Workaround for "phantom" key presses in the C128 "1351" mouse driver.

libsrc/c128/mou/callback.inc

@@ -0,0 +1,29 @@
+;
+; Callback routine called from the IRQ handler after the ROM IRQ handler
+; had been run.
+;
+; Christian Groessler, 24.04.2014
+;
+; Check if there was joystick activity before and/or after the ROM handler.
+; If there was activity, discard the key presses since they are most
+; probably "phantom" key presses.
+
+callback:
+        ldx     old_key_count
+        cpx     KEY_COUNT
+        beq     @nokey
+
+        lda     OLD_BUTTONS             ; keypress before?
+        bne     @discard_key            ; yes, discard key
+
+        lda     #$7F
+        sta     CIA1_PRA
+        lda     CIA1_PRB                ; Read joystick #0
+        and     #$1F
+        eor     #$1F                    ; keypress after
+        beq     @nokey                  ; no, probably a real key press
+
+@discard_key:
+        stx     KEY_COUNT               ; set old keyboard buffer fill level
+
+@nokey: rts