A C Based Emulator for the One-Page CPU

2015-02-26 - By Robert Elder

     One of my favourite side-projects is a collection of compiler, microkernel and emulator tools, that I've collectively called RECC.  A core part of the project is a specification for a very simple and easy to emulate 14 instruction machine language.  The project includes a compiler that works for a very large subset of ANSI C89 and can cross-compile to the op-cpu assembly language.  Here is an example of the One-Page CPU assembly language:

ll r1 0x0
ll r2 0x10
shl r1 r2
ll r2 0x4C
or r1 r1 r2
loa r2 SP

     In addition to the 14 machine instructions that the One-Page CPU supports it also features preemptive context switching, a timer interrupt, and I/O interrupts.  Traditional manuals that document CPUs are hundreds of pages, so I wanted to create one that would be simple enough that you could describe it on one sheet of paper.  Admittedly, however, if you haven't written a kernel before you might have trouble understanding the specification at first.

     As far as compilers go, they aren't particularly useful unless they can compile non-trivial programs, so I wrote a small microkernel that supports context switching and interrupts, which I've cross compiled and run successfully inside a javascript emulator.

     Since the machine language is so simple, an emulator for the One-Page CPU can be written in a few hundred lines.  Recently, I decided to make a C based implementation of the emulator, which now means you can run a trivial microkernel as if it were a single program inside of C or Javascript.  In the future, I hope to write emulators for a large number of languages, so that C programs and microkernels can be embedded into the majority of other languages.

     The kernel doesn't do much at the moment, but since it supports inter-process communication and preemptive context switching, you can easily add your own user tasks.  You can try it out for yourself by running the commands below (assuming you're on Linux):

#  Clone the repo
git clone https://github.com/RobertElderSoftware/recc.git && cd recc && git checkout 1657a285a190316a94a734e9a5fbc7e3986d6e40
#  Build the kernel and the linux-based C emulator
make bootstrap-datatypes && make build-kernel && make emulators/linux-emulator-example
#  Use the emulator to run the kernel directly from the command-line
emulators/linux-emulator-example kernel/kernel.l1

     After running these commands, you should see something similar to this:

     You'll also get the same thing if you run the kernel in the Javascript emulator:

     A live version of the Javascript emulator running the full kernel is can be found here.  The Javascript version of the emulator also comes with a nice visualization for debugging:

     You're not limited to running the kernel either.  If you want, you can create arbitrary programs that have multiple external symbols and link them together.  As long as they have a main method, they should run fine in either emulator.

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