From 8e223c82644a91fd7372e1aa6d06ef0f4ac967da Mon Sep 17 00:00:00 2001
From: Peter Ferrie <peter.ferrie@gmail.com>
Date: Sat, 21 Dec 2019 16:28:59 -0800
Subject: [PATCH] documentation

---
 src/ui.common.a | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/src/ui.common.a b/src/ui.common.a
index 71262b9b9..391ecd42a 100644
--- a/src/ui.common.a
+++ b/src/ui.common.a
@@ -68,6 +68,7 @@ Home
          +READ_ROM_NO_WRITE
          sta   DHIRESOFF             ; get out of DHGR mode
          sta   CLR80VID              ; get out of DHGR mode
+                                     ; write-order matters for RGB-card
          jsr   $FB3C ;ROM_TEXTish    ; clear screen but don't show it
          jsr   ROM_HOME              ; HOME
          lda   PAGE1
@@ -91,12 +92,20 @@ BlankDHGR
          sta   WRITEAUXMEM
          jsr   ClearHGR1             ; clear hi-res screen 1 in auxmem
          sta   WRITEMAINMEM
+
+         ; magic sequence to set colour mode on an RGB card
+         ; SET80VID clears the RGB-card shift-register
+         ; DHIRESON/OFF shifts that bit into the mode register
+         ; we do that twice because the mode register is two bits wide
+         ; we need mode 00 for colour
+
          sta   SET80VID
          sta   DHIRESON
          sta   DHIRESOFF
          sta   DHIRESON
          sta   DHIRESOFF
-         sta   DHIRESON
+
+         sta   DHIRESON              ; then turn DHGR on
          bit   PAGE1
          jmp   HGRMode