clean up Get/SetSlot

document and make arguments consistent

lib/register/instructions/get_slot.rb

@@ -1,16 +1,28 @@
 module Register
-  # offset memory get access
-  # so the value to be set must be given as the first register
-  # the second argument holds the base address
-  # and the third a possible (small) offset into the "object"
+
+  # GetSlot moves data into a register from memory.
+  # SetSlot moves data into memory from a register.
+  # Both use a base memory (a register)
+
+  # While the virtual machine has only one instruction (Set) to move data between slots,
+  # the register has two, namely GetSlot and SetSlot
   #
-  # if for example the value is pointed to by a register, a VariableGet (load) is needed first
+  # This is because that is what cpu's can do. In programming terms this would be accessing
+  #  an element in an array, in the case of GetSlot setting the value in the array.
+
+  # btw: to move data between registers, use RegisterTransfer
+
   class GetSlot < Instruction
-    def initialize value , reference , index = 0
-      @value = value
-      @reference = reference
+
+    # 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 Instruciton name
+    # So GetSlot means the slot (array and index) moves to the register (last argument)
+    def initialize array , index , register
+      @register = register
+      @array = array
       @index = index
     end
-    attr_accessor :value , :reference , :index
+    attr_accessor :register , :array , :index
   end
 end