Interruptor support

git-svn-id: svn://svn.cc65.org/cc65/trunk@3492 b7a2c559-68d2-44c3-8de9-860c34a00d81

libsrc/apple2/crt0.s

@@ -161,11 +161,12 @@ msglen = * - errmsg
         .segment        "LOWCODE"
 
         ; ProDOS TechRefMan, chapter 6.2:
-        ; "Each installed routine must begin with a CLD instruction"
+        ; "Each installed routine must begin with a CLD instruction."
 intrpt: cld
 
-        ; Call interruptors
+        ; Call interruptors and check for success
         jsr     callirq
+        bcc	:+
 
         ; ProDOS TechRefMan, chapter 6.2:
         ; "When the routine that can process the interrupt is called, it
@@ -173,6 +174,12 @@ intrpt: cld
         clc
         rts
 
+        ; ProDOS TechRefMan, chapter 6.2:
+        ; "When a routine that cannot process the interrupt is called,
+        ;  it should return (via an RTS) with the cary flag set ..."
+:       sec
+        rts
+
 ; ------------------------------------------------------------------------
 
         .code