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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

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.

maguirre 11y ago
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"
- tptacek 11y ago
  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?
  - maguirre 11y ago
    I was not surprised that it was called a VM. I was surprised that I didn't know that!
- bitwize 11y ago
  "Virtual machine" here means "abstract machine" as in JVM or CLR, not "multiplexed hardware".
tptacek 11y ago
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?
dandrews 11y ago
That's a little ungenerous of you. The toy VM has presented an aha! moment to all of us at one time - or more than once, when you learn about Turing equivalence and the rabbit hole that leads from that. Skip the articles beneath your skill level and move along.
- ternaryoperator 11y ago
  Agreed, it was a bit ungenerous, esp. b/c I didn't realize til someone mentioned it below that the author was a teenager. My objection, though, is not what's above/below me, it's that given the title, I was expecting a deeper article only to find one that has appeared numerous times. It was frustration I was expressing more than condescension. But you're quite right in using the term "ungenerous." I appreciate the gentle reproof.
tylermac1 11y ago
Considering the author is 16, I think it's a great write up/exercise.
- becausecomputer 11y ago
  Thank you for raising this. My faith in humanity has been restored.
rounak 11y ago
You're being unnecessarily harsh. If you got frustrated means it wasn't targeted towards you. Would appreciate if you did an advanced version of this writeup :)
titanix2 11y ago
Since I also wrote a toy VM where the toolchain supports linking of different code files (albeit the runtime doesn't implement CALL / RET) I decided to make a blog post about this specific topic. https://netspring.wordpress.com/2015/05/10/toy-virtual-machine-modular-compilation/ https://netspring.wordpress.com/2015/05/10/toy-virtual-machi...

maguirre 11y ago

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"

tptacek 11y ago
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?
- maguirre 11y ago
  I was not surprised that it was called a VM. I was surprised that I didn't know that!
bitwize 11y ago
"Virtual machine" here means "abstract machine" as in JVM or CLR, not "multiplexed hardware".

tptacek 11y ago

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?

dandrews 11y ago

That's a little ungenerous of you. The toy VM has presented an aha! moment to all of us at one time - or more than once, when you learn about Turing equivalence and the rabbit hole that leads from that. Skip the articles beneath your skill level and move along.

ternaryoperator 11y ago
Agreed, it was a bit ungenerous, esp. b/c I didn't realize til someone mentioned it below that the author was a teenager. My objection, though, is not what's above/below me, it's that given the title, I was expecting a deeper article only to find one that has appeared numerous times. It was frustration I was expressing more than condescension. But you're quite right in using the term "ungenerous." I appreciate the gentle reproof.

tylermac1 11y ago

Considering the author is 16, I think it's a great write up/exercise.

becausecomputer 11y ago
Thank you for raising this. My faith in humanity has been restored.

rounak 11y ago

You're being unnecessarily harsh. If you got frustrated means it wasn't targeted towards you. Would appreciate if you did an advanced version of this writeup :)

titanix2 11y ago

Since I also wrote a toy VM where the toolchain supports linking of different code files (albeit the runtime doesn't implement CALL / RET) I decided to make a blog post about this specific topic. https://netspring.wordpress.com/2015/05/10/toy-virtual-machine-modular-compilation/ https://netspring.wordpress.com/2015/05/10/toy-virtual-machi...