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1
Homework
Reading (linked
from my web page)
S and S Extracts
National Semiconductor
UART Data Sheet
Machine Projects
mp2 due at start
of class 12
Labs
Continue labs in
your assigned section
2
Addressing I/O
Devices
Intel I/O devices
have addresses assigned in an 00rthogonal00space from memory addresses
Remember the M/IO#
signal that is used with the address bus to select memory versus I/O
devices?
Use I/O instructions
for I/O device addresses
inw inb
outw outb
3
Addressing I/O
Devices
The 00nput00
instruction 00direct addressing
inw
$0xdd, %ax # 8 bit address
inb
$0xdd, %al # 8 bit address
The 00nput00
instruction 00indirect addressing
movw
$0x3f8, %dx
inw
(%dx), %ax # 16 bit address
inb
(%dx), %al # 16 bit address
Reads from an I/O
device to a register
4
Addressing I/O
Devices
The 00utput00
instruction 00direct addressing
outw
%ax, $0xdd # 8 bit address
outb
%al, $0xdd # 8 bit address
The 00utput00
instruction 00indirect addressing
movw
$0x3f8, %dx
outw
%ax, (%dx) # 16 bit address
outb
%al, (%dx) # 16 bit address
Writes from a register
to an I/O device
5
Addressing I/O
Devices
In some processor
architectures (Motorola), there is no M/IO# signal in the control bus
This is called using
00emory mapped I/O00/font>
I/O device addresses
are in the same address space as memory
I/O device ports
are accessed as memory addresses
Use equivalent of
00ove00instructions to write or read data to or from I/O device
registers as memory
6
Addressing I/O
Devices
COM port addresses
(Neveln, Table 4.2)
* corrects an error in the text
COM Port
I/O Addresses
1
2
3
4
0x3F8-0x3FF
0x2F8-0x2FF
0x3E8-0x3EF
0x2E8-0x2EF *
7
Accessing the
Serial Port
PC specification
allows up to four serial ports
COM1: base address
is 0x3f8
COM2: base address
is 0x2f8
0x3f8
0x3fc
0x3fb
Write
Read
D7
D6
D5
D4
D3
D2
D1
D0
D7
D6
D5
D4
D3
D2
D1
D0
DLAB
Set
Brk
Evn
Par
Par
Enb
#
Stop
Loop
Out2
Out1
RTS
DTR
Len
Sel 1
Len
Sel 0
0x3fd
0x3fe
Same as Write
Same as Write
- - -
- - -
0
0
Stk
Par
DCD
RI
DSR
CTS
DCD
CHG
DSR
CHG
CTS
CHG
TE
RI
RX
ERR
TX
EMP
THRE
BRK
Int
FRM
ERR
PAR
ERR
OVRN
ERR
Data
RDY
0
8
Accessing the
Serial Port
Don00 want to
use hard coded numbers!
Look at $pcinc/serial.h
for symbolic constants
#define COM1_BASE 0x3f8
#define COM2_BASE 0x2f8
#define UART_TX 0
/* send data */
#define UART_RX 0
/* recv data */
. . .
