From 80a5fe182aa7af429e5a8c37e2553c36ae274e86 Mon Sep 17 00:00:00 2001
From: cuz <cuz@b7a2c559-68d2-44c3-8de9-860c34a00d81>
Date: Wed, 15 Jan 2003 17:19:12 +0000
Subject: [PATCH] Keep unknown flag bits, restore known flags on exit

git-svn-id: svn://svn.cc65.org/cc65/trunk@1901 b7a2c559-68d2-44c3-8de9-860c34a00d81
---
 libsrc/common/_sys.s | 25 +++++++++++++++++++------
 1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/libsrc/common/_sys.s b/libsrc/common/_sys.s
index 09856b9bf..0698ae42e 100644
--- a/libsrc/common/_sys.s
+++ b/libsrc/common/_sys.s
@@ -21,12 +21,21 @@ __sys:	sta	ptr1
 	lda	(ptr1),y
 	sta	jmpvec+1
 
-; Get the flags, mask unnecessary bits and push them. Push a
+; Remember the flags so we can restore them to a known state after calling the
+; routine
 
-	dey
-	lda	(ptr1),y
-	and	#%11001111      ; Mask out break and unused flag
-	pha
+        php
+
+; Get the flags, keep the state of bit 4 and 5 using the other flags from
+; the flags value pased by the caller. Push the new flags and push A.
+
+ 	dey
+        php
+        pla                     ; Current flags -> A
+        eor     (ptr1),y
+        and     #%00110000
+        eor     (ptr1),y
+	pha                     ; Push new flags value
 	ldy	#0
 	lda	(ptr1),y
 	pha
@@ -46,7 +55,7 @@ __sys:	sta	ptr1
 	plp
 	jsr	jmpvec
 
-; Back from the routine. Save the flags and a
+; Back from the routine. Save the flags and A.
 
 	php
 	pha
@@ -66,6 +75,10 @@ __sys:	sta	ptr1
 	pla
 	sta	(ptr1),y
 
+; Restore the old flags value
+
+        plp
+
 ; Done
 
 	rts