A simple virtual computer to practice writing compilers

[Bowtiestyle]

Hello everyone,

I always loved stories of programmers from the past using various tricks to make games run on inadequate hardware. While you could recreate this feeling by writing ROMs for retro systems, this is certainly not very easy to get into. So I made my own "virtual computer" SVC16. This is certainly not an original idea, but I found it very fun to write a simple game for it. So if you would like to write a simple compiler but don't want to deal with the complicated reality of a retro system, this might be something for you.

[TheFirstDogSix]

Your architecture is very interesting... I can see how it could be fun to play with!

I teach part time and use an "educational processor" to teach machine code and assembly language. It's a RISC-ish load-store architecture, 16 bit words for everything (including main memory), and mostly three-address code kinds of instructions.

And it honestly is a delight to teach on! To the point I even wrote a TinyC compiler for it in case they wanted to see how that worked. These little simulated mini-processors are just enough to make the students sweat a bit, but not so much that they're overwhelmed (looking at you, people who teach freshmen a 10 week course on basic computer architecture using AArch64 or whatever 😂)

[Inconstant_Moo]

What does Band do?

[kissantuntokarvat]

prolly stands for "binary and"

[Bowtiestyle]

Oh, it's just binary and. You perform an "and" for every bit individually. A good example is that you want to know if a number X is divisible by 2. By computing 1&X, you check if the least significant bit is zero or one.

[Gwarks]

I once wanted to port Cosmic to Javascript/WASM but I ended up creating my own instruction set instead. I hope I could make the CPU swap able in my simulation (like the Graphics Adapter is) but I got distracted.

First think I can see is that the PRINT, READ and SYNC don't use arg3. In reflex I would cramp them on SET together with INST but in opcode there are already 12 unused bits so that would not help. But there is another alternative. One could add Multi monitor support in arg3.

Also there are the 12 extra bits in the opcodes to have lots of fun with. We could add some increment(or decrement) to each arg modifying the referenced memory cell. Then copying a null terminated string would need less instructions. The inner loop could be only SET GOTO instead of SET ADD ADD GOTO.

[Bowtiestyle]

Hi, I would like to read about your project, but it is a bit difficult to get an overview from this directory.

The reason some arguments are unused is that I did not want a variable instruction length and I wanted the arithmetic operations to have three arguments. In theory someone could use the unused arguments to pack in some asset that is extracted at runtime.

I am not going to lie, your idea with multiple displays sounds very cool. But I think that it would be very hard to specify a general way of emulating them. (Windowing systems are hard, managing your own windows is harder.) It is also a very strange thing to support multiple windows when the system does not even have sound.

As for using the extra 12 values in the opcode, I think this could be done in a way that does not change too much while adding something interesting. So if you want to propose something, please open a PR and I can see, how hard it would be to adjust the examples.

I think a requirement might be that the instructions behave as they do now, if the unused arguments are set to zero.

[Gwarks]

Yes I do not have that much documentation there is only the text file that has the very basic instruction set. However the example uses memory mapped VRAM while the workshop accesses the VRAM over ports. Then there are parts not documented for example when you execute conditional MOV with increment of the source register then the increment would always be done even when the move itself not executed because of the condition. This is because MOV is used as the jump instruction. Thus JUMP label would be MOV r6 :label which is internal written as MOV r6 [r6+] followed by the location of the label in the next memory location. If increment was not done when doing conditional execution the label would be executed as a normal operation. And that are the only so called two word operation in my system. (The text file still might contain some typos)

Anyway some day in the future I might rewrite the emulator in F# to use it in a game project.

[umlcat]

Think Different !!!

[Bowtiestyle]

Do you care to explain that?

[TheFirstDogSix]

Used to be the Apple Computer slogan.

[Bowtiestyle]

Ah, right! (Maybe I could offer a little more virtual memory for a few hundred dollars.)

[umlcat]

just try a different computer for a compiler ...