tinyvm.rb

# run some assembly code like
code = <<-eof
          # stack
  push 1  # [1]
  push 2  # [1, 2]
  add     # [3]
  print   # []
  push 2  # [2]
cond:
  dup     
  push 3
  equals?
  if-true-jump ret
  push 3
  goto cond
ret:
  print
eof


class AsmParser
  attr_accessor :instr
  def initialize
    @instr = []
  end

  def parse str
    labels = {}
    unresolved_labels = {}
    get_label = proc do|l|
      labels[l] || ((unresolved_labels[l] ||= []) << instr.size; nil)
    end

    iter = str.each_line

    loop do
      l = iter.next
      case l
      when /push (\d+)/i
        instr << :push
        instr << $1.to_i
      when /add/i
        instr << :add
      when /print/i
        instr << :print
      when /equals\?/i
        instr << :equals?
      when /pop/i
        instr << :pop
      when /dup/i
        instr << :dup
      when /if-true-jump (\S+)/i
        instr << :iftrue
        instr << get_label[$1]
      when /if-false-jump (\S+)/i
        instr << :iffalse
        instr << get_label[$1]
      when /(\S+):/i
        labels[$1] = instr.size
      when /goto (\S+)/i, /jump (\S+)/i
        instr << :jump
        instr << get_label[$1]
      else
        puts "unrecognized line: #{l}"
      end
    end

    unresolved_labels.each do |k,v|
      loc = labels[k]
      v.each do |idx|
        instr[idx] = loc
      end
    end

    instr
  end
end

class VM
  attr_accessor :instr
  def load instrs
    @instr = instrs
    self
  end

  def run
    ip = 0
    stack = []
    while ip < @instr.size
      op = @instr[ip]
      ip += 1
      case op
      when :push
        stack << instr[ip]
        ip += 1
      when :dup
        stack << stack.last
      when :add
        stack << (stack.pop + stack.pop)
      when :print
        puts stack.pop
      when :jump
        ip = instr[ip]
      when :equals?
        stack << (stack.pop == stack.pop)
      when :pop
        stack.pop
      when :iftrue
        jumpto = instr[ip]
        if stack.pop
          ip = jumpto
        else
          ip += 1
        end
      when :iffalse
        jumpto = instr[ip]
        if stack.pop
          ip += 1
        else
          ip = jumpto
        end
      when :nop
      else
        puts "error: not understand this much"
        return
      end
    end
  end
end


VM.new
  .load(AsmParser.new.parse(code))
  .run