diff --git a/src/constants.a b/src/constants.a
index cf61aa537..060f75fbe 100644
--- a/src/constants.a
+++ b/src/constants.a
@@ -88,11 +88,11 @@ CLR80VID =     $C00C      ; 40 columns (also used to get out of DHGR mode)
 SET80VID =     $C00D      ; 80 columns (also used to get into DHGR mode)
 PRIMARYCHARSET= $C00E     ; no mousetext for you
 SLOT3STATUS =  $C017      ; bit 7 only
-TBCOLOR =      $C022      ; IIgs text foreground and background colors (also VidHD)
+TBCOLOR =      $C022      ; IIgs text foreground and background colors (also VidHD but write-only)
 NEWVIDEO =     $C029      ; IIgs graphics modes (also VidHD)
 SPEAKER =      $C030      ; chirp chirp
-CLOCKCTL =     $C034      ; bits 0-3 are IIgs border color (also VidHD)
-SHADOW =       $C035      ; IIgs auxmem-to-bank-E1 shadowing (also VidHD)
+CLOCKCTL =     $C034      ; bits 0-3 are IIgs border color (also VidHD but write-only)
+SHADOW =       $C035      ; IIgs auxmem-to-bank-E1 shadowing (also VidHD but write-only)
 TEXTMODE =     $C051
 PAGE1 =        $C054      ; page 1 (affects text, HGR, DHGR)
 PAGE2 =        $C055      ; page 2 (affects text, HGR, DHGR)