FIFO_SIZE
&& sp->_sysmouse_tail < FIFO_SIZE
MEVENT _sysmouse_fifo[FIFO_SIZE];
MEVENT _drv_mouse_fifo[FIFO_SIZE];
int _fifo[FIFO_SIZE]; /* input push-back buffer */
(head >= FIFO_SIZE-1) \
? head = FIFO_SIZE-1 \
(tail >= FIFO_SIZE-1) \
? tail = FIFO_SIZE-1 \
(peek >= FIFO_SIZE-1) \