操作系统基础知识

linux系统组成部分：linux内核、shell、实用工具utility、应用程序、文件系统。

原语：若干机器指令构成的一段代码，原语在执行期间不可分割，一旦开始执行，不能被中断。

PV原语：与进程间通信机制信号量对应的话，当信号量大于0时，表示可供并发进程使用的资源数，小于0时，表示等待中的进程数。P操作信号量-1，V操作信号量+1。PV操作每个进程只能成对使用一次。

进程调度：先进先出、时间片轮询、最高优先级和多级队列反馈。

进程间通信方法：

信号量（特殊地，互斥信号量和同步信号量为初始值为1和0的信号量）、

共享内存、

消息机制（包括消息缓冲和信箱）、

管道通信（也叫共享文件通信）。

http://en.wikipedia.org/wiki/Deadlock

进程死锁的四个条件：互斥条件、请求与保持条件、不剥夺条件和循环等待条件。

A deadlock situation can arise if and only if all of the following conditions hold simultaneously in a system:^[1]

1. Mutual Exclusion: At least one resource must be non-shareable.^[1] Only one process can use the resource at any given instant of time.
2. Hold and Wait or Resource Holding: A process is currently holding at least one resource and requesting additional resources which are being held by other processes.
3. No Preemption: The operating system must not de-allocate resources once they have been allocated; they must be released by the holding process voluntarily.
4. Circular Wait: A process must be waiting for a resource which is being held by another process, which in turn is waiting for the first process to release the resource. In general, there is a set of waiting processes, P = {P₁, P₂, ..., P_N}, such that P₁ is waiting for a resource held by P₂, P₂ is waiting for a resource held by P₃ and so on till P_N is waiting for a resource held by P₁.^[1][7]

These four conditions are known as the Coffman conditions from their first description in a 1971 article by Edward G. Coffman, Jr.^[7]Unfulfillment of any of these conditions is enough to preclude a deadlock from occurring.

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存储管理：可变分区管理、页式管理、虚拟页式管理（又叫请求页式管理）。

可变分区管理：编程时，所用地址都是逻辑地址，当程序装载到内存时，通过存储管理得到该进程内存空间的物理基地址和限定长度，并将两者存储在该进程的进程控制块PCB中。进程运行时，所有地址的访问，均会“逻辑地址+基地址”得到程序或数据的物理地址。逻辑上相邻的地址，物理上也相邻。

页式管理：内存等长地分为若干物理页面（块），程序的逻辑空间按照同样大小划分为等长逻辑页面（页）。每个进程有一张页表，记录逻辑页面和物理页面的对应关系，页表行数等于逻辑页面数。逻辑上相邻的页面，物理上不一定相邻。页表基址和页表长度存储在该进程的进程控制块中。如果程序占用存储空间较大，则页表占用空间可能超过一个页面，此时可采用多级页表结构。