102 lines
4.9 KiB
Markdown
102 lines
4.9 KiB
Markdown
---
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layout: page
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title: PDP-11 Machine Emulation Module (PDPjs)
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permalink: /modules/pdp11/
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---
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PDP-11 Machine Emulation Module (PDPjs)
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=======================================
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Overview
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---
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PDPjs, our [PDP-11 Machine](/devices/pdp11/machine/) emulation module, was written in 2016. It was adapted from
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the [PDP-11/70 Emulator (v1.4)](http://skn.noip.me/pdp11/pdp11.html) written by Paul Nankervis.
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PDPjs is currently comprised of the following PDP-11 components, as listed in [package.json](../../package.json)
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(see the *pdp11Files* property):
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* [bus.js](/modules/pdp11/lib/bus.js)
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* [computer.js](/modules/pdp11/lib/computer.js)
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* [cpu.js](/modules/pdp11/lib/cpu.js)
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* [cpuops.js](/modules/pdp11/lib/cpuops.js)
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* [cpustate.js](/modules/pdp11/lib/cpustate.js)
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* [debugger.js](/modules/pdp11/lib/debugger.js)
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* [defines.js](/modules/pdp11/lib/defines.js)
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* [device.js](/modules/pdp11/lib/device.js)
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* [disk.js](/modules/pdp11/lib/disk.js)
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* [keyboard.js](/modules/pdp11/lib/keyboard.js)
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* [memory.js](/modules/pdp11/lib/memory.js)
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* [messages.js](/modules/pdp11/lib/messages.js)
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* [panel.js](/modules/pdp11/lib/panel.js)
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* [pc11.js](/modules/pdp11/lib/pc11.js)
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* [ram.js](/modules/pdp11/lib/ram.js)
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* [rk11.js](/modules/pdp11/lib/rk11.js)
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* [rl11.js](/modules/pdp11/lib/rl11.js)
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* [rom.js](/modules/pdp11/lib/rom.js)
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* [serial.js](/modules/pdp11/lib/serial.js)
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Since this module was written in 2016, it seemed appropriate to start adopting some of the more useful features of
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[ECMAScript](http://www.ecma-international.org/ecma-262/6.0/index.html) 2015 (aka ES6), including:
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* Classes
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* *const* and *let*
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* Computed Properties
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* Default Parameters
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* Octal and Binary Constants
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* Template Literals
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- String Interpolation (i.e., ${*expr*})
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* New Built-in Methods
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- String.repeat()
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* *import* and *export*
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However, I've still configured the Closure Compiler to "transpile" to ECMAScript 5 (aka ES5), because some people
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may still be using older browsers that don't support ES6 -- or at least the subset of ES6 features I'm currently
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using.
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Eventually, I need to do some performance testing and determine whether the ES6 version performs any faster and/or
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consumes fewer resources than the ES5 version. If it does, then I should either bite the bullet and generate ES6 code,
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or generate both versions and use a loader that detects the browser's capabilities and loads the appropriate version.
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Caveats
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-------
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### Shared modules
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All PCjs machines rely on shared modules that are normally stored in [/shared/lib](/shared/lib/). However, until
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ALL the machines have been converted to use ES6 classes, shared code must now exist in two flavors:
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[/shared/lib](/shared/lib/) and [/shared/es6](/shared/es6/).
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Once all the other machines have been converted to use ES6 classes, the shared ES6 code will be folded back into
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[/shared/lib](/shared/lib/), and the temporary ES6 folder will go away. Obviously, there is incentive for me to do
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this sooner rather than later, since in the interim, I must make any changes to shared code in both places.
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There's also a less obvious problem: if you load a web page that attempts to load two or more PCjs machines, one of
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which uses [/shared/lib](/shared/lib/) and another of which uses [/shared/es6](/shared/es6/), at least one of them will
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fail to start, because the two sets of shared code cannot coexist. Well, they *could* have coexisted if I had been
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willing to change the names of all the shared global objects (like **Component**), but I wasn't.
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### *import* and *export*
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With regard to *import* and *export* statements, the main reason I use them is to inform my development environment
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(WebStorm) about each file's dependencies, thereby preventing inspection warnings. And ultimately, I plan to make PDPjs
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run as a Node application, so explicitly declaring all imports and exports will be required, but for now, it's just
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a web application, so strictly speaking, they're not required.
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When loading uncompiled PDPjs files into a web browser, the Node-based web server bundled with PCjs still relies on
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the `<script>` tag to load all JavaScript files, and as far as I know, no browser currently knows what to do with the
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*import* and *export* keywords under those conditions. Chrome, for example, will immediately throw an exception when
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it encounters a file containing them.
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As a work-around, the bundled web server intercepts all requests for .js files and inserts line comments in front of
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every *import* and *export* statement, so that your web browser won't barf on them. The statements are completely
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superfluous anyway, since the web server generates `<script>` tags for all the necessary scripts, in the order they are
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listed in [package.json](../../package.json).
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This work-around assumes that all *export* statements appear AFTER the object they're exporting; e.g.:
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export default ComputerPDP11;
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and NOT as part of the object declaration; e.g.:
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export default class ComputerPDP11 extends Component { ... }
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