diff --git a/constants/SP2000.inc b/constants/SP2000.inc
index 13e7569..12e240c 100644
--- a/constants/SP2000.inc
+++ b/constants/SP2000.inc
@@ -1650,8 +1650,9 @@ CTC:
 SIO:
 .Ch_A.Data              EQU     #18       ; Data register                        COM
 .Ch_A.Ctrl              EQU     #19       ; Control register                     COM
-.Ch_B.Data              EQU     #1A       ; Data register                        keyboard & mouse
-.Ch_B.Ctrl              EQU     #1B       ; Control register                     keyboard & mouse
+.Ch_B.Data              EQU     #1A       ; Data register                        keyboard & mouse  DMOUSE
+.Ch_B.Ctrl              EQU     #1B       ; Control register                     keyboard & mouse  CMOUSE
+
 
 PIO:
 .Port_A.Data            EQU     #1C       ; Data register                        LPT 1