I/O System Fundamentals

I/O System Fundamentals

1. The Basic Concepts of the I/O System

The Definition of the I/O System

I/O system:

• Realises the exchange of information between the computer and the peripheral devices
• Has both a hardware part and a software part
• Is an essential part of the computer system

I/O devices:

• Input devices: bring external information into the computer
• Output devices: send the information of the computer out to the outside world
• External storage: stores large amounts of data

The Functions of the I/O System

Data exchange:

• Transfers data between the CPU and the I/O devices
• Supports different data formats
• Guarantees the reliability of data transfer

Device control:

• Controls the working state of the I/O devices
• Manages the start and the stop of a device
• Handles device exceptions

Information conversion:

• Converts the data formats
• Adapts to different device interfaces
• Handles signal level conversion

2. Peripheral Devices

Input Devices

Keyboard:

• A character input device
• Mechanical keyboards, membrane keyboards, capacitive keyboards
• Supports many input methods

Mouse:

• A graphical interface input device
• Mechanical mice, optical mice, laser mice
• Supports clicking, dragging, and other operations

Scanner:

• An image input device
• Flatbed scanners, handheld scanners
• Supports OCR text recognition

Camera:

• A video input device
• Digital cameras, network cameras
• Supports real-time video transmission

Output Devices

Monitor:

• An image output device
• CRT monitors, LCD monitors, LED monitors
• Supports different resolutions and refresh rates

Printer:

• A text and image output device
• Dot-matrix printers, inkjet printers, laser printers
• Supports colour and black-and-white printing

Speakers:

• An audio output device
• Active speakers, passive speakers
• Supports stereo and multi-channel audio

Projector:

• A large-screen display device
• LCD projectors, DLP projectors
• Supports high-resolution projection

External Storage

Hard disk:

• A large-capacity storage device
• Mechanical hard disks, solid state drives
• Supports fast reading and writing

Optical disc:

• A portable storage device
• CDs, DVDs, Blu-ray discs
• Supports both read-only and writable media

USB flash drive:

• A portable storage device
• Flash memory
• Supports hot plugging

Memory card:

• A small storage device
• SD cards, CF cards, memory sticks
• Used in digital devices

3. The I/O Interface

The Functions of the I/O Interface

Signal conversion:

• Converts the signal levels
• Adapts to different device interfaces
• Handles the signal timing

Data buffering:

• Buffers the data being transferred
• Balances the difference in speed
• Raises the transfer efficiency

Device control:

• Controls the working state of the device
• Manages the device registers
• Handles device interrupts

The Structure of the I/O Interface

Data lines:

• Transfer data information
• Support different data widths
• Transfer in both directions

Control lines:

• Transfer control signals
• Include read/write, interrupt, and other signals
• Coordinate the data transfer

Status lines:

• Transfer the status information of the device
• Include ready, busy, error, and other states
• Monitor the working state of the device

I/O Ports and Their Addressing

I/O port:

• Used for communication between the host and the peripherals
• Every port has a unique address
• Supports read and write operations

The addressing methods:

• Unified addressing: memory and I/O share one address space
• Independent addressing: memory and I/O are addressed separately
• Hybrid addressing: combines the two methods

The port types:

• Data port: transfers data
• Control port: transfers control information
• Status port: transfers status information

4. I/O Modes

Programmed I/O

Basic concept:

• The CPU actively polls the state of the device
• Checks whether the device is ready
• The efficiency is low

The working process:

1. The CPU sends a command to the device
2. The CPU polls the state of the device
3. The data is transferred once the device is ready
4. The polling repeats until the transfer is complete

Characteristics:

• Advantages: simple to implement, simple to control
• Disadvantages: low CPU utilisation, low efficiency
• Applications: simple systems, low-speed devices

Interrupt-Driven I/O

Basic concept:

• The device interrupts the CPU when it becomes ready
• The CPU responds to the interrupt and handles it
• Raises the CPU utilisation

The working process:

1. The CPU sends a command to the device
2. The CPU continues to execute other tasks
3. The device raises an interrupt when it becomes ready
4. The CPU responds to the interrupt and handles the data transfer

Characteristics:

• Advantages: high CPU utilisation, high efficiency
• Disadvantages: complex to implement, interrupt handling is needed
• Applications: most I/O devices

The DMA Mode

Basic concept:

• Direct memory access
• The data is transferred directly between memory and the peripheral
• The CPU only initiates and terminates the transfer

The working process:

1. The CPU sets the parameters of the DMA controller
2. The DMA controller takes over the bus
3. The data is transferred directly between memory and the peripheral
4. The DMA controller releases the bus once the transfer is complete

Characteristics:

• Advantages: a light load on the CPU, the highest efficiency
• Disadvantages: complex hardware, high cost
• Applications: high-speed devices, bulk data transfers

5. I/O System Performance

The Performance Metrics

Throughput:

• The amount of data transferred per unit of time
• Unit: MB/s, GB/s
• Reflects the overall performance of the I/O system

Response time:

• The time from the request to its completion
• Includes the queueing time and the service time
• Impacts the user experience

Utilisation:

• The rate at which the I/O devices are used
• Reflects the efficiency of the devices
• Impacts the performance of the system

Performance Optimisation

Hardware optimisation:

• Use high-speed interfaces
• Add more cache
• Optimise the device scheduling

Software optimisation:

• Optimise the I/O scheduling algorithms
• Reduce the number of I/O operations
• Use asynchronous I/O

System optimisation:

• Balance the performance of the components
• Reduce the I/O bottlenecks
• Raise the overall efficiency

Exercises

Exercise 1

Briefly describe the basic functions of the I/O system.

Exercise 2

Give examples of input devices, output devices, and external storage.

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Glossary