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I/O System

The I/O system manages communication between the CPU and external hardware devices. It provides a uniform interface to the variety of devices attached to the computer.

Device Classes​

  • Character Devices: Handle data one byte at a time (e.g., keyboards, mice, serial ports).
  • Block Devices: Handle data in fixed-size blocks (e.g., hard drives, SSDs).
  • Network Devices: Handle packets of data (e.g., Ethernet, Wi-Fi cards).

Communicating with Hardware​

The kernel uses several techniques to interact with devices:

Polling​

The CPU repeatedly checks the device status.

  • Pros: Simple to implement.
  • Cons: Wasteful of CPU cycles (busy waiting).

Interrupts​

The device signals the CPU when it's ready or an event occurs.

  • Interrupt Handler (ISR): A kernel function executed in response to a specific interrupt.
  • Top Halves & Bottom Halves: Modern OSs split interrupt handling into a fast part (Top Half) that disables further interrupts and a deferred part (Bottom Half) that does the heavy work.

Direct Memory Access (DMA)​

Allows devices to transfer data directly to/from RAM without involving the CPU.

  • Mechanism: The CPU tells the DMA controller the address and size of the data to transfer. Once finished, the DMA controller sends an interrupt.

Device Drivers​

A Device Driver is a kernel module that understands how to communicate with a specific hardware device and provides a standardized API to the rest of the kernel.

I/O Performance and Buffering​

Buffering​

Storing data temporarily in memory before it's sent to the device or application.

  • Double Buffering: Using two buffers so that one can be filled while the other is being emptied.

Caching​

Storing frequently used data in fast memory to avoid slow I/O operations.

Blocking vs. Non-blocking vs. Async I/O​

  • Blocking I/O: The process is suspended until the I/O operation finishes.
  • Non-blocking I/O: The system call returns immediately. If data isn't ready, it returns an error (e.g., EWOULDBLOCK). The process must try again.
  • Asynchronous I/O: The process starts the I/O operation and continues execution. When the operation finishes, the kernel notifies the process (e.g., via a signal or callback).