diff --git a/src/arch/micro_asm.py b/src/arch/micro_asm.py
index e36daf862..d9d9d1b21 100644
--- a/src/arch/micro_asm.py
+++ b/src/arch/micro_asm.py
@@ -64,9 +64,14 @@ class Micro_Container(object):
             string += "  %s\n" % microop
         return string
 
-class Macroop(Micro_Container):
+class Combinational_Macroop(Micro_Container):
     pass
 
+class Rom_Macroop(object):
+    def __init__(self, name, target):
+        self.name = name
+        self.target = target
+
 class Rom(Micro_Container):
     def __init__(self, name):
         super(Rom, self).__init__(name)
@@ -310,6 +315,9 @@ def p_block(t):
 # Defines a section of microcode that should go in the current ROM
 def p_rom_block(t):
     'rom_block : DEF ROM block SEMI'
+    if not t.parser.rom:
+        print_error("Rom block found, but no Rom object specified.")
+        raise TypeError, "Rom block found, but no Rom object was specified."
     for statement in t[3].statements:
         handle_statement(t.parser, t.parser.rom, statement)
     t[0] = t.parser.rom
@@ -317,7 +325,12 @@ def p_rom_block(t):
 # Defines a macroop that jumps to an external label in the ROM
 def p_macroop_def_0(t):
     'macroop_def : DEF MACROOP ID LPAREN ID RPAREN SEMI'
-    t[0] = t[4]
+    if not t.parser.rom_macroop_type:
+        print_error("ROM based macroop found, but no ROM macroop class was specified.")
+        raise TypeError, "ROM based macroop found, but no ROM macroop class was specified."
+    macroop = t.parser.rom_macroop_type(t[3], t[5])
+    t[0] = macroop
+
 
 # Defines a macroop that is combinationally generated
 def p_macroop_def_1(t):
@@ -438,13 +451,15 @@ def p_error(t):
 
 class MicroAssembler(object):
 
-    def __init__(self, macro_type, microops, rom):
+    def __init__(self, macro_type, microops,
+            rom = None, rom_macroop_type = None):
         self.lexer = lex.lex()
         self.parser = yacc.yacc()
         self.parser.macro_type = macro_type
         self.parser.macroops = {}
         self.parser.microops = microops
         self.parser.rom = rom
+        self.parser.rom_macroop_type = rom_macroop_type
 
     def assemble(self, asm):
         self.parser.parse(asm, lexer=self.lexer)
diff --git a/src/arch/micro_asm_test.py b/src/arch/micro_asm_test.py
index 858ac511e..f0aebe2b2 100755
--- a/src/arch/micro_asm_test.py
+++ b/src/arch/micro_asm_test.py
@@ -26,7 +26,7 @@
 #
 # Authors: Gabe Black
 
-from micro_asm import MicroAssembler, Macroop, Rom
+from micro_asm import MicroAssembler, Combinational_Macroop, Rom_Macroop, Rom
 
 class Bah(object):
     def __init__(self):
@@ -52,7 +52,7 @@ microops = {
     "dah": Dah
 }
 
-class TestMacroop(Macroop):
+class TestMacroop(Combinational_Macroop):
     def tweak(self):
         microops["bah"] = Bah_Tweaked
     def untweak(self):
@@ -68,7 +68,7 @@ class TestMacroop(Macroop):
             "print": self.print_debug
         }
 
-assembler = MicroAssembler(TestMacroop, microops, Rom('main ROM'))
+assembler = MicroAssembler(TestMacroop, microops, Rom('main ROM'), Rom_Macroop)
 
 testAssembly = '''
 # Single line comment