Implementing a Virtual Machine in C • Hacker News Board

Re: Implementing a Virtual Machine in C

#11

Post by ddfreyne » Sat, May 09, 2015, 8:31 PM UTC

jcoffland wrote: post #9

I'd like to see such an article on a register based VM. Pawn and Lua are nice examples. Most VMs are stack based but this is mainly because they are conceptually easier to understand. Register based machines have some real advantages, like requiring far fewer instructions inside tight loops.

I created a register-based VM named RCPU, for educational purposes. You can find it at https://github.com/ddfreyne/rcpu. It has most of what you’d expect from a register-based VM, and even has video output (rcpu-assemble samples/video.rcs and then rcpu-emulate the resulting file with --video).

I intend to write up my findings, but haven’t gotten around to it yet. I do have slides for a talk for this project though: https://speakerdeck.com/ddfreyne/simulating-a-cpu-with-ruby

Re: Implementing a Virtual Machine in C

#12

Post by voidiac » Sat, May 09, 2015, 8:34 PM UTC

jcoffland wrote: post #9

I'd like to see such an article on a register based VM. Pawn and Lua are nice examples. Most VMs are stack based but this is mainly because they are conceptually easier to understand. Register based machines have some real advantages, like requiring far fewer instructions inside tight loops.

Register based VMs are not more complex than stack based VMs. You can even have a stack and the only difference is that your instructions operate on registers and not directly on the stack. I only consider generating code more complex with a register based VM.

If you are interested here is a toy vm I wrote myself: https://github.com/byo3rn/ire

PS: I suck at documentation.

Re: Implementing a Virtual Machine in C

#13

Post by neverartful » Sat, May 09, 2015, 9:13 PM UTC

Contrary to the naysayers, I like seeing stuff like this. Why? Because it's a simple, gentle introduction. It's easily digestible for the newcomer. And it might be easy enough to encourage a newcomer to start building their own VM that goes on to be something real.

For those that criticize it and find faults with it -- I'm sure the author would consider pull requests. Or you could provide your own fork with all the improvements that you believe are necessary.

Re: Implementing a Virtual Machine in C

#14

Post by phodo » Sat, May 09, 2015, 9:13 PM UTC

While seemingly simple, the simple non-turing example is not too far off from the (simple) Forth-like stack-based programming language found and executed in bitcoin transactions.

https://en.bitcoin.it/wiki/Script

Re: Implementing a Virtual Machine in C

#15

Post by ternaryoperator » Sat, May 09, 2015, 9:24 PM UTC

I find these kinds of very basic intro articles frustrating. They till the same ground over and over: a tiny instruction set implemented with a switch statement. None of the more difficult issues are addressed: exception handling, linking to libraries or other programs written for the same VM, portability of programs across architectures, accessing the OS for services like file I/O, time, etc.-- All the things that make a toy not a toy.

Every CS student in the world has written a toy VM just like this one.

Re: Implementing a Virtual Machine in C

#16

Post by maguirre » Sat, May 09, 2015, 9:36 PM UTC

ternaryoperator wrote: post #15

I find these kinds of very basic intro articles frustrating. They till the same ground over and over: a tiny instruction set implemented with a switch statement. None of the more difficult issues are addressed: exception handling, linking to libraries or other programs written for the same VM, portability of programs across architectures, accessing the OS for services like file I/O, time, etc.-- All the things that m…

I feel the same way. I went to read the post expecting a lot more than what I found and came away feeling both more knowledgeable than I thought I was and more ignorant for not knowing that I could get away with calling what a saw a "Virtual Machine"

Re: Implementing a Virtual Machine in C

#17

Post by tptacek » Sat, May 09, 2015, 9:38 PM UTC

ternaryoperator wrote: post #15

I find these kinds of very basic intro articles frustrating. They till the same ground over and over: a tiny instruction set implemented with a switch statement. None of the more difficult issues are addressed: exception handling, linking to libraries or other programs written for the same VM, portability of programs across architectures, accessing the OS for services like file I/O, time, etc.-- All the things that m…

The details you're talking about are pretty much just details; for instance, a foreign function interface for C functions (and thus system calls) is a pretty straightforward exercise that involves just a bit about the C ABI for your platform.

It's much easier to teach someone who knows how to build a basic register machine how to set up the stack for a call to a C function than it is to teach someone who doesn't know how a register machine works how to build one.

Also: there are reasons you might want to build a simple VM that has no access to system services and no notion of an exception. BPF is a good example of such a virtual machine.

If there's something frustrating about this article to me, it's that it lacks a motivating example. Why would you want to build this VM? What are you compiling down to it from?

Re: Implementing a Virtual Machine in C

#18

Post by tptacek » Sat, May 09, 2015, 9:39 PM UTC

maguirre wrote: post #16

ternaryoperator wrote: post #15

I find these kinds of very basic intro articles frustrating. They till the same ground over and over: a tiny instruction set implemented with a switch statement. None of the more difficult issues are addressed: exception handling, linking to libraries or other programs written for the same VM, portability of programs across architectures, accessing the OS for services like file I/O, time, etc.-- All the things that m…

I feel the same way. I went to read the post expecting a lot more than what I found and came away feeling both more knowledgeable than I thought I was and more ignorant for not knowing that I could get away with calling what a saw a "Virtual Machine"

It's an instruction set, with an instruction dispatcher, a stack, and a register file. Why would it surprise you that someone would call it a VM?

Are you maybe getting your signals crossed between the kind of VM this article is talking about (in the p-code sense of a VM) and virtualization systems?

Re: Implementing a Virtual Machine in C

#19

Post by voidiac » Sat, May 09, 2015, 9:43 PM UTC

aftbit wrote: post #2

I'm a bit disappointed - this VM doesn't have instructions for looping or branching, nor does it really use the registers in any way. I was hoping to read a writeup that introduced some concepts that were used in real (non-toy) systems.

Branching would be something like:

  case JMP: {
    ip = program[ip + 1] - 1;
    break;
  }

Re: Implementing a Virtual Machine in C

#20

Post by tptacek » Sat, May 09, 2015, 9:46 PM UTC

jcoffland wrote: post #9

I'd like to see such an article on a register based VM. Pawn and Lua are nice examples. Most VMs are stack based but this is mainly because they are conceptually easier to understand. Register based machines have some real advantages, like requiring far fewer instructions inside tight loops.

Stack VMs aren't used just because they're easier to understand:

* In interpreted environments, registers are stored in memory anyways, so the advantage of simulating them isn't as great

* It is easier to generate code for stack machines, because you don't need to run register allocation

* There's a tradeoff in instruction complexity versus number of instructions between stack and register machines