Computer component :

Input/OutputInput/Output• I/O Modules•External Devices• I/O Modules Function and

Structure• I/O Operation Techniques• I/O Channels and Processors•External Interface

I/O Modules

• The third key element of a computer system.• Each module interfaces to the system bus

and controls one or more peripheral devices.• The reasons to use I/O Module :

– A wide variety of peripherals with various methods of operation

– The difference of data transfer rate– The difference of data formats and word

lengths...

External Devices

• An external device connected to an I/O modules is a peripheral device or a peripheral.

• Three categories of peripheral– Human readable : for user e.g. monitor, printer– Machine readable : for equipment e.g.

magnetic disk and tape system– Communication : for remote devices e.g.

terminal, another computer

Typical I/O Device Data Rates

External Devices• The link is used to exchange

control, status and data between theI/O module and the external device– Data : the form of a set of bits– Status : the state of the device– Control : the function that the device will perform

• Control Logic controls the device’s operation response to direction of I/O module.

• Transducer converts data from electrical to other forms of energy during output and vice versa during input.

ControlfromI/O Module

StatustoI/O Module

Data (bits)to and fromI/O Module

Buffer

Transducer

Control

Logic

Data (Device-unique)to and fromEnvironment

An External Device

I/O Modules

The two major functions• Interface to CPU and memory

via the system bus• Interface to one or more

peripheral devices by data links

Address Line

Data Line

System Bus

I/O

Module Links to Peripheral Devices

Control Line

Generic Model of an I/O Module

I/O Modules Function

• Control and timing • Processor communication• Device communication• Data buffering• Error detection

Control and Timing• To coordinate the flow of traffic between internal resources

and external devices. For example, the control of the transfer data from an external device to CPU involving the followings steps:1. CPU interrogates the I/O module to check the status of the device.2. The I/O module returns the device status.

3. If the device is operational and ready to transmit, CPU requests th e transfers of

data by mean of a command to the I/O module. 4. The I/O module obtains a unit of data from the external device.

5. The data are transferred from the I/O module to CPU

Processor communication• Each of the interactions between the processor and

the I/O module involves one or more bus arbitration• I/O module must communication with the CPU

involving :– Command decoding :accepts command from the CPU

(read,write,seek)– Data : exchanges between the CPU and the I/O module

via the data bus– Status reporting : send status signal (busy, ready, error) – Address recognition : I/O module must recognize one

unique address for each I/O device it controls

Device communication

• I/O module must be able to communication with I/O devices involving commands, status information, and data.

• The transfer rate into and out of memory or CPU is quite high, the rate of peripheral devices is lower.

• I/O module must be able to operate at both device and memory speeds by buffering data in itself.

Data buffering

Error detection

• I/O module is responsible for error detection and reporting errors to the CPU.

• Type of error– mechanical and electrical malfunctions

(e.g.paper jam,bad disk track) – unintentional changes to the bit pattern as it is

transmitted from device to I/O module.• Some form of error-detecting code is used to

detect transmission errors (e.g. the use of a parity bit on each character of data).

I/O Module Structure

Block Diagram of an I/O Module

Data registers

Status/Control registers

I/Ologic

External Device

Interface Logic

External Device

Interface Logic

...

ControlLines

AddressLines

DataLines

Data

StatusControl

Interface toSystem Bus

Interface toExternal Device

Data

StatusControl

I/O Module Structure

• I/O modules vary in complexity and the nu mber of external devices that they control.

• I/O module takes on most of the detailed p - rocessing burden, presenting a high level i

nterface to the CPU is an I/O channel or I/O processor (in mainframes).

• I/O module that is quite primitive and requires detail control is an I/O controller or device controller (in PCs).

I/O Commands and Instructions

• I/O Commands : four types that I/O module may receiv e when it is addressed by a processor:

– Control : used to activate a peripheral and tell it what to do.– Test :used to test varios status condition associated with I/O

module&peripherals– Read : causes the I/O module to obtain data from the periph

eral– Write : causes the I/O module to take data to the peripheral

• I/O Intsructions : to execute, CPU issues an address, sp ecifying the particular I/O module and external device

and I/O command.

I/O Operation Techniques

• Programmed I/O• - Interrupt Driven I/O• Direct Memory Access (DMA)

Programmed I/O

• Data are exchanged between the CPU and the I/O module.

