Chapter 14: I/O Systems

I/O Hardware

Common concepts ：

• Port
• Bus (daisy chain or shared direct access)
• Controller (host adapter) 操作端口和总线的设备

Devices have addresses, used by ：

• Direct I/O instructions
• Memory-mapped I/O

Polling：

Determines state of device：

• command-ready
• busy
• Error

Busy-wait cycle to wait for I/O from device（忙等待）

Direct Memory Access

• Used to avoid programmed I/O for large data movement
• Requires DMA controller
• Bypasses CPU to transfer data directly between I/O device and memory

Application I/O Interface

Kernel I/O Subsystem

Scheduling ：

• Some I/O request ordering via per-device queue
• Some OSs try fairness

Buffering ：

store data in memory while transferring between devices

• To cope with device speed mismatch
• To cope with device transfer size mismatch
• To maintain “copy semantics”

Caching -：

fast memory holding copy of data

• Always just a copy
• Key to performance

Spooling（假脱机）

hold output for a device

• If device can serve only one request at a time
• i.e., Printing

Device reservation -provides exclusive access to a device

• System calls for allocation and deallocation
• Watch out for deadlock

I/O Protection

User process may accidentally or purposefully attempt to disrupt normal operation via illegal I/O instructions.

• All I/O instructions defined to be privileged
• I/O must be performed via system calls

Performance

I/O a major factor in system performance:

• Demands CPU to execute device driver, kernel I/O code
• Context switches due to interrupts
• Data copying
• Network traffic especially stressful

Improving Performance:

• Reduce number of context switches
• Reduce data copying
• Reduce interrupts by using large transfers, smart controllers, polling
• Use DMA
• Balance CPU, memory, bus, and I/O performance for highest throughput

DMA原理

DMA原理：DMA 是所有现代电脑的重要特色，他允许不同速度的硬件装置来沟通，而不需要依于 CPU 的大量 中断 负载。否则，CPU 需要从来源把每一片段的资料复制到暂存器，然后把他们再次写回到新的地方。在这个时间中，CPU 对于其他的工作来说就无法使用。 DMA 传输重要地将一个内存区从一个装置复制到另外一个。当 CPU 初始化这个传输动作，传输动作本身是由 DMA 控制器 来实行和完成。典型的例子就是移动一个外部内存的区块到芯片内部更快的内存去。像是这样的操作并没有让处理器工作拖延，反而可以被重新排程去处理其他的工作。DMA 传输对于高效能 嵌入式系统 算法和网络是很重要的。　
　　在实现DMA传输时，是由DMA控制器直接掌管总线，因此，存在着一个总线控制权转移问题。即DMA传输前，CPU要把总线控制权交给DMA控制器，而在结束DMA传输后，DMA控制器应立即把总线控制权再交回给CPU。

后记：至此，OS的主要课程就结束啦，感谢zwshen老师井井有条的线上教学，以及学长和同学们的帮助。很欢迎大家和我讨论关于OS的问题呀！联系方式 [email protected]