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PDPjs and support for the PDP-11
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@jeffpar jeffpar released this 06 Oct 17:57
· 2750 commits to master since this release
v1.30.0
6e6afd5

PDPjs is the newest addition to the PCjs family of emulators, which includes PCx86, PC8080, and C1Pjs.

While PDPjs may eventually support a range of DEC PDP machines, 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, neutering features as needed.

The code has already undergone a lot of refactoring. Opcodes are now decoded by function table rather than one gigantic 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 now 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 must now be converted to the new I/O registration model.

The first new device component is serialport.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 custom boot code written by Paul, 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.

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