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I recently finished this book called The Elements of Computing Systems where you build a working computer system from the ground up, starting from basic logic gates, to creating your own machine code and Assembly language, to intermediate code, and finally a simple object-oriented programming language that compiles down to VM code. I enjoyed it a lot and I'd like to create something similar in JavaScript, but with more features. I've already written an emulator for the Hack machine in JS:

  // Creates a new CPU object that is responsible for processing instructions
  var CPU = function() {

var D = 0;    // D Register    
var A = 0;    // A Register
var PC = 0;   // Program counter


// Returns whether an instruction is valid or not
var isValidInstruction = function(instruction) {
    if (instruction.length != 32)
        return false;

    instruction = instruction.split(""); 

    for (var c = 0; c < instruction.length; c++)
    {
        if (instruction[c] != "0" && instruction[c] != "1")
            return false;
    }

    return true;
};  


// Given an X and Y input and 6 control bits, returns the ALU output
var computeALU = function(x, y, c) {

    if (c.length != 6)
        throw new Error("There may only be 6 ALU control bits");

    switch (c.join(""))
    {
        case "000000": return 0; 
        case "000001": return 1; 
        case "000010": return -1; 
        case "000011": return x; 
        case "000100": return y; 
        case "000101": return ~x;
        case "000110": return ~y;
        case "000111": return -x; 
        case "001000": return -y; 
        case "001001": return x+1; 
        case "001010": return y+1;
        case "001011": return x-1;
        case "001100": return y-1;
        case "001101": return x+y;
        case "001110": return x-y;
        case "001111": return y-x;
        case "010000": return x*y;
        case "010001": return x/y;
        case "010010": return y/x;
        case "010011": return x%y;
        case "010100": return y%x;
        case "010101": return x&y;
        case "010110": return x|y;
        case "010111": return x^y;
        case "011000": return x>>y;
        case "011001": return y>>x;
        case "011010": return x<<y;
        case "011011": return y<<x;

        default: throw new Error("ALU command " + c.join("") + " not recognized"); 
    }
}; 


// Given an instruction and value of Memory[A], return the result
var processInstruction = function(instruction, M) {

    if (!isValidInstruction(instruction))
        throw new Error("Instruction " + instruction + " is not valid");

    // If this is an A instruction, set value of A register to last 31 bits
    if (instruction[0] == "0")
    {
        A = parseInt(instruction.substring(1, instruction.length), 2);

        PC++; 

        return {
            outM: null,
            addressM: A,
            writeM: false,
            pc: PC
        }; 
    }

    // Otherwise, this could be a variety of instructions
    else
    {
        var instructionType = instruction.substr(0, 3);
        var instructionBody = instruction.substr(3);

        var outputWrite = false; 

        // C Instruction - 100 c1, c2, c3, c4, c5, c6 d1, d2, d3 j1, j2, j3 (000..000 x16)
        if (instructionType == "100")
        {
            var parts = [ "a", "c1", "c2", "c3", "c4", "c5", "c6", "d1", "d2", "d3", "j1", "j2", "j3" ];
            var flags = {}; 

            for (var c = 0; c < parts.length; c++)
                flags[parts[c]] = instructionBody[c]; 

            // Compute the ALU output
            var x = D;
            var y = (flags["a"] == "1") ? M : A; 
            var output = computeALU(x, y, [flags["c1"], flags["c2"], flags["c3"], flags["c4"], flags["c5"], flags["c6"]]); 

            // Store the result
            if (flags["d1"] == "1") A = output; 
            if (flags["d2"] == "1") D = output;
            if (flags["d3"] == "1") outputWrite = true; 

            // Jump if necessary
            if ((flags["j1"] == "1" && output < 0) || (flags["j2"] == "1" && output == 0) || (flags["j3"] == "1" && output > 0)) 
                PC = A;
            else
                PC++; 

            // Return output
            return {
                outM: output,
                addressM: A,
                writeM: outputWrite,
                pc: PC
            }; 
        }

        else throw new Error("Instruction type signature " + instructionType + " not recognized");
    }
}; 


// Reset the CPU by setting all registers back to zero
this.reset = function() {
    D = 0;
    A = 0;
    PC = 0;
}; 


// Set the D register to a specified value
this.setD = function(value) {
    D = value;
}; 


// Set the A register to a specified value
this.setA = function(value) {
    A = value;
}; 


// Set PC to a specified value
this.setPC = function(value) {
    PC = value;
};


// Processes an instruction and returns the result
this.process = function(instruction, M) {
    return processInstruction(instruction, M); 
}; 
}; 

I was thinking about adding things like a filesystem, sound, Internet connectivity, and an RGBA screen output (currently it's only black and white). But how feasible would this be, really?

Because what I'm thinking about doing is starting completely from scratch. And what I mean by that is create my own machine code, then work all the way up towards a C-like language and actually create working programs and stuff.

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11  
It's entirely feasible. bellard.org/jslinux –  World Engineer Mar 3 '13 at 5:32
4  
Just go for it and see how far you get. Even if you fail in your ultimate goal I'm sure you'll learn tons, and it sounds like that's your main motivation. –  James Mar 3 '13 at 8:30
2  
Don't use strings, javascript supports 32 bit integers and bitwise operations on them –  Esailija Mar 3 '13 at 11:07
    
Number is the only real "bad" part IMO. –  Erik Reppen Mar 5 '13 at 5:47
    
Also, this makes me want to ask. Has any dynamic interpreted language ever not had a layer between it and machine language? –  Erik Reppen Mar 5 '13 at 5:51
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1 Answer

up vote 2 down vote accepted

You could certainly do it. You'd need to implement certain components of your operating system, such as the boot loader, and interrupts in a lower level language.

Have a look at the approach taken by the Singularity Operating System by Microsoft on how to develop an operating system that runs on Managed Code.

Of course, there is no requirement that you have to bolt on memory management to JavaScript, you could add an API for memory management to JavaScript. You could choose to write a compiler for JavaScript or write a virtual machine.

Singularity has source code available so you could gain valuable insight from looking at the design decisions that Microsoft made.

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