Virtual machine in C++

Posted on October 18, 2013

This is not a tutorial. This post is a flashback I had today. It might be a bit fiction as my memory about events tends to be fuzzy at times. But I promise it at least resembles the real story. I was in elementary school and just found out about programming and was learning about c++. After reading “C++ na kolenih” by Goran Bervar I was empowered by knowledge and tried to do all sorts of projects. Mostly in console. Stuff like subtitle format converter - NIH syndrome. I was a bit frustrated because I couldn’t find any books about windows programming in the library. Yes, library was may primary source of information, because my English was not nearly good enough for technical stuff. I might add here I worked on Windows 98(and later XP) with DevC++. I found out about Visual Studio in a few years and did some Windows development. I digressed a bit. Then came the most optimistic idea. A virtual machine. Something quite high level(instruction to print) an eventually an assembler. I now realize I was always into language stuff. So a designed a machine language with just enough instructions to do Hello World, that is PRINT and END.


At first I thought about doing a monolithic structure - switch case(in fact what I’ve done with scrat recently). But I had some considerations. What if number of of instruction rises a lot? I’ll be left maintaining spaghetti code. Or at least I thought that’s what spaghetti code looks like, but in retrospective I believe I had a good taste anyway. 

But I tried that anyway. Just for kicks. Did whole machine as one class that had an array for memory and a single point of entry - boot. It run a loop a while loop with PC<-PC+1, fetched instruction from memory, switched on them, called appropriate method to implement that instruction and looped. Even had registers. I think my current professor of Computer Architecture(this course brought back the memory) might actually be proud if he heard what I did back then. 


I was always quite comfortable with pointers. I don’t now, they’re mathematicky concept. I like such stuff. Or perhaps it was because I was young when I was introduced into the matter and wasn’t spoiled with automatic memory management(which I quite like nowadays). 

So I tried with function pointers. C is cool enough to let you have pointers to functions! And that means higher order functions. But I didn’t know about math enough to appreciate the concept as I do now. But still - I thought it’s extremely cool. So I did a function that halted execution and printed out “no such instruction”. Why you ask? Well I did a 256-cell table(8-bit instruction) of pointers to functions. Now I didn’t have to switch - just a look-up and invocation. Great. Apart from the fact it doesn’t work. 

Compiler said something along the lines of “You cannot make a table of pointers to functions!”. I was puzzled. Skip 10 years into the future. Today I was rethinking this and thought about casting the pointer. All the functions would be void->void so I can cast back no problem. A table of void pointers and casting. Yay!

Now 10 years back. I didn’t think about casting the pointer. Type info was sacred to me!

So I “invented” function objects.


I swear to god I have not heard about function objects back then. It wasn’t until this year reading Bloch’s Efficient Java where he talks about strategy objects. I immediately recognized my idea. So now I had many classes, every one implementing execute method. And I had an array of these objects. Now I did a look-up and invocation on an object. Sweet. And it even worked. But sadly I dropped the project and went on to graphics. Learnt SDL and did Tic-Tac-Toe. And dreamed about doing a vector 3D engine(curves baby!). Which until this day I didn’t try to implement. Maybe I’ll try in near future. 

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