A JavaScript processor simulator to design and execute any assembly language architecture
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A Javascript processor implementation that can simulate any load-store architecture

How to Use

To use the processor, you must first create an ISA.

Creating your ISA

Your ISA must be a function which returns a list of your supported instructions. The function takes as input the processor's register file, the memory unit, and the program counter:

var isa = function (Register, Memory, ProgramCounter) {
  // ...

Again, you have to return a list of supported instructions. The format for instructions is:

Instruction Format

  cmd: "[name of instruction]",
  desc: "[description of instruction]",
  syntax: [
      {src1: Register | Memory | Number},
      {src2: Register | Memory | Number},
      {dest: Register | Memory}
  eval: function (src1, src2) {
    // operate on src1 and src2 and return the result

The syntax field is an array of supported syntax structures for the instruction. You can define multiple syntax structures for the same instruction; the decoder will move forward with whichever one is satisfied first.

The src1, src2, and dest fields should contain the type of the operand supported for this structure. For instance, the following defines a syntax structure for an instruction that can take a register descriptor as its first operand, a number as its second operand, and will store the result of the eval function into a memory cell supplied as the third operand:

syntax: [
    {src1: Register},
    {src2: Number},
    {dest: Memory}

If dest is not supplied, the processor will store the result (if it exists) into src1.

The eval function doesn't need to return a result; nothing will be stored if this is the case. This is useful for functions that mutate some internal state but don't require results to be stored anywhere. More on that below.

Internal State

Since your ISA is a function, you can use local variables to mimic processor internal components like condition bits, overflow registers, etc. You could even simulate an accumulator or stack architecture in the same way.

var isa = function (Register, Memory, ProgramCounter) {
  var overflow = 0;
  var condition = false;

  var accum = 0;

  return [
    // ...


Since the ProgramCounter is supplied as an argument to the ISA function, you are free to manipulate its value. Here's an example of an instruction format for an unconditional branch:

  cmd: "bra",
  desc: "Unconditional branch by a relative instruction offset",
  syntax: [
      {src1: Number}
  eval: function (src1) {
    ProgramCounter.set(ProgramCounter.get() + src1);

Instantiating the Processor

var processor = new Processor(isa, regCount, regSize, memCellCount, memCellSize);

To instantiate the processor, you call the function exposed by the archsim module and supply the ISA generator (see above) and the following arguments:

  • regCount: The number of registers in your processor
  • regSize: The size (in bits) of your registers (values are truncated to fit!)
  • memCellCount: The number of memory cells in the memory unit
  • memCellSize: How many bits each memory cell stores (8 should be common)

Writing code

Operands are fetched in the same order they are defined in the syntax, so if you define src1, then src2, and then dest, the instruction can be used like this:

instr   {src1}, {src2}, {dest}

Register and Memory descriptors

To access registers, use r[i], where i is the number of the register you wish to access. If i is out of range according to regCount, an error is thrown.

To access memory cells, use m[i], where i is the "address" of the memory cell you wish to access. If i is out of range according to memCellCount, an error is thrown.

Executing Code

To execute a program, just use load() followed by exec(). These operations can be chained:


Where asm is a string containing the assembly language program, with each instruction separate by a newline (blank lines are ignored in execution, but can be included for clarity in the source code).

Event Listeners

You can implement event listeners for the following functions:

  • processor.onProgramLoaded = function (instrs, regs, mem) {};
  • processor.onProgramComplete = function (regs, mem) {};
  • processor.onInstructionComplete = function (instr, regs, mem) {};
  • processor.onDecodeComplete = function (instr) {};
  • processor.onError = function (err) {};

Hopefully these are straightforward. onInstructionComplete is called after the program counter is incremented. regs and mem allow access to the contents of the register file and memory unit, respectively.


The onError handler will be invoked once an error is encountered, and execution of the program halts. The possible errors are:

Processor level

  • Program Not Loaded: Nothing to execute -- when exec() is called before a program has been loaded with load().
  • Illegal Instruction: {instr} -- when a program uses an instruction with an unsupported syntax structure, or an instruction that it otherwise can't decode
  • Unsupported instruction: {instr} -- when a program uses an instruction that is not supported, i.e. if no add instruction is given in the ISA, but a program uses it.

Register File level

  • Register File Error: Invalid register descriptor: {} -- when a register is expected but the operand is not of the form r[i].
  • Register File Error: Accessing a nonexistent register: {} -- when the program attempts to access a register outside of the allocated range, i.e. r[10] when the register count is 8..

Memory Unit level

  • Memory Error: Invalid memory cell descriptor: {} -- see corresponding register file error
  • Memory Error: Accessing a nonexistent memory cell: {} -- see corresponding register file error