def arm_index( index )
      index = index.index if index.is_a?(Risc::Instruction)
      raise "index error #{index}" if index < 0
      index * 4
    end
def arm_index( index )
      index = index.index if index.is_a?(Risc::Instruction)
      raise "index error #{index}" if index < 0
      index * 4
    end
arm indexes are
  in bytes, so *4
 if an instruction is passed in we get the index with index function