[OS] Chapter 13 — I/O System

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Notion 好讀版

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。

  • Depending on who to control the transfer:

    • Programmed I/O: transfer controlled by CPU

      → 需要一直用 CPU

    • Direct memory access (DMA) I/O: controlled by DMA controller (a special purpose controller)

      → 只負責 I/O

      • Design for large data transfer

Interrupt

  • Interrupt Vector Table

    數字越小越重要

  • Interrupt Prioritization

    • Maskable interrupt — 有一些不重要的 interrupt 會被 mask 掉

    • Non-maskable interrupt (NMI): highest-priority, never masked

      → Often used for power-down, memory error

  • Interrupt-Driven I/O

  • CPU and device Interrupt handshake

    1. Device asserts interrupt request (IRQ) → I/O 要註冊他的 interrupt，這樣 OS 才知道要去處理誰
    2. CPU checks the interrupt request line at the beginning of each instruction cycle
    3. Save the status and address of interrupted process
    4. CPU acknowledges the interrupt and search the interrupt vector table for interrupt handler routines
    5. CPU fetches the next instruction from the handler routine
    6. Restore interrupted process after executing interrupt handler routine

DMA (Direct Memory Access)

File System → （我要讀多少資料，要讀到 X）→ device driver → （說「我要讀多少資料，要讀到 X」，並 register 一個 interrupt 說「做完之後跟我說欸」） → disk controller → （初始化 DMA，因為他只能傳很少的資料，所以 DMA 就開始不停地送搬移的指令到 disk controller，然後存到 X）→ DMA → （Through interrupt 到 CPU 說我搬完了）→ CPU → 看到搬完了，開心：）

注意那個 X 通常是 physical 的 address，不然就要去看 page table，那就需要 CPU 去跑 OS 來看。

→ 這樣 Virtual Memory 就有點失效，因為那塊就被 fix 住了。

Review Slides ( I )

  • Definition of I/O port? Bus? Controller?
  • I/O device and CPU communication?
    • Port-mapped vs. Memory-mapped
    • Poll vs. Interrupt
    • Programmed I/O vs. DMA
  • Steps to handle an interrupt I/O and DMA request?

Kernel I/O Subsystem

  • I/O Scheduling

  • Buffering — Bridge 速差

  • Caching — Always just a copy; Key to performance

  • Spooling — 像 buffer，但是是 all or nothing

  • Error handling — when I/O error happens

  • I/O protection — Privileged instructions

  • Device-status Table

  • Life Cycle of An I/O Request

Performance

  • Intercomputer Communications

Application Interface

Textbook Questions

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