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Comment by euroclydon | Hacker News Reader

Thomas, You've mentioned that your new-programming-language-exercise du jour is to write a virtual machine (or microcontroller emulator), right? I'd like to try that in C. How would you go about writing this in a simple way. I know I could google this, but I'm worried I'll find results with too many details and pollute my discovery process.

Really, I'd just like some opinionated answers to the following from anyone who cares:

1) What is a simple instruction set that I can support?

2) What existing programs are available for that instructions set, or what set of tools are there to compile to that instructions set?

3) What are the high level tasks that I will need to accomplish to write this virtual machine?

4) What should I look up or borrow exclusively versus figure out for myself?

Thanks!

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1) AVR

2) You'd compile simple C programs, of which there are zillions, rather than writing complex programs in assembly.

3) Make a struct that captures the state of the CPU: an array of integers for the register file, a flag word for the CPU flags, &c. Decode instructions (to a struct or something, which captures all the options of the instruction). Execute one instruction (pick it yourself): resolve its operands into temporary variables, execute the logic (almost invariably trivial), set the appropriate processor flags (overflow, zero, &c), and then store the result. Test lightly, and then repeat for all the other instructions. Most will be the same except for a single line of code. Make a big u_char array to represent memory. Write a HEX file loader, which will take a .hex file and populate memory with its contents; the GCC toolchain will compile C programs to HEX files. Now write the code to load an instruction, execute it, set the program counter appropriately, and repeat. Spend the next 2 weeks debugging.

4) I say, do it yourself. You can get yourself tied up in knots reading all the different ways to implement a VM. To start with, write a naive VM yourself with no help. Then go back and read that stuff if you want; it'll make much more sense.

Incidentally, "AVR" == "Arduino", so if you want a HEX file to execute you can have the Arduino toolkit drive the GCC cross-compiler for you, while you sit back sipping mojitos. Download the Arduino toolkit, open up the "Blink" demo, and press "Verify / Compile". Watch the little message window as your code compiles: Near the end of the process it will cough up a line like this:

/var/folders/4x/cwc9yvnx071_9whsft75cjvc0000gn/T/build8742301187783484704.tmp/Blink.cpp.hex

That file seems like it would be a fun thing to explore.

Scheew! I think I'll close the nongnu.org tab I have open and go get that mojito right now. Thanks!!!

If you don't mind I'll add my 2c here as well, since I've written my own compiler, parser and VM.

I just read a book about compiler construction (can't remember the name, but it's yellow - I can find out if you want) and that was all I needed.

It's actually a lot easier than you might expect to write a compiler for a C-like language (I added some object-oriented features, but it's basically C). I don't even have a CS degree. Although you can use tools like yacc and lex to help with the parsing, I decided to just do it myself. The compiler construction books tells you how to do this and it's fairly straightforward.

If you've done assembly language programming before you will already have a fair idea of what instructions you need. Just look at the z80 or 6502 or other simple cpu. I just used the stack for all operations (no registers) to keep things simple. Also I don't support all C features to keep things simple (no debugging, no pointers-to-pointers, all declarations must come before any definitions, etc).

The virtual machine is very easy to write - you basically just read in the object, do stuff like load string tables, then just have a loop reading each instruction and dispatch it using a function table. You probably also want to have a system library for doing stuff like accessing files, network i/o or whatever.

If you want to discuss further, let me know.

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