• The CPU executes a program that gives it direct control of the I/O operation.

• When the CPU issues to the I/O modules, it must wait until the I/O operation is complete.

• If the CPU is faster than the I/O module, this is wasteful of CPU time.

Issue read command to I/O module

Read statusof I/O module

Check status

Read word fromI/O module

Write word intomemory

Done?

Yes

No

Ready

Error ConditionNotReady

CPU --> I/O

I/O --> CPU

I/O --> CPU

CPU --> Memory

Programmed I/O Programmed I/O

Interrupt-Driven I/O

• The CPU send START signal to I/O cont roller for start working.

• After send signal, the CPU can continu e their jobs.

• When I/O controller finish work, it sen d interrupt signal to the CPU.

• The CPU suspend their works and load data into memory.

Interrupt-Driven I/O Interrupt-Driven I/O

Issue read command to I/O module

Read statusof I/O module

Check status

Read word fromI/O module

Write word intomemory

Done?

Yes

No

Ready

Error Condition

Do Something Else

CPU --> I/O

I/O --> CPU

I/O --> CPU

CPU --> Memory

Interrupt

Direct Memory Access

• CPU requests to read/write by sent signal to read/write control line.

• The address of the I/O device involved, sent on the data lines.

• The number of words to be read/written, stored in data count register

• The starting location in memory to read/write, sent on the data lines and stored in address regist

er.• When finishing data transfer, it send interrupt sign

al to the CPU.

Data Count

Data Register

Address Register

ControlLogic

Data lines

Address lines DMA Request DMA Acknowledge

InterruptReadWrite

Issue read blockcommand

to DMA module

Read statusof DMA module

Do Something Else

CPU --> DMA

DMA --> CPU

Direct Memory AccessDirect Memory Access

Interrupt

I/O Channel and Processors

• The Evolution of the I/O Function– The CPU directly controls a peripheral device.– A controller or I/O module is added. The CPU uses pro

grammed I/O without interrupt.– Interrupts are employed.– I/O module is given direct access to memory via DMA.– I/O is enchanced to become a processor in itself. (I/O

Channel)– I/O module has a local memory of its own. (I/O Proces

sor)

I/O Channel and Processors

• Characteristics of I/O Channels– I/O channel is able to execute I/O instructions,

completing I/O operations– The CPU initiates an I/O transfer by instructing

the I/O channel to execute a program in memory.• Two types of I/O channel

– A Selector Channel controls one of multiple high-speed devices at any one time.

– A Multiplexor Channel can handle I/O with multiple devices at the same time.

Selector

Channel

Data and Address Channel to Main Memory

Control Signal path to CPU

....I/O Controller

I/O ControllerMultip

lexorChann

el

To Memory

To CPU. . . .

I/O ControllerI/O ControllerI/O

ControllerI/O Controller Two types of I/O channel Two types of I/O channel

The External Interface

• Type of interfaces between a peripheral and I/O module– Parallel interface : multiple lines

connecting the I/O module and the peripheral, multiple bits are transferred simultaneously.

– Serial interface : only one line used to transmit data and bits must be transmitted one at a time.Buffer

I/O Moduleto system bus

to peripheral

Parallel I/O

Buffer

I/O Moduleto system bus

to peripheral

Serial I/O

The External Interface

• Type of interfaces between external device s and I/O module– - - Point to point interface provides a ded

icated line. (e.g. keyboard, printer, extern -al modem) : EIA 323

– Multipoint interfaces are in effect exter nal buses, and exhibit the same type of lo gic as the buses. (e.g. external mass stor

age, multimedia devices) : SCSI, FireWire

The External Interface

• Small Computer System Interface– SCSI is the standard (multipoint) interface for CD-

ROM drives, audio equipment and external mass storage devices.

– Uses a parallel interface with 8,16,32 data lines.– All devices are daisy chained together. – Each SCSI device has 2 connectors for input and

output.– All devices function independently and can

exchange data with each other as well as with the host system.

The External Interface

• FireWire– Serial Interface which could use with digit

al camera, VCRs, televisions– High speed, low cost, easy to implement– - Mouse, Laser printer, SCSI External disk d

rives, sound, LAN card, etc. could be rep laced with single simple connector single simple connector

– - Daisy chain up to 63 devices connected o ff single port

IEEE-1394