start on fake memory

get index checks on the array access of parfait

lib/risc/fake_memory.rb

@@ -0,0 +1,40 @@
+module Risc
+  # simulate memory during compile time.
+  #
+  # Memory comes in chunks, power of 2 chunks actually.
+  #
+  # Currently typed instance variables map to ruby instance variables and so do not
+  # end up in memory. Memory being only for indexed word aligned access.
+  #
+  # Parfait really does everything else, apart from the internal_get/set
+  # And our fake memory (other than hte previously used array, does bound check)
+  class FakeMemory
+    attr_reader :min
+    def initialize(from , size)
+      @min = from
+      @memory = Array.new(size)
+      raise "only multiples of 2 !#{size}" unless size == 2**(Math.log2(size).to_i)
+    end
+    def set(index , value)
+      range_check(index)
+      @memory[index] = value
+      value
+    end
+    alias :[]=  :set
+
+    def get(index)
+      range_check(index)
+      @memory[index]
+    end
+    alias :[] :get
+
+    def size
+      @memory.length
+    end
+
+    def range_check(index)
+      raise "index too low #{index} < #{min}" if index < min
+      raise "index too big #{index} < #{min}" if index >= size
+    end
+  end
+end