comment principle behind new tables in power_insn.py

https://libre-soc.org/irclog/%23libre-soc.2022-09-18.log.html#t2022-09-18T16:22:37

diff --git a/src/openpower/decoder/power_insn.py b/src/openpower/decoder/power_insn.py
index 059d39b33aa4f51482ae979c16d35d2d13da4c88..96077e148bdd1f44c5eb351779cfcf39e1c9a4ca 100644 (file)
--- a/src/openpower/decoder/power_insn.py
+++ b/src/openpower/decoder/power_insn.py
@@ -1751,6 +1751,12 @@ class RM(BaseRM):
     def select(self, record, Rc):
         rm = self
 
+        # the idea behind these tables is that they are now literally
+        # in identical format to insndb.csv and minor_xx.csv and can
+        # be done precisely as that.  the only thing to watch out for
+        # is the insertion of Rc=1 as a "mask/value" bit and likewise
+        # regtype detection (3-bit BF/BFA, 5-bit BA/BB/BT) also inserted
+        # as the LSB.
         table = None
         if record.svp64.mode is _SVMode.NORMAL:
             # concatenate mode 5-bit with Rc (LSB) then do a mask/map search