This document discusses input/output (I/O) in computer systems. It defines I/O as the transfer of data between a computer's CPU and memory and external peripheral devices. Peripherals connect to the computer via I/O modules and operate more slowly than the CPU and RAM. The document outlines different types of peripherals and describes generic I/O module functions like controlling data transfer and buffering. It also explains the three main I/O techniques: programmed I/O, interrupt-driven I/O, and direct memory access (DMA), where the I/O module transfers data directly between memory and peripherals without CPU involvement.

1. +
William Stallings
Computer Organization
and Architecture
10th Edition

2. + Chapter 5
Input-Output

3. +
Input / Output Function
- Transfer data between the computer (CPU and Memory) and the connected
“peripherals (I/O devices)”.
◼ I/O operations are made by a wide variety of external
devices, simply called “peripherals”, these peripherals are
attached to the computer by a link to an I/O module.
◼ Delivering different amounts of data
◼ At different speeds
◼ In different formats
◼ All peripherals are slower than CPU and RAM
◼ Peripherals need I/O modules through which the exchange
of data can be done.

4. +
Generic Model of I/O Module
An Interface is required between the CPU bus and an I/O device to arrange
data transfer called “ports (I/O Module) ”

5. + External Devices classification :
1-Human readable: suitable for communicating with users.
◼ Screen, printer, keyboard
2- Machine readable, suitable for communicating with
equipment.
◼ Monitoring and control
3- Communication, suitable for communicating with
remote devices.
◼ Modem
◼ Network Interface Card (NIC)

6. +
The I/O devices
◼ The I/O Device Contains:
◼ 1- buffer: to temporary store data
◼ 2- Logic circuit to:
◼ receive command from I/O module for specific function as Read
data,Write data, disk head position.
◼ Provide the I/O device state as “Ready, Not Ready”.
◼ 3- Transducer: convert electrical data to other form of energy
during output and vise versa during input.
For Example: Keyboard and Monitor
When a user press a key , a corresponding binary
code is generated and transmitted to the computer ,
then same code transmitted to the Monitor to
display.

7. +
I/O Module Functions
◼ The major function of an I/O module are:
◼ 1- Control & Timing, to coordinate the flow of traffic between internal
resources & external devices.
◼ 2- CPU Communication: Involves command decoding, data, status
reporting, address recognition
◼ 3- Device Communication: Involves commands, status information, and
data
◼ 4- Data Buffering: Performs the needed buffering operation to balance
device and memory speeds
◼ 5- Error Detection: Detects and reports transmission errors

8. +
I/O Steps (transfer of data between
cpu & external device)
1- CPU asks the I/O module to check the device status
2- The I/O module returns the device status
3- If ready, the CPU requests the transfer of data
4- I/O module gets data from external device (8 or 16 bits,…)
5- The I/O module transfers data to the CPU

9. +
I/O Module Diagram

10. +
I/O Techniques
1- Programmed
2- Interrupt driven
3- Direct Memory Access (DMA)
◼ General terms :
◼ I/O channels = I/O processor = I/O module
◼ Are commonly seen on mainframes
◼ I/O controller = device controller =I/O module
◼ Are commonly seen on microcomputers

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12. +
1- I/O using CPU
a) Programmed I/O
◼ when the CPU issues I/O command, the CPU follows the
performing of this command.
◼ This cause the CPU wait until the I/O command is
completed
b) Interrupt driven I/O
◼ When the CPU issues a command and then continues to
execute other instructions and the I/O module issues an
interrupt when it I/O command is performed
© 2016 Pearson Education, Inc., Hoboken, NJ. All rights reserved.

13. +
Example: 82C55 Programmable
Peripheral Interface(continued)
◼ The Control register is loaded
by the CPU to control the
Operation mode:
◼ Mode 0 : programmed I/O
◼ Ports A,B, and C programmed
as buffered input or output .
◼ Mode 1: Interrupt driven I/O
◼ Port A and B programmed as
buffered input or output. Port C
used for handshaking, strobe ,
and Interrupt request signals

14. +
USING 82C55A with Mode1 operation to
interface with Keyboard/Display
Port A is input from keyboard, PortB is output to display, ½ Port C
is used for handshaking and Interrupt request for the keyboard, the
other half for the display.

15. + Direct Memory Access (DMA)
◼ In the previous techniques, interrupt driven and
programmed I/O require active CPU intervention to transfer
data between memory and an I/O module, and any data
transfer must traverse through the cpu, so:
◼ The transfer rate is limited by the speed with which the cpu
can test and service a device.
◼ The CPU is tied up, a number of instructions must be
executed for each I/O transfer.
◼ The DMA module is the solution
◼ In this mode, the I/O module and main memory exchange
data directly, without involving the processor.

16. +
DMA Function and operation
◼ The DMA involved additional Module (hardware) on the system bus.
◼ The DMA module must use the bus only when the cpu does not need it,
or it forces the cpu to suspend operation temporarily.
◼ When the cpu wishes to read/write a block of data it sends the following
information to the DMA module:
◼ The requested operation Read or Write
◼ The I/O device address
◼ The starting address of memory block for data to be R/W
◼ The amount of data to be transferred
◼ Then the CPU carries on with other work
◼ DMA controller deals with data transfer and sends interrupt when
finished

17. +
Typical DMA Block Diagram

18. +
To be continued

19. +
Direct Memory Access (DMA 8237)
◼ Used to exchange large amounts of data between The I/O devices and
main memory .
◼ 1- The CPU initiate the transfer (determine location of data on I/O
device - the starting location in memory - the size of the block -
read/write)
◼ 2- The CPU do other operations while the transfer is in progress.
◼ 3-The DMA Controller supplies the memory and I/O with control
signals and memory address information during the DMA transfer.
◼ 4-the CPU receive an interrupt from the DMA controller once the
operation has been done.
◼ The DMA 8237 has 8 channels, each channel is dedicated to I/O device
as disk drive controllers, graphics cards, network cards and sound
cards.
◼ South bridge in ISA bus contains two DMA . (not in PCI bus)

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© 2016 Pearson Education, Inc., Hoboken, NJ. All rights reserved.

21. +
© 2016 Pearson Education, Inc., Hoboken, NJ. All rights reserved.