1. CS-401 Computer Architecture & Assembly Language Programming
Lecture-38
Serial Port Programming

2. Serial Port

3. Serial Port Connector 1 – Carrier Detect (CD) 2 – Received Data (RD)
3 – Transmitted Data (TD)
4 – Data Terminal Ready (DTR)
5 – Signal Ground
6 – Data Set Ready (DSR)
7 – Request to Send (RTS)
8 – Clear to Send (CTS)
9 – Ring Indicator (RI)

4. Frame Format data bits (5-8) Start bit (1) parity bit (1) Stop bits
(1-2)

5. Connection Diagram TD RD
Ground TD RD
Ground

6. Transmission setting

7. int 0x14 Serial Initialize Port AH = 0x00 AL = port parameters
DX = port number (0x00 – 0x03)
Return
AH = line status
Al = modem status

8. Port Parameters - AL baud rate 000-110 001-150 010-300 011-6007 6 5 4 3 2 1
baud rate
parity
00-N
10-N
01-O
11-E
data bits
00-5
01-6
10-7
11-8
stop bits
0-1
1-2

9. Write Character to Port
int 0x14 Serial
Write Character to Port
AH = 0x01
AL = character
DX = port number (0x00 – 0x03)
Return
AH (bit 7)= set on error
AH (bits 0-6)= port status

10. Read Character from Port
int 0x14 Serial
Read Character from Port
AH = 0x02
AL = character
DX = port number (0x00 – 0x03)
Return
AH = line status
AL = received character (AH bit 7 clear)

11. int 0x14 Serial Get Port Status AH = 0x03 AL = character
DX = port number (0x00 – 0x03)
Return
AH = line status
AL = modem status

12. Line Status 7 6 5 4 3 2 1 0 rx ready time out tx shift register empty
time out
rx ready
tx shift register empty
over run
tx holding register empty
parity error
break detect
framing error

13. Serial Ports IO ports

14. Serial Port I/O Ports
COM1: 0x03F8-0x03FF
COM2: 0x02F8-0x02FF

15. Serial Port I/O Ports Port 0x3F8/0x2F8 Transmit Holding Register (THR)
Receiver Buffer Register (RBR)

16. Serial Port I/O Ports Port 0x3F9/0x2F9 Interrupt Enable Register (IER)
Bit 0 = Received Data Available
Bit 1 = Transmitter Holding Register Empty

17. Interrupt Identification Register
Serial Port I/O Ports
Port 0x3FA/0x2FA
Interrupt Identification Register
Bit 0 = Interrupt Pending Status
(0) Interrupt Pending
(1) Interrupt Not Pending
Bit 1-3 = Pending Interrupt Source
(001) THR Empty
(010) Received a Character

18. Serial Port I/O Ports Port 0x3FB/0x2FB Line Control Register (LCR)
DLAB
data bits
set break enable
stop bit
parity

19. Serial Port I/O Ports Port 0x3FD/0x2FD Line Status Register (LCR)
reserved
data ready
tx shift register empty
over run
tx holding register empty
parity error
break detect
framing error