PDPjs blog post

_posts/2016-10-06-introducing-pdpjs-a-pdp-11-emulator.md

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+---
+layout: post
+title: Introducing PDPjs, a PDP-11 Emulator
+date: 2016-10-06 15:00:00
+permalink: /blog/2016/10/06/
+machines:
+  - id: test1170
+    type: pdp11
+    debugger: true
+    config: /devices/pdp11/machine/1170/machine.xml
+    connection: dl11->vt100.serialPort
+  - id: vt100
+    type: pc8080
+    config: /devices/pc8080/machine/vt100/machine.xml
+    connection: serialPort->test1170.dl11
+---
+
+[PDPjs](http://www.pcjs.org/devices/pdp11/machine/) is the newest addition to the PCjs family of emulators, joining
+PCx86, PC8080, 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)](http://skn.noip.me/pdp11/pdp11.html) written by
+[Paul Nankervis](mailto:paulnank@hotmail.com), 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
+large 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](https://github.com/jeffpar/pcjs/blob/master/modules/pdp11/lib/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](https://github.com/jeffpar/pcjs/blob/master/modules/pdp11/lib/device.js), which must now be converted
+to the new I/O registration model.
+
+The first new device component is
+[serialport.js](https://github.com/jeffpar/pcjs/blob/master/modules/pdp11/lib/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](http://www.pcjs.org/devices/pdp11/machine/1170/vt100/), by clicking the "Run"
+button on the test machine.  The test machine is running
+[custom boot code](https://github.com/jeffpar/pcjs/blob/master/devices/pdp11/rom/custom/boot.mac) 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.
+
+{% include machine.html id="test1170" %}
+
+{% include machine.html id="vt100" %}
+
+*[@jeffpar](http://twitter.com/jeffpar)*  
+*Oct 6, 2016*