diff --git a/synthDC/Makefile b/synthDC/Makefile
index 76531d950..118536bf6 100755
--- a/synthDC/Makefile
+++ b/synthDC/Makefile
@@ -84,6 +84,14 @@ endif
 endif
 	
 # adjust config if synthesizing with any modifications
+# This code is subtle with ifneq.  It successively turns off a larger
+# set of features in order of cycle time limiting.
+# When mod = orig, all features are ON
+# When mod = PMP0, the number of PMP entries is set to 0
+# when mod = noPriv, the privileged unit and PMP are disabled
+# when mod = noFPU, the FPU, privileged unit, and PMP are disabled
+# when mod = noMulDiv, the MDU, FPU, privileged unit, and PMP are disabled.
+
 ifneq ($(MOD), orig)
 	# PMP 0
 	sed -i 's/PMP_ENTRIES \(64\|16\|0\)/PMP_ENTRIES 0/' $(CONFIGDIR)/wally-config.vh