extern __xdata union ao_monitor ao_monitor_ring[AO_MONITOR_RING];
#define ao_monitor_ring_next(n) (((n) + 1) & (AO_MONITOR_RING - 1))
+#define ao_monitor_ring_prev(n) (((n) - 1) & (AO_MONITOR_RING - 1))
extern __data uint8_t ao_monitoring;
extern __data uint8_t ao_monitor_head;