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A lightweight Virtual Machine for dynamic, object-oriented languages. It aims to be fast, as simple as possible, easily optimizable, with LLVM support, and easily targetable for language designers and implementors. That's why its interface (instruction set and bytecode format) is extensively documented and an example compiler is provided under the compiler folder.

Before anything, I want to give special thanks to my awesome mentors Jeremy Tregunna, Brian Ford, Dirkjan Bussink and Evan Phoenix. Without their teachings and patience I would never have started this in the first place.


I'd love to discuss literally anything about my choices regarding the design and the implementation of TerrorVM. Feel free to ping me on twitter, drop me an email, or if you are in Berlin, just grab some beers together :) After all,

I have no idea what I'm doing

Object model

In TerrorVM, everything is an object, and every object may have a prototype. The basic value types that the VM provides are:

  • Number: Double-precision floating point numbers.
  • String: Immutable strings.
  • Vector: Dynamically sized vectors that may contain any type.
  • Map: Hashmaps (for now only strings are supported as keys).
  • Closure: A first-class function.
  • True: True boolean.
  • False: False boolean.
  • Nil: Represents nothingness. It is falsy just like false.

These basic types are objects themselves (of type Object). They are the prototype for any objects of their own kind, and are provided with all the functionality that those objects will need -- this is done in the preludes (alpha and beta), I'll explain what those are a bit further ahead.

Objects are simply collections of slots that may contain any kind of object.

I'm considering adding Traits, although I'll wait until I see a need for it. In the simplicity of Terror lies its power.

The VM runtime object

TerrorVM exposes as much of itself as possible at runtime. The goal of this is to make it easily targetable and flexible. For example, the toplevel object VM exposes two subobjects (types and primitives). VM.types is a map of all the VM types like this:

  :object => Object,
  :vector => Vector,
  :number => Number,

Primitives contains all the native functions that the VM exposes (such as puts, print, clone, arithmetic primitives, etc):

  :clone => #<Closure ...>,
  :puts => #<Closure ...>,


As you already know, TerrorVM tries to implement as much as possible in its own code, rather than C. This makes it a perfect candidate as a multi-language VM to implement any language on top of it. You can find the high-level alpha and beta preludes and their respective compiled versions alpha and beta.

The alpha prelude wires up the VM primitives to the real objects at runtime, so that your code can use them conveniently. This is our current prelude in high-level Ruby (interpreted by the VM in the bootstrap phase):

VM.types[:object].clone = VM.primitives[:clone]
VM.types[:object].print = VM.primitives[:print]
VM.types[:object].puts  = VM.primitives[:puts]

VM.types[:number][:+] = VM.primitives[:'number_+']
VM.types[:number][:-] = VM.primitives[:'number_-']
VM.types[:number][:/] = VM.primitives[:'number_/']
VM.types[:number][:*] = VM.primitives[:'number_*']

VM.types[:vector][:[]] = VM.primitives[:'vector_[]']
VM.types[:vector][:to_map] = VM.primitives[:vector_to_map]

Beautiful, isn't it? :)

In later stages, such as beta, we define other high-level functions like Vector#map.

If you wish to change any kernel files such as the prelude, you'll have to recompile the files to the native TVM format, like this:

$ make kernel

And if you add more high-level examples (in Ruby) under the compiler/examples folder, you must recompile them as well:

$ make examples

Debugging your TerrorVM programs

TerrorVM ships with a debugger that you can use to debug your programs. The debugger can set breakpoint at specific lines and step through either high-level lines of code or low-level bytecode instructions.

To use the debugger, pass -d as a second argument to terror:

$ bin/terror -d examples/functions.tvm

Here's an example session:

1    > a = 123
2      foo = 123
3      self.fn = -> foo {
4        # foo is shadowed because it's a local argument

> n
DEBUG src/terror/vm.c:82: PUSH 0
DEBUG src/terror/vm.c:284: SETLOCAL 0

1      a = 123
2    > foo = 123
3      self.fn = -> foo {
4        # foo is shadowed because it's a local argument
5        a + foo


The debugger will always show you the high-level code (in compiler/examples/functions.rb) so you know where you are at every point.

The commands for the debugger are:

h: show help
s: step to the next bytecode instruction
n: step to the next line of code
c: continue execution
d: show the stack
l: show locals
t: show backtrace
b: set breakpoint in a line. Example: b 30

Implementing your own dynamic language running on TerrorVM

TerrorVM is designed to run dynamic languages. You can easily implement a compiler of your own that compiles your favorite dynamic language down to TVM bytecode.

I've written a demo compiler in Ruby under the compiler/ folder, just to show how easy it is to write your own. This demo compiler compiles a subset of Ruby down to TerrorVM bytecode, so you can easily peek at the source code or just copy and modify it.

You can write your compiler in whatever language you prefer, of course.

