execute1: Use truth table embedded in instruction for CR logical ops

It turns out that CR logical instructions have the truth table of
the operation embedded in the instruction word.  This means that we
can collect the two input operand bits into a 2-bit value and use
that as the index to select the appropriate bit from the instruction
word.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

diff --git a/execute1.vhdl b/execute1.vhdl
index ae13c72d4d77c61a322751e0626c5739f4d709d5..e41a743a5dbbbbb32fc20247ca7861039359e2d6 100644 (file)
--- a/execute1.vhdl
+++ b/execute1.vhdl
@@ -200,6 +200,7 @@ begin
        variable bo, bi : std_ulogic_vector(4 downto 0);
        variable bf, bfa : std_ulogic_vector(2 downto 0);
        variable cr_op : std_ulogic_vector(9 downto 0);
+        variable cr_operands : std_ulogic_vector(1 downto 0);
        variable bt, ba, bb : std_ulogic_vector(4 downto 0);
        variable btnum, banum, bbnum : integer range 0 to 31;
        variable crresult : std_ulogic;
@@ -532,27 +533,10 @@ begin
                    btnum := 31 - to_integer(unsigned(bt));
                    banum := 31 - to_integer(unsigned(ba));
                    bbnum := 31 - to_integer(unsigned(bb));
-                   case cr_op(8 downto 5) is
-                   when "1001" => -- CREQV
-                       crresult := not(e_in.cr(banum) xor e_in.cr(bbnum));
-                   when "0111" => -- CRNAND
-                       crresult := not(e_in.cr(banum) and e_in.cr(bbnum));
-                   when "0100" => -- CRANDC
-                       crresult := (e_in.cr(banum) and not e_in.cr(bbnum));
-                   when "1000" => -- CRAND
-                       crresult := (e_in.cr(banum) and e_in.cr(bbnum));
-                   when "0001" => -- CRNOR
-                       crresult := not(e_in.cr(banum) or e_in.cr(bbnum));
-                   when "1101" => -- CRORC
-                       crresult := (e_in.cr(banum) or not e_in.cr(bbnum));
-                   when "0110" => -- CRXOR
-                       crresult := (e_in.cr(banum) xor e_in.cr(bbnum));
-                   when "1110" => -- CROR
-                       crresult := (e_in.cr(banum) or e_in.cr(bbnum));
-                   when others =>
-                       crresult := '0';
-                       report "BAD CR?";
-                   end case;
+                    -- Bits 5-8 of cr_op give the truth table of the requested
+                    -- logical operation
+                    cr_operands := e_in.cr(banum) & e_in.cr(bbnum);
+                    crresult := cr_op(5 + to_integer(unsigned(cr_operands)));
                    v.e.write_cr_mask := num_to_fxm((31-btnum) / 4);
                    for i in 0 to 31 loop
                        if i = btnum then