Bitfields for serial line status:
Bit(s)	Description	)
 7	timeout
 6	transmit shift register empty
 5	transmit holding register empty
 4	break detected
 3	framing error
 2	parity error
 1	overrun error
 0	receive data ready
Note:	for COMM-DRV, if bit 7 is set, an error occurred, and may be retrieved
	  through a separate call (see AX=8000h"COMM-DRV")