Overview

• 電腦不是在做 I/O 就是在做計算

• I/O devices: tape, HD, mouse, joystick, network card, screen, flash disks, etc

• I/O subsystem: the methods to control all I/O devices

• Two conflicting trends

  • 我們希望他是 interface，這樣才可以 standardization 的控制
  • 但是他又很多樣性

• Device drivers: a uniform device-access interface to the I/O subsystem

  ⇒ Similar to system calls between apps and OS

• Device categories

  • Storage devices: disks, tapes
  • Transmission devices: network cards, modems
  • Human-interface devices: keyboard, screen, mouse
  • Specialized devices: joystick, touchpad

I/O Hardware

• Port: A connection point between I/O devices and the host. 每個 port 都有自己的 ID，那就是 device 對於電腦的 identifier。

  → E.g.: USB ports

• Controller: A collection of electronics that can operate a port, a bus, or a device

  ⇒ A controller could have its own processor, memory, etc. (E.g.: SCSI controller)

• Bus: A set of wires and a well-defined protocol that specifies messages sent over the wires（連接的方式，除了物理相接，還有 protocol 的部分）

  → E.g.: PCI bus

I/O Methods

• 透過 port address 做 什麼事
• Port address 用一個簡單的 table 就好了，反正也沒有很大
• Each I/O port consists of four registers (1~4 Bytes)
  • Data-in register — In buffer
  • Data-out register — Out buffer
  • Status register — 狀態的值
  • Control register — command

I/O Methods Categorization

• Depending on how to address a device

  • Port-mapped I/O

    • 直接用 I/O instruction (e.g. IN, OUT)

  • Memory-mapped I/O

    • Access by standard data-transfer instruction (e.g. MOV)

    🙂 More efficient for large memory I/O (e.g. graphic card)

    ☹️ Vulnerable to accidental modification, error

• Depending on how to interact with a device:

  • Poll (busy-waiting): processor periodically check status register of a device. 不停地確認好了沒，啥時可以作下一個

    → 如果量不多，用這個

  • Interrupt: device notify processor of its completion. I/O 好了他會直接 call interrupt 跟你的程式講。

  ⇒ 通常會 Memory-mapped I/O ＋ Interrupt 或是另外兩個相加，不然會互相抵銷好處。