进程的概念

进程:运行的程序(runing program)

通常情况下，一台pc上需要同时运行多个进程，以便提供易用性。

需要解决的问题：如何提供多个CPU的假象
->虚拟化CPU技术(分时复用CPU time sharing of CPU)

底层机制(mechanisms eg.上下文切换)+上层策略(policies eg.进程调度策略) 实现分时复用

进程的组成: 地址空间(包含指令和数据等)、寄存器(eg. IP、SP等)、持久存储设备

进程API:

Create: An operating system must include some method to create new processes. When you type a command into the shell, or double-click on an application icon, the OS is invoked to create a new process to run the program you have indicated.
Destroy: As there is an interface for process creation, systems also provide an interface to destroy processes forcefully. Of course, many processes will run and just exit by themselves when complete; when they don’t, however, the user may wish to kill them, and thus an interface to halt a runaway process is quite useful.
Wait: Sometimes it is useful to wait for a process to stop running; thus some kind of waiting interface is often provided.
Miscellaneous Control: Other than killing or waiting for a process, there are sometimes other controls that are possible. For example,most operating systems provide some kind of method to suspend a process (stop it from running for a while) and then resume it (continue it running).
Status: There are usually interfaces to get some status information about a process as well, such as how long it has run for, or what state it is in.

进程创建: A Little More Detail

从磁盘将程序加载到内存地址空间
早期操作系统: 一次全部加载指令和数据 -> 现代操作系统: 懒加载，每次只加载进程运行时需要的部分指令和数据(paging和swapping技术)
分配stack空间(C语言中用于局部变量、函数参数、返回地址),操作系统还需要将main；函数参数填充到stack中(eg. argc&argv)
分配heap(C语言中用于动态分配内存，malloc分配，free释放，heap大小根据请求动态增长)
I/O相关操作(eg. 在unix系统中，每个进程默认关联三个打开的文件描述符:标准输入、标准输出和标准出错，更多I/O相关内存位于持久化部分)

进程状态

Running: In the running state, a process is running on a processor.This means it is executing instructions.
Ready: In the ready state, a process is ready to run but for some reason the OS has chosen not to run it at this given moment.
Blocked: In the blocked state, a process has performed some kind of operation that makes it not ready to run until some other event takes place. A common example: when a process initiates an I/O request to a disk, it becomes blocked and thus some other process can use the processor.

操作系统数据结构

Information an OS needs to track about each process in the xv6 kernel:

// the registers xv6 will save and restore
// to stop and subsequently restart a process
struct context {
int eip;
int esp;
int ebx;
int ecx;
int edx;
int esi;
int edi;
int ebp;
};
// the different states a process can be in
enum proc_state { UNUSED, EMBRYO, SLEEPING,
RUNNABLE, RUNNING, ZOMBIE };
// the information xv6 tracks about each process
// including its register context and state
struct proc {
char *mem; // Start of process memory
uint sz; // Size of process memory
char *kstack; // Bottom of kernel stack
// for this process
enum proc_state state; // Process state
int pid; // Process ID
struct proc *parent; // Parent process
void *chan; // If !zero, sleeping on chan
int killed; // If !zero, has been killed
struct file *ofile[NOFILE]; // Open files
struct inode *cwd; // Current directory
struct context context; // Switch here to run process
struct trapframe *tf; // Trap frame for the
// current interrupt
};

一些术语：

The process is the major OS abstraction of a running program. At any point in time, the process can be described by its state: the contents of memory in its address space, the contents of CPU registers (including the program counter and stack pointer, among others),and information about I/O (such as open files which can be read or written).
The process API consists of calls programs can make related to processes. Typically, this includes creation, destruction,and other useful calls.
Processes exist in one of many different process states, including running, ready to run, and blocked. Different events (e.g., getting scheduled or descheduled, or waiting for an I/O to complete) transition a process from one of these states to the other.
A process list contains information about all processes in the system. Each entry is found in what is sometimes called a process control block (PCB), which is really just a structure that contains information about a specific process.

进程API (实用内容)

//todo

底层机制: Limited Direct Execution

虚拟化CPU->同时运行多个进程(的假象)
idea: 分时复用CPU
challenges:

performance : 高性能
control : OS保留控制能力

实现: 硬件支持+软件实现

基本技术: 限制直接执行(Limited Direct Execution)

the problem without “Limited”:

限制操作
进程切换

problem 1 : Restricted Operations
通过系统调用限制特权指令的执行：
即中断机制

problem 2 : Switching Between Processes

A Cooperative Approach: Wait For System Calls
通过系统调用陷入内核
A Non-Cooperative Approach: The OS Takes Control
时钟中断:使内核获得CPU控制

由时钟中断构造的进程切换有两次寄存器的保存和恢复，第一次发生在时钟中断发生时，由硬件将user registers保存到进程内核栈中，第二次发生在操作系统进行进程切换时，软件将kernel registers保存到进程控制块中。

处理中断/陷入时发生中断/陷入
并行部分将做详细讨论: 主要是关中断和锁机制

上下文切换(context switch) -> 进程切换