From 68766cb60b07cf1ba1429e2c4d239be7e404f48c Mon Sep 17 00:00:00 2001
From: mgcaret <mgcaret@users.noreply.github.com>
Date: Sat, 28 Apr 2018 16:00:08 -0700
Subject: [PATCH] Rewrite quit_code to use ca65 '816 mnemonics and provide some
 analysis (#79)

* Convert '816 quit code to use ca65 65816 mode.
Add commentary on this code.
---
 desktop/desktop.s | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/desktop/desktop.s b/desktop/desktop.s
index 8f0e473..3950252 100644
--- a/desktop/desktop.s
+++ b/desktop/desktop.s
@@ -7514,11 +7514,18 @@ stack_data:
         .addr   $DEAF,$DEAD     ; ???
 
 quit_code:
-        .byte   $18,$FB,$5C,$04,$D0,$E0
-        ;; 65816 code:
-        ;; 18           clc             ; clear carry
-        ;; FB           xce             ; exchange carry/emulation (i.e. turn on 16 bit)
-        ;; 5C 04 D0 E0  jmp $E0D004     ; long jump
+        ;;; note that GS/OS GQUIT is called by ProDOS at $E0D000
+        ;;; to quit back to GS/OS.
+        ;;; since DeskTop pre-dates GS/OS, it's likely that this does
+        ;;; a similar function for ProDOS 16.
+        ;;; with GS/OS 5 and 6, this code potentially messes up
+        ;;; the shadow register, $C035
+        .pushcpu
+        .setcpu "65816"
+        clc
+        xce                             ; enter native mode
+        jmp     $E0D004                 ; ProDOS 8->16 QUIT, presumably
+        .popcpu
 
         DEFINE_QUIT_PARAMS quit_params