diff --git a/common.vhdl b/common.vhdl
index 3e9da69..44198b0 100644
--- a/common.vhdl
+++ b/common.vhdl
@@ -1,10 +1,21 @@
 library ieee;
 use ieee.std_logic_1164.all;
+use ieee.numeric_std.all;
 
 library work;
 use work.decode_types.all;
 
 package common is
+
+    -- SPR numbers
+    subtype spr_num_t is integer range 0 to 1023;
+
+    function decode_spr_num(insn: std_ulogic_vector(31 downto 0)) return spr_num_t;
+
+    constant SPR_LR    : spr_num_t := 8;
+    constant SPR_CTR   : spr_num_t := 9;
+    constant SPR_TB    : spr_num_t := 268;
+
     type ctrl_t is record
 	lr: std_ulogic_vector(63 downto 0);
 	ctr: std_ulogic_vector(63 downto 0);
@@ -216,4 +227,8 @@ package common is
 end common;
 
 package body common is
+    function decode_spr_num(insn: std_ulogic_vector(31 downto 0)) return spr_num_t is
+    begin
+	return to_integer(unsigned(insn(15 downto 11) & insn(20 downto 16)));
+    end;
 end common;
diff --git a/execute1.vhdl b/execute1.vhdl
index 6632783..862c631 100644
--- a/execute1.vhdl
+++ b/execute1.vhdl
@@ -269,16 +269,17 @@ begin
 		    v.e.write_cr_data(hi downto lo) := newcrf;
 		end loop;
 	    when OP_MFSPR =>
-		if std_match(e_in.insn(20 downto 11), "0100100000") then
+		case decode_spr_num(e_in.insn) is
+		when SPR_CTR =>
 		    result := ctrl.ctr;
-		    result_en := '1';
-		elsif std_match(e_in.insn(20 downto 11), "0100000000") then
+		when SPR_LR =>
 		    result := ctrl.lr;
-		    result_en := '1';
-		elsif std_match(e_in.insn(20 downto 11), "0110001000") then
+		when SPR_TB =>
 		    result := ctrl.tb;
-		    result_en := '1';
-		end if;
+		when others =>
+		    result := (others => '0');
+		end case;
+		result_en := '1';
 	    when OP_MFCR =>
 		if e_in.insn(20) = '0' then
 		    -- mfcr
@@ -308,11 +309,13 @@ begin
 		end if;
 		v.e.write_cr_data := e_in.read_data3(31 downto 0);
 	    when OP_MTSPR =>
-		if std_match(e_in.insn(20 downto 11), "0100100000") then
+		case decode_spr_num(e_in.insn) is
+		when SPR_CTR =>
 		    ctrl_tmp.ctr <= e_in.read_data3;
-		elsif std_match(e_in.insn(20 downto 11), "0100000000") then
+		when SPR_LR =>
 		    ctrl_tmp.lr <= e_in.read_data3;
-		end if;
+		when others =>
+		end case;
 	    when OP_POPCNTB =>
 		result := ppc_popcntb(e_in.read_data3);
 		result_en := '1';