Entries tagged simple-vm

Related tags: golang, kindle, random, travel.

Another golang port, this time a toy virtual machine.

Monday, 2 July 2018

I don't understand why my toy virtual machine has as much interest as it does. It is a simple project that compiles "assembly language" into a series of bytecodes, and then allows them to be executed.

Since I recently started messing around with interpreters more generally I figured I should revisit it. Initially I rewrote the part that interprets the bytecodes in golang, which was simple, but now I've rewritten the compiler too.

Much like the previous work with interpreters this uses a lexer and an evaluator to handle things properly - in the original implementation the compiler used a series of regular expressions to parse the input files. Oops.

Anyway the end result is that I can compile a source file to bytecodes, execute bytecodes, or do both at once:

I made a couple of minor tweaks in the port, because I wanted extra facilities. Rather than implement an opcode "STRING_LENGTH" I copied the idea of traps - so a program can call-back to the interpreter to run some operations:

int 0x00  -> Set register 0 with the length of the string in register 0.

int 0x01  -> Set register 0 with the contents of reading a string from the user


This notion of traps should allow complex operations to be implemented easily, in golang. I don't think I have the patience to do too much more, but it stands as a simple example of a "compiler" or an interpreter.

I think this program is the longest I've written. Remember how verbose assembly language is?

Otherwise: Helsinki Pride happened, Oiva learned to say his name (maybe?), and I finished reading all the James Bond novels (which were very different to the films, and have aged badly on the whole).

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Writing your own e-books is useful

Wednesday, 8 October 2014

Before our recent trip to Poland I took the time to create my own e-book, containing the names/addresses of people to whom we wanted to send postcards.

Authoring ebooks is simple, and this was a useful use. (Ordinarily I'd have my contacts on my phone, but I deliberately left it at home ..)

I did mean to copy and paste some notes from wikipedia about transport, tourist destinations, etc, into a brief guide. But I forgot.

In other news the toy virtual machine I hacked together got a decent series of updates, allowing you to embed it and add your own custom opcode(s) easily. That was neat, and fell out naturely from the switch to using function-pointers for the opcode implementation.



Before I forget, a simple virtual machine

Sunday, 5 October 2014

Before I forget I had meant to write about a toy virtual machine which I'ce been playing with.

It is register-based with ten registers, each of which can hold either a string or int, and there are enough instructions to make it fun to use.

I didn't go overboard and write a complete grammer, or a real compiler, but I did do enough that you can compile and execute obvious programs.

First compile from the source to the bytecodes:

$ ./compiler examples/loop.in

Mmm bytecodes are fun:

$ xxd  ./examples/loop.raw
0000000: 3001 1943 6f75 6e74 696e 6720 6672 6f6d  0..Counting from
0000010: 2074 656e 2074 6f20 7a65 726f 3101 0101   ten to zero1...
0000020: 0a00 0102 0100 2201 0102 0201 1226 0030  ......"......&.0
0000030: 0104 446f 6e65 3101 00                   ..Done1..

Now the compiled program can be executed:

$ ./simple-vm ./examples/loop.raw
[stdout] register R01 = Counting from ten to zero
[stdout] register R01 = 9 [Hex:0009]
[stdout] register R01 = 8 [Hex:0008]
[stdout] register R01 = 7 [Hex:0007]
[stdout] register R01 = 6 [Hex:0006]
[stdout] register R01 = 5 [Hex:0005]
[stdout] register R01 = 4 [Hex:0004]
[stdout] register R01 = 3 [Hex:0003]
[stdout] register R01 = 2 [Hex:0002]
[stdout] register R01 = 1 [Hex:0001]
[stdout] register R01 = 0 [Hex:0000]
[stdout] register R01 = Done

There could be more operations added, but I'm pleased with the general behaviour, and embedding is trivial. The only two things that make this even remotely interesting are:

  • Most toy virtual machines don't cope with labels and jumps. This does.
    • Even though it was a real pain to go patching up the offsets.
    • Having labels be callable before they're defined is pretty mandatory in practice.
  • Most toy virtual machines don't allow integers and strings to be stored in registers.
    • Now I've done that I'm not 100% sure its a good idea.

Anyway that concludes todays computer-fun.



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