05. Input/Output System

The I/O system is an important part of the operating system responsible for external device management and data exchange. This chapter systematically explains the basic concepts of the I/O system, device classification and characteristics, I/O control methods, buffer management, device management and drivers, and typical I/O subsystem structures.

1. Basic Concepts of the I/O System

• I/O System (Input/Output System): The part of the OS responsible for managing various input/output devices and data exchange between the host and external devices.
• I/O Devices: Include input devices (e.g., keyboard, mouse), output devices (e.g., monitor, printer), and input/output devices (e.g., disk, USB drive).

2. Device Classification and Characteristics

• By data exchange method: block devices (e.g., disk), character devices (e.g., keyboard, serial port).
• By speed: high-speed devices (disk), low-speed devices (printer), medium-speed devices (terminal).
• By function: dedicated devices (e.g., printer), shared devices (e.g., disk).

3. I/O Control Methods

• Programmed I/O: CPU actively polls device status, low efficiency, high CPU usage.
• Interrupt-driven I/O: Device notifies CPU via interrupt when ready, CPU responds to interrupt for data transfer, improves efficiency.
• DMA (Direct Memory Access): Data is transferred directly between main memory and device without CPU intervention, suitable for large data transfers.

4. Buffer Management

• Purpose of buffers: Coordinate speed differences between CPU and I/O devices, improve data transfer efficiency.
• Single buffer: Only one data block at a time, simple but limited efficiency.
• Double/multiple buffers: Multiple buffers used in turn, improves concurrency and efficiency.

5. Device Management and Drivers

• Exclusive vs. shared devices: Some devices can only be used by one process at a time (e.g., printer), others can be shared (e.g., disk).
• Device allocation and release: OS is responsible for allocating and releasing device usage rights.
• Device drivers: Implement communication between OS and hardware devices.

6. Typical I/O Subsystem Structure

• User-level I/O interface: e.g., standard library functions (read/write, etc.).
• OS I/O management module: e.g., buffer management, device allocation, error handling, etc.
• Device drivers: Interact with specific hardware.
• Hardware controller: Electronic circuits that directly control devices.

Exercises

1. Briefly describe the main functions of the I/O system.
2. Compare the advantages and disadvantages of programmed I/O, interrupt-driven I/O, and DMA.
3. Explain the purpose of buffer management and give an example of the advantages of multiple buffers.
4. What is the role of a device driver?
5. Draw the hierarchical structure of a typical I/O subsystem and briefly describe the function of each layer.