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An emulator for old / retro computers
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erc

(Emulator of old, Retro Computers)


Erc is software that allows you to emulate computers from the days of yore. (The "days of yore" may be defined as the 1980s.) You may find it interesting if you feel nostalgia for those old machines, or if you want to see how those computers worked.

Erc is also a sound you might make when feeling slightly frustrated, or if you were a small cat that happens to make funny sounds that aren't quite meows.

Goals

I've long had an interest in retro computing, collecting books on the 6502 processor in particular, and this program has been written and rewritten in fits and starts over several years (and in several languages!). Being able to emulate the Apple II, which was my childhood computer, has been a dream of mine for some time!

Erc is written in C, largely because I have long loved that language, though I have not had much need to use it in my professional career. C is a somewhat cranky language, and not one that lends itself to modern engineering practices, so erc was partly written as a challenge to myself to try and elevate the language.

In particular, it's a goal of mine to ensure that erc:

  • is written for other developers to read, especially as a reference for those who are interested in emulation in general and the platforms emulated here in particular;
  • is modular, allowing as much code reuse within the application as is practical;
  • is unit-testable, to the extent that C allows, and maintains a high level of code coverage.

Updates (Jan 18, 2018)

I thought I'd write something on the state of the emulator. Here's what's done so far:

  • We have fully implemented the MOS 6502 processor support for the Apple II, and we have much of the infrastructure to emulate an Apple II machine in place now.
  • Memory organization is principally complete. There's a lot of memory in the Apple II! You have main memory, auxiliary memory, bank-switchable memory, read-only memory. Early on, we implemented memory map support for vm_segments, which is something that has been flexible enough to support all of the types of soft-switches that the Apple II has to control access to said memory (as well as for many other functions).
  • There's a basic disassembler in place for the 6502 support, which has been incredibly helpful in identifying where we have had functional breakdowns in terms of proper execution and bootstrapping of the "machine".
  • We're up to 169 tests as of the time of this writing, which is awesome!

We do have a lot of stuff done for the graphics system, but there's a lot more to go; getting graphics and text working as intended is my next goal. I'm hopeful this will lead us to a point where erc is usable with general disk images of software.

Some ideas for the future:

  • An assembler that could splice a program into multiple disk images
  • Commadore 64 support (which will be difficult, as I've used a Commadore maybe...one time? Two times?)

Running

Running erc right now is a bit hazardous; not in the sense that it will break anything, but more in the sense that you may not know what you should be doing. Since it is still somewhat in flux, I won't go into too much detail on how you can run it; I will point you to the help printout, which you can see by doing this:

./erc --help

(The above assumes you have built the software in the build subdir, and are actually in that subdir.) Generally speaking, you can indicate what you have in one of two "disk drives", given a valid disk image for an Apple II (e.g. something ending in .dsk, .do, etc., and having the proper length for said image).

Compiling and installing

This software uses CMake to build its makefiles, so if you're familiar with that, you should feel somewhat at home.

If you've never compiled any C code before, you will need to install a few things first. If you have not done so, you should install the excellent Homebrew if using a Mac. If you are using Linux, you are probably already acquainted with your local package manager. You will also need to have XCode installed in a Mac environment. You can do so through the App Store.

The following other things you must install are given as Homebrew commands. If you are using Linux, I will leave the appropriate commands you must run as an exercise for the reader.

brew install cmake
brew install snaipe/soft/criterion

Once all that is accomplished, you can do this:

cd build
cmake ..
make

This should produce an executable of the emulator, which you can install wherever you wish.

Testing

If you're feeling a bit nosy, you can run unit tests by doing the following:

cd tests/build
cmake ..
make; ./erc-test

This will execute the testing build of the software, which is handled through Criterion.

Contributing

Right now, I am not accepting pull requests as so much of the design of erc is in flux. I am happy to receive any issues you may file.