Added a serial driver for the builtin 6551 of the Plus/4 (untested, but taken

mostly from the C64 version which is tested and works).
Improved interrupt handling (was needed for the driver).


git-svn-id: svn://svn.cc65.org/cc65/trunk@2754 b7a2c559-68d2-44c3-8de9-860c34a00d81

libsrc/plus4/break.s

@@ -55,11 +55,12 @@ uservec:    	jmp	$FFFF  	   	; Patched at runtime
 
 
 
-; Break handler, called if a break occurs. Note: Y is not on the stack!
+; Break handler, called if a break occurs. 
 
 .proc	brk_handler
 
-       	sty	_brk_y
+	pla
+       	sta	_brk_y
 	pla
 	sta	_brk_x
 	pla