From 84aa972ef20a8edda0ebc46d1f12d0d7577dc04b Mon Sep 17 00:00:00 2001
From: Matthew Daniels <108370479+gm-matthew@users.noreply.github.com>
Date: Tue, 16 May 2023 02:49:35 +0100
Subject: [PATCH] Add description of swtrilgy patch from MAME

Star Wars Trilogy will sometimes write to a JTAG register and then read it back to verify it, but this doesn't work correctly with Supermodel's current JTAG implementation; it will try again and never succeed, getting stuck in an infinite loop. This patch stops the code from branching up and trying the write/read cycle again, allowing the subroutine to continue normally.

We need to implement shift register widths correctly for each JTAG device (Mercury, Venus, Earth, etc.) for the unpatched code to work properly... right now Supermodel implements one JTAG device with a huge 197-bit data shift register
---
 Config/Games.xml | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/Config/Games.xml b/Config/Games.xml
index cc95b8b..411001a 100644
--- a/Config/Games.xml
+++ b/Config/Games.xml
@@ -2273,8 +2273,8 @@
     <roms>
       <patches>
         <!--
-          The purpose of this patch from MAME is no longer clear. Force feedback
-          or JTAG related? If anyone knows, please update this note.
+          Patch from MAME: forces JTAG write/read tests to succeed, otherwise
+          the game spins in a loop forever (until we implement JTAG properly)
         -->
         <patch region="crom" bits="32" offset="0xf6e44" value="0x60000000" />
       </patches>
@@ -2362,10 +2362,7 @@
     </hardware>
     <roms>
       <patches>
-        <!--
-          The purpose of this patch from MAME is no longer clear. Force feedback
-          or JTAG related? If anyone knows, please update this note.
-        -->
+        <!-- Patch from MAME, see swtrilgy for details -->
         <patch region="crom" bits="32" offset="0xf6dd0" value="0x60000000" />
       </patches>
       <region name="crom" stride="8" chunk_size="2" byte_swap="true">
@@ -3301,4 +3298,4 @@
       </region>
     </roms>
   </game>
-</games>
\ No newline at end of file
+</games>