Garbage collection

The algorithm of choice for TerrorVM was Baker's treadmill, an incremental, real-time, non-moving GC algorithm, implemented in libtreadmill.

Unfortunately I couldn't make it work so for now I'm using a simple Mark and Sweep implemented in libsweeper as a separate library and included via Git submodules.


This is a really important topic these days, not to be overlooked. Although its concurrency support is not in place yet, it will feature forking, threads and coroutines, but I might change my mind as I learn more.

Bytecode format

The bytecode format might change to be more compact, but I'll describe what it is for now. A file must contain a main block, and may contain other blocks (functions defined there). This is how a block looks like (if you're curious, it's just a hello world):

"hello world
128 SEND
144 RET

As you can see, _main, defines the entry point of the file. Then these mysterious numbers :2:8 mean that this block has two literals and eight lines of instructions. There are actually only 5 instructions, but the operands for these instructions count as well, so we're in a total of 8.

Right after these counts, we have the literals, each one in its own line. There are two kinds of literals: numbers and strings. Numbers are just numbers, but strings must be preceded by a ".

And finally we get to eight lines of numbers, namely the instructions and their operands. The labels you see beside every instruction (PUSHSELF) are totally optional, the VM doesn't read them, but they help debugging when looking at a bytecode file manually.

After that there might be more functions. Imagine our hello world defined an empty closure, then we'd have right after 144 RET:

144 RET

That's it! :)

Examples (high-level Ruby code and its Terror compiled counterpart)

Instruction set

  • NOOP: no operation -- does nothing.


  • PUSHSELF: pushes the current self to the stack.
  • PUSHLOBBY: pushes the Lobby (toplevel object) to the stack.
  • PUSH A: pushes the literal at index A to the stack.
  • PUSHTRUE: pushes the true object to the stack.
  • PUSHFALSE: pushes the false object to the stack.
  • PUSHNIL: pushes the nil object to the stack.

Local variables

  • PUSHLOCAL A: pushes the local at index A to the stack.
  • SETLOCAL A: sets the current top of the stack to the local variable A. Does not consume any stack.
  • PUSHLOCALDEPTH A, B: pushes the local at index B from an enclosing scope (at depth A) to the stack.
  • SETLOCALDEPTH A, B: sets the current top of the stack to the local variable B in an enclosing scope (at depth A). Does not consume any stack.


  • JMP A: Skips as much as A instructions.
  • JIF A: Pops the top of the stack and skips as much as A instructions if it is falsy (false or nil).
  • JIT A: Pops the top of the stack and skips as much as A instructions if it is truthy (any value other than false or nil).

Slots (attributes)

  • GETSLOT A: Pops the object at the top of the stack and asks for its slot with name A (a literal), pushing it to the stack if found -- if not, it'll raise an error.
  • SETSLOT A: Pops a value to be set, then pops the object at the top of the stack and sets its slot with name A (a literal) to the value that was first popped. Then pushes that value back to the stack.


  • POP N: pops N values off the stack.
  • DEFN A: takes the closure with the name A (a literal) and pushes it to the stack.
  • MAKEVEC A: Pops as much as A elements off the stack and pushes a vector with all of them in the order they were popped (the reverse order they were pushed in the first place).

Call frames

  • SEND A, B: Pops as much as B arguments off the stack, then the receiver, and sends it the message with the name A (a literal) with those arguments.


  • DUMP: Prints the contents of the value stack to the standard output.

Building the VM

Since TerrorVM makes use of Clang's block extension, you'll need at least clang 3.4 to compile it.

$ git clone git://
$ cd terrorvm
$ make

To run the tests:

$ make dev

And to clean the mess:

$ make clean

If you want to run the tests under Valgrind to ensure there are no memory leaks:

$ make valgrind

Running programs

TerrorVM runs .tvm bytecode files such as the numbers.tvm under the examples directory.

$ bin/terror examples/numbers.tvm

It ships with a simple compiler written in Ruby (Rubinius) that compiles a tiny subset of Ruby to .tvm files. Check out the compiler directory, which has its own Readme, and the compiler/examples where we have the hello_world.rb file used to produce the hello_world.tvm.

TerrorVM doesn't need Ruby to run; even the example compiler is a proof of concept and could be written in any language (even in C obviously).

The terror executable acts as a wrapper for the example compiler as well. To compile and run on the fly a file written in our subset of Ruby:

$ bin/terror -x compiler/examples/numbers.rb

To compile a file yourself:

$ bin/terror -c output.tvm path/to/my/input.rb

In case of doubt, -h is your friend :)

$ bin/terror -h

Who's this

This was made by Josep M. Bach (Txus) under the MIT license. I'm @txustice on twitter (where you should probably follow me!).


  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Added some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request


Lightweight, fast Virtual Machine for dynamic, object-oriented languages.







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