diff --git a/crhelpers.vhdl b/crhelpers.vhdl
index f51110c..1e6160f 100644
--- a/crhelpers.vhdl
+++ b/crhelpers.vhdl
@@ -5,13 +5,16 @@ library work;
 use work.common.all;
 
 package crhelpers is
-    function fxm_to_num(fxm: std_ulogic_vector(7 downto 0)) return integer;
-    function num_to_fxm(num: integer) return std_ulogic_vector;
+    subtype crnum_t is integer range 0 to 7;
+    subtype crmask_t is std_ulogic_vector(7 downto 0);
+
+    function fxm_to_num(fxm: crmask_t) return crnum_t;
+    function num_to_fxm(num: crnum_t) return crmask_t;
 end package crhelpers;
 
 package body crhelpers is
 
-    function fxm_to_num(fxm: std_ulogic_vector(7 downto 0)) return integer is
+    function fxm_to_num(fxm: crmask_t) return crnum_t is
     begin
         -- If multiple fields are set (undefined), match existing
         -- hardware by returning the first one.
@@ -27,7 +30,7 @@ package body crhelpers is
         return 7;
     end;
 
-    function num_to_fxm(num: integer) return std_ulogic_vector is
+    function num_to_fxm(num: crnum_t) return crmask_t is
     begin
         case num is
             when 0 =>
diff --git a/execute1.vhdl b/execute1.vhdl
index 002bdac..6d3a936 100644
--- a/execute1.vhdl
+++ b/execute1.vhdl
@@ -109,8 +109,8 @@ begin
 	variable newcrf : std_ulogic_vector(3 downto 0);
 	variable result_with_carry : std_ulogic_vector(64 downto 0);
 	variable result_en : integer;
-	variable crnum : integer;
-	variable scrnum : integer;
+	variable crnum : crnum_t;
+	variable scrnum : crnum_t;
 	variable lo, hi : integer;
 	variable sh, mb, me : std_ulogic_vector(5 downto 0);
 	variable sh32, mb32, me32 : std_ulogic_vector(4 downto 0);