Implement "pop argument n"

Author: leocassarani

lib/code_writer.rb

@@ -80,7 +80,7 @@ def write_push_pop(command, segment, index)
     when Parser::C_POP
       output.puts <<-EOF
         // Get base address of the local segment
-        @LCL        // A=1
+        @#{base_address(segment)} // A=1
         D=M         // D=RAM[1]=300
         // Add the index offset to the base address
         @#{index}   // A=2
@@ -116,6 +116,13 @@ def operand(command)
     }.fetch(command)
   end
 
+  def base_address(segment)
+    {
+      'local'    => 'LCL',
+      'argument' => 'ARG'
+    }.fetch(segment)
+  end
+
   def generate_label
     @counter += 1
     "LABEL#{@counter}"