all” component, device.js, which has now been converted to the new I/O registration model.

The first new device component is serial.js, which is currently the only means PDPjs has of communicating with the outside world. So you can try PDPjs connected to a VT100 Terminal, by clicking the Run button on the test machine. The test machine is running a custom Boot Monitor included with Paul’s emulator, but due to the lack of other device support, nothing can be booted yet.

Obviously PDPjs is very much a work-in-progress. Before I proceed much farther, I really want to put the CPU through some rigorous testing, so I’ll be on the lookout for some comprehensive PDP-11 instruction tests. Or I’ll write my own, and compare results across 1 or 2 other PDP-11 emulators.

Jeff Parsons
Oct 6, 2016