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Input/Output Control Methods

The control method of I/O (input/output) devices is an important mechanism for processing data transfer between peripheral devices and main memory in computer systems. Its goal is to minimize the CPU's intervention in I/O operations, thereby improving CPU utilization and overall system efficiency. There are four common I/O control methods:

  • Direct program control method
  • Interrupt-driven control method
  • Direct memory access (DMA) control method
  • Channel control method

Direct program control method

Principle

  • Direct program control is the most basic I/O method. The CPU directly controls peripheral devices by executing input/output instructions.

  • The CPU will continuously check the status of the device (busy/idle flag) through polling to decide whether to transfer data.

Process

  1. The CPU sends instructions to check the status of the I/O device (such as whether it is busy).

  2. If the device is ready (not busy), data is transferred (one byte or one data block).

  3. Repeat the above steps until all data transfers are completed.

Advantages and Disadvantages

  • Advantages:
  • Simple implementation, no need for complex hardware support.
  • Disadvantages:
  • Low efficiency: The CPU must wait for the peripheral device to complete the operation, causing waste.
  • Limited scope of application: Suitable for simple, small systems (scenarios where the CPU and I/O speeds are slightly different).

Application scenarios: Suitable for simple devices such as early computer systems or microcontrollers (MCUs).

Interrupt-driven control method

Principle

  • The interrupt-driven control method introduces an interrupt mechanism. When the peripheral device needs CPU processing, it will send an interrupt request (IRQ) to the CPU.
  • The CPU can process other tasks in parallel. Only when it receives an interrupt signal will it suspend the current task and process the I/O operation instead.

Process

  1. After the CPU issues an I/O request, it continues to execute other tasks instead of waiting.
  2. The peripheral device sends an interrupt signal to the CPU when it is ready or completes the operation.
  3. The CPU responds to the interrupt, saves the current context, and executes the interrupt handler to complete the data transfer.
  4. After completing the I/O operation, the CPU resumes the original task.

Advantages and Disadvantages

  • Advantages:
  • Improves CPU utilization because the CPU no longer needs to poll the device status.
  • Suitable for medium-speed I/O devices.
  • Disadvantages:
  • Frequent interrupts: Each transmission of a data byte or character will trigger an interrupt, increasing the CPU's interrupt processing overhead.
  • To reduce the number of interrupts, it can be alleviated by increasing the buffer.

Application scenarios: Commonly used in keyboard input, mouse operation, low-speed serial port communication, etc.

Direct Memory Access (DMA) Control Mode

Principle

  • The DMA (Direct Memory Access) mode establishes a direct data transmission channel between the CPU and the main memory through the DMA controller.
  • The DMA controller takes over some functions of the CPU and is responsible for directly transferring data between the device and the main memory. The CPU only needs to intervene at the beginning and end of the transfer.

Process

  1. The CPU sends a command to the DMA controller to specify the data source address, target address and transfer length.
  2. The DMA controller takes over the bus control and performs data transfer. The CPU can continue to handle other tasks.
  3. After the transfer is completed, the DMA controller notifies the CPU to end the operation through an interrupt.

Composition

  • Command/Status Register (CR): Stores command or device status information.
  • Memory Address Register (MAR): Stores the target/source address of the transferred data.
  • Data Register (DR): Used to temporarily store data.
  • Data Counter (DC): Records the number of bytes remaining to be transferred.

Advantages and Disadvantages

  • Advantages:
  • Efficient: Suitable for bulk data transfer of high-speed devices, reducing the CPU burden.
  • Disadvantages:
  • The function is relatively simple and cannot handle complex I/O tasks.

Application scenarios: high-speed hard disk data transmission, graphics card video data transmission, network equipment, etc.

Channel control method

Principle

  • The channel control method realizes further parallelization of I/O operations by introducing I/O channels (Channel).
  • I/O channels can manage multiple devices and independently perform data transmission tasks, reducing the direct involvement of the CPU.

Process

  1. The CPU sends a start instruction to the channel through the operating system, and passes the I/O request information and the address of the channel program to the channel.

  2. The channel reads the channel program (containing a series of I/O operation instructions) and controls the device to perform data transmission.

  3. After the transmission is completed, the channel sends an interrupt signal to the CPU to notify the end of the task.

Channel instruction format

  • Command code: specifies the operation type (read, write, etc.).
  • Data main memory address: the target/source address of data transmission.
  • Number of bytes to be transmitted: the number of bytes to be transmitted.

Advantages and Disadvantages

  • Advantages:
  • Highly parallelized: enables parallel work of CPU, channels and devices, reducing CPU intervention.
  • Suitable for the management of multiple devices and complex tasks.
  • Disadvantages:
  • Complex design and high hardware cost.

Application scenarios: large computers, servers and other scenarios that require efficient data transmission.

Summary and comparison

Control method Advantages Disadvantages Applicable scenarios
Direct program control Simple implementation, low hardware requirements Low CPU efficiency, low device utilization Simple system, low-speed equipment
Interrupt-driven control Improve CPU utilization Frequent interrupts will affect system performance Keyboard, mouse, serial port communication, etc.
Direct memory access Efficient, suitable for large-scale high-speed data transmission Unable to handle complex tasks Hard disk, graphics card, network equipment, etc.
Channel control Highly parallelized, suitable for complex tasks Complex hardware, high design cost Large computers, multi-tasking systems