XMTRDY
while ((inb(early_serial_base + LSR) & XMTRDY) == 0 && --timeout)
while ((inb(early_serial_base + LSR) & XMTRDY) == 0 && --timeout)
while ((static_call(serial_in)(early_serial_base, LSR) & XMTRDY) == 0 && --timeout)