#define UART_LCR 3
/* line control */
#define UART_MCR 4
/* modem control */
#define UART_LSR 5
/* line status */
#define UART_MSR 6
/* modem status */
9
Parallel Serial
Conversion
UART performs double
buffered, bidirectional, parallel-to-serial / serial-to-parallel conversion:
Transmit Holding
Register
Transmit Shift
Register
TXD
(Serial)
THRE
TX Empty
Data Bus
(Parallel)
RXD
(Serial)
Data Ready
Receive Shift
Register
Receive Holding
Register
Overrun Error
10
Strategies for
I/O Driver Code
Two Basic Strategies
for I/O Driver Code
Status Polling
Interrupt Driven
Status Polling
Uses only the port
addresses on the I/O device
Ties up the entire
processor for the duration of I/O
Interrupt Driven
Adds an interrupt
line from I/O device to processor
Allows processor
to do other work during I/O
11
Status Polling
Review the serial
port details:
Status and Control
Registers
We will look at
assembly language driver to send and receive data in 00ull duplex00
mode
Half Duplex 00
Sending or receiving alternately (data going only one direction at a
time)
Full Duplex 00
Sending and receiving at same time (data going both directions simultaneously)
12
Initializing
the UART
Tutor does this
for us on COM1: and COM2:
Select speed, data
bits, parity, and number of stop bits
Turn on DTR and
wait for DSR on
Half duplex mode
modem signal handshake:
Transmit: Turn on
RTS and wait for CTS on
Receive: Turn off
RTS and wait for DCD on
Full duplex mode
modem signal handshake:
Turn on RTS and
leave it on
Transmit whenever
CTS on
Receive whenever
DCD on
13
Status Polling
Loop on send/receive
data to/from COM2:
(Assume Tutor has initialized
bit rate and line control)
1. Turn on DTR & RTS,
wait for DSR, CTS, & DCD
2. Read data ready (DR)
3. If data is ready, read
a byte of receive data
4. Read transmit holding
register empty (THRE)
5. If THR is empty, write
a byte of transmit data
6. Jump back to step 2
Processor loop is
much faster than byte transfer rate
But, hard to do
other work while looping on status
14
Status Polling
Assembly Code
Step 1a: Turn on
DTR and RTS
movw $0x2fc, %dx # modem
control
inb (%dx), %al # get
current
orb $0x03, %al # or on
2 lsbs
outb %al, (%dx) # set
control
15
Status Polling
Assembly Code
Step 1b: Wait for
DSR, CTS, and DCD
movw $0x2fe, %dx # modem
status
loop1:
inb (%dx),
%al # get current
andb $0xb0, %al #
get 3 signals
xorb $0xb0, %al #
check all 3
jnz loop1 #
some missing
# all 3 are on now
16
Status Polling
Assembly Code
Step 2: Read Data
Ready
Step 3: If ready,
read a byte from receive data
loop2:
movw $0x2fd, %dx #
line status
inb (%dx),
%al # get data ready
andb $0x01, %al #
look at dr
jz
xmit # if recv data
movw $0x2f8, %dx #
i/o data addr
inb (%dx),
%al # move rx to %al
movb %al, somewhere #
save it somewhere
movw $0x2fd, %dx #
line status
17
Status Polling
Assembly Code
Step 4: Read transmit
holding register empty
Step 5: If empty,
write a byte to transmit data
xmit:
inb (%dx),
%al # get thre
andb $0x20, %al #
look at thre
jz
loop2 # if tx hr empty
movb somewhere,
%al # get data to send
movw $0x2f8, %dx #
i/o data addr
outb %al, (%dx) #
send it
jmp loop2 #
and loop
18
COM Port Driver
in C
#include <serial.h>
void pollputc(unsigned char ch)
{
/* polling loop, waiting for THRE
bit to go on */
while ((inpt(COM1_BASE +
UART_LSR) & UART_LSR_THRE) == 0)
;
/* output character */
outpt(COM1_BASE + UART_TX,
ch);
}
19
Implementing
a Comm Protocol
You can now write
a 00ow-level00serial port driver!
But, can you put
a sequence of bytes out the serial port to implement standard communication
protocol?
Not without a lot
more information about the protocol format for messages and procedures
that are used to interact in a standard fashion with other devices
That is a higher
level 00rotocol00that is usually coded in some other portion of
the system than the 00river00/font>
Example of a protocol
procedure is 00low control00/font>
20
SW and HW Flow
Control
A flow control mechanism
allows receiver to stop transmitter from sending more characters than
it is ready to handle (e.g. out of buffers)
Software - Use X-ON
and X-OFF characters
Send X-OFF (ASCII
DC3) to stop remote TX
Send X-ON
(ASCII DC1) to start remote TX
Hardware - Use control
signal leads
Turn off RTS (crossed
to CTS) to stop remote TX
Turn on RTS
(crossed to CTS) to start remote TX
Homework
85.5k 
