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Space Invaders Revisited

When I first started working on PCjs, JavaScript features like Classes and requestAnimationFrame() weren’t widely available. Over the next 8 or so years, things changed a lot, I learned a lot, and PCjs slowly grew to support more machines.

Unfortunately, that growth resulted in a lot of duplicated code, along with some out-dated and kludgy code. While I did try to factor out common chunks of logic into a set of PCjs shared modules modules, those efforts were limited, partly to save time, but also to minimize the risk of breaking old machines while creating new ones. Getting a new emulator up and running is time-consuming enough without constantly testing and fixing all the others.

I decided to make a break with that code base a couple years ago, when I wrote a web-based emulation of the TI-57 Calculator. I created a new set of Machine library modules hierarchy, including a new time management class to “clock” all the internal devices, which could be driven either by setTimeout() or requestAnimationFrame(), and I used simple JSON and HTML markup to define the machine configuration and layout, instead of the older XML-based PCjs configuration scheme, which had become increasingly clunky and difficult to maintain.

A New 8080 Emulator and Debugger

A few months ago, I decided to continue the evolution of those new classes, starting with a fairly simple machine that I had previously emulated: the 8080-based arcade machine Space Invaders.

First, since I always like to start with an operational debugger, I took the most useful features common to all the PCjs debuggers and packed them into a new Debugger base class, which provides most of the commands that the new 8080 Debugger needs.

Then I separated management of the browser display elements into a new Monitor base class, so that the Space Invaders Video device could focus on the graphics hardware. And the handful of machine I/O ports are implemented by a Ports device that extends a standard Ports class, which plugs into the new Bus class, which implements as many buses as a machine needs (eg, memory and I/O).

And of course, beating at the heart of every machine is the CPU, and for Space Invaders, the 8080 CPU was a straight-forward port of the original PCjs emulation.

One significant change in this new architecture is that every internal device is an instance of the Device class, which in turn builds upon a simple inheritance chain that includes all the functionality that any device might need. If a device wants to call printf(), for example, it can do so by simply invoking this.printf(), rather than having to first “include” (ie, import or require) the class that contains printf().

I’ve also done away with specialized PCjs printing functions like printMessage() and printMessageIO(). printf() is the preferred method, and if a device wants to assign certain print operations to certain message groups (ie, sets of messages that can be turned on or off through the debugger), it simply includes the MESSAGE id as the first parameter to printf().

Debugger input and output controls have been unified into a single textarea “window”, there’s improved breakpoint management for setting read and write breakpoints on any valid memory or I/O address, an execution history buffer can be enabled and dumped with the built-in dump (“d”) command, and all the debugger’s commands can be accessed from any browser debug console window via a global window.command() function.

Time to Kill

Now, as much as I love Space Invaders – it was the first arcade game I became addicted to back in 1979 – the goal here wasn’t really to make yet another clone of Space Invaders. I just wanted to make it easier to build more web-based emulators, fix some things that have long bugged me, make the animation smoother, improve debugging and machine configuration, and so on.

The new Space Invaders emulation should be running below. Keys are mapped by the Input device to the machine’s buttons using “map” data provided in the machine configuration file. Here’s a summary:

For touch-screen devices like the iPhone and iPad, I’ve implemented a quick-and-dirty mapping, where regions across the top of the monitor correspond to first three buttons:

and regions across the bottom of the monitor correspond to the last three buttons:

This is purely experimental and may only work in portrait mode; landscape and full-screen modes will probably need more work to make them usable.

An emulator is never really done, because an emulation can always be made just a little bit better. But I feel like this is a nice fresh start.

DIP Switches

Jeff Parsons
Sep 28, 2019