introduce base class for get_slot and get_byte called getter

lib/register/instructions/getter.rb

@@ -0,0 +1,34 @@
+module Register
+
+  # Getter is a base class for get instructions (GetSlot and GetByte , possibly more coming)
+  #
+  # The instruction that is modelled is loading data from an array into a register
+  #
+  # Getter has a
+  # - an array where the data comes from
+  # - and (array) index
+  # - Register that the data is moved to
+
+  # Getter and Setter api follow the pattern from -> to
+
+  class Getter < Instruction
+
+    # If you had a c array and index offset
+    # the instruction would do register = array[index]
+    # The arguments are in the order that makes sense for the Instruction name
+    # So GetSlot means the slot (array and index) moves to the register (last argument)
+    def initialize source , array , index , register
+      super(source)
+      @array = array
+      @index = index
+      @register = register
+      raise "index 0 " if index == 0
+      raise "Not integer or reg #{index}" unless index.is_a?(Numeric) or RegisterValue.look_like_reg(index)
+      raise "Not register #{register}" unless RegisterValue.look_like_reg(register)
+      raise "Not register #{array}" unless RegisterValue.look_like_reg(array)
+    end
+    attr_accessor :array , :index , :register
+
+  end
+
+end