com_rbuf_size
sc->sc_rbavail = com_rbuf_size;
scc = cc = com_rbuf_size - sc->sc_rbavail;
if (cc == com_rbuf_size) {
get -= com_rbuf_size << 1;
get -= com_rbuf_size << 1;
u_int com_rbuf_size = COM_RING_SIZE;
sc->sc_rbuf = malloc(com_rbuf_size << 1, M_DEVBUF, M_WAITOK);
sc->sc_rbavail = com_rbuf_size;
sc->sc_ebuf = sc->sc_rbuf + (com_rbuf_size << 1);