bring the blocks down to mom level

reusing message_setup, but adding yield specific instructions

lib/mom/instruction/block_yield.rb

@@ -0,0 +1,31 @@
+module Mom
+
+  # A BlockYield calls an argument block. All we need to know is the index
+  # of the argument, and the rest is almost as simple as a SimpleCall
+
+  class BlockYield < Instruction
+    attr :arg_index
+
+    def initialize(index)
+      @arg_index = index
+    end
+
+    def to_s
+      "BlockYield[#{arg_index}] "
+    end
+
+    def to_risc(compiler)
+      block = compiler.use_reg( :Block )
+      return_label = Risc.label(self, "continue_#{object_id}")
+      save_return =  SlotLoad.new([:message,:next_message,:return_address],[return_label],self)
+      moves = save_return.to_risc(compiler)
+      moves << Risc.slot_to_reg( self , Risc.message_reg ,:arguments , block)
+      moves << Risc.slot_to_reg( self , block ,arg_index , block)
+
+      moves << Risc.slot_to_reg(self, Risc.message_reg , :next_message , Risc.message_reg)
+      moves << Risc::DynamicJump.new(self, block )
+      moves << return_label
+    end
+  end
+
+end