Keep unknown flag bits, restore known flags on exit

git-svn-id: svn://svn.cc65.org/cc65/trunk@1901 b7a2c559-68d2-44c3-8de9-860c34a00d81

diff --git a/libsrc/common/_sys.s b/libsrc/common/_sys.s
index 09856b9bf3f05334aff71e0e13ab17d2d2bb16b0..0698ae42e81678ec6438e475d64afa203616ff31 100644 (file)
--- 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