本文是我在上UCSD的 CSE 120: Principles of Operating Systems (Winter 2020) 整理的笔记，这一课主要介绍了操作系统里面输入系统系统的概念，包括硬件上的组成以及软件的结构。

I/O basic

1. Intro

   • I/O = Input/Output

     • Input from attached device to CPU/memory
     • Output from CPU/memory to device

   • Synchronization and transferring data

2. Issues

   • Problems:

     • So many different types of I/O devices
     • Wide range: speed, operation, data transfer units

   • Questions:

     • How does a process initiate I/O?
     • How is synchronization achieved?
     • How is data transferred?

3. Background: I/O Hardware

   • CPU and device (controller) communicate via

     • I/O instructions
     • Memory instructuions (memory-mapped)

   • Data transfer: progammed I/O vs. DMA (direct memory access)

   • Synchronization: polling vs. interrupts

4. Buffered/Unbuffered I/O

   • Pros:

     • What if pages containing buffer are paged out?
     • What if entire process is swapped out?
     • Can pin pages, but if too many processes do this?

   • Cons:

     • Memory copying is expensive
     • Consider effect on caches

5. Dealing with Complexity of Devices

   • Many different types of devices

     • Classify by shared characteristics
     • Imposes structure: shared code, lower complexity

   • Dimensions

     • Varaible vs. fixed size units
     • Sequential vs. random-access
     • Synchronous vs. asynchronous
     • Speed of operation

I/O system

1. I/O system Intro

   • Software that deals with I/O

     • Mostly in the kernel
     • Also in processes (in form of library, e.g., stdio)

   • Separated into two portions

     • Device-dependent
     • Device- independent

   • Structure: Layered

2. Device Dependent: Device Drivers

   • Encapsulates device-dependent code

     • Contains device-specific register reads/write

   • Implements a standard interface

     • open(), close(), read(), write()

   • Interrupt handlers

     • Executes when I/O completes
     • Updates data structure
     • Wakes up waiting process

3. Device-Independent I/O

   • Uniform interfacing for device drivers
   • Naming, protection
   • Uniform block size
   • Buffering, caching
   • Storage allocation
   • Locking
   • Error handling

4. User-space I/O

   • Convenient interface

     • printf() vs. write()

   • User-level buffering

     • Unix: stdio library

   • Spooling daemons

     • Printer

5. Overall Operation

Example: UNIX

1. I/O Model

   • Uses file system interface

   • stdio.h: C standard I/O library

   • Block devices (disks, USB cameras, …)

     • Fixed-size blocks
     • Randomly addressable
     • Uses buffer cache

   • Character devices (serial ports, parallel ports, sound cards,…)

     • Variable sequence of bytes
     • For non-block devices

2. I/O System Call Interface

   • fd = open("/dev/devname", …)
   • close(fd)
   • nr = read(fd, buf, n)
   • nw = write(fd, buf, n)
   • ioctl(fd, cmd, buf) //(input/output control)

3. Standard I/O Library

   • fopen, fread, fwrite, fprintf, fscanf, fclose, …
   • Private buffer kept in user space
   • Minimizes the number of I/O system calls

4. Software Block Cache Design

   • Has copies of blocks that are also on disk

   • Upon read or write

     • Check if a buffer contains the block
     • If not, get from disk
     • To make room, remove LRU block