一.操作系统工作概述

1. 存储程序计算机工作模型，计算机系统最最基础性的逻辑结构；

2. 函数调用堆栈，高级语言得以执行的基础；

3. 中断。多道程序操作系统的基点。

二.代码分析

在上一篇博文《搭建OS kernel环境方法》的基础上进行时间片轮转多道程序的小os.

主要对mypcb.h,  mymain.c 和myinterrupt.c这三个文件进行分析。

<pre name="code" class="cpp"><span style="font-size:12px;">//mypcb.h
</span>

<span style="font-size:12px;">#define MAX_TASK_NUM        4
#define KERNEL_STACK_SIZE   1024*8
/* CPU-specific state of this task */
struct Thread {//给任务定义一个eip和esp
    unsigned longip;
    unsigned longsp;
};
typedef struct PCB{
    int pid;//任务编号
    volatile long state;/* -1 unrunnable, 0 runnable, >0 stopped */
    char stack[KERNEL_STACK_SIZE];     //定义栈空间
    /* CPU-specific state of this task */
    struct Thread thread;       //定义进程的结构体thread, 当中有eip和esp
    unsigned longtask_entry;//任务的函数起始处, 也就是任务第一次运行的起始位置
    struct PCB *next;//一个任务链表, 指向下一个任务
}tPCB;</span>

//mymain.c
#include <linux/types.h>
#include <linux/string.h>
#include <linux/ctype.h>
#include <linux/tty.h>
#include <linux/vmalloc.h>
#include "mypcb.h"    //引入当中两个结构体表示
tPCB task[MAX_TASK_NUM];//定义两个数组
tPCB * my_current_task = NULL;
volatile int my_need_sched = 0;//定义是否调度, 1则调度, 0则不调度
void my_process(void);
void __init my_start_kernel(void)    //起始函数位置
{
    int pid = 0;
    int i;
    <strong>/* Initialize process 0*/</strong>
    task[pid].pid = pid;
    task[pid].state = 0;/* -1 unrunnable, 0 runnable, >0 stopped */
    task[pid].task_entry = task[pid].thread.ip = (unsigned long)my_process;
    task[pid].thread.sp = (unsigned long)&task[pid].stack[KERNEL_STACK_SIZE-1];   <strong>//0号进程栈在最開始的位置</strong>
    task[pid].next = &task[pid];
    
   <strong> /*fork more process */</strong>
    for(i=1;i<MAX_TASK_NUM;i++)
    {
        memcpy(&task[i],&task[0],sizeof(tPCB));//复制0号进程的结构形式
        task[i].pid = i;
        task[i].state = -1;//初始的任务(除0号进程外)都设置成未运行
        task[i].thread.sp = (unsigned long)&task[i].stack[KERNEL_STACK_SIZE-1];
        task[i].next = task[i-1].next;<strong>//新fork的进程加到进程链表的尾部,  该新建任务的next指向上一个任务的next,也就是自己（最后一个）</strong>
        task[i-1].next = &task[i];  <strong>//配置上一个任务的next指向这时候新创建的任务</strong>
    }
    /* start process 0 by task[0] */
    pid = 0;
    my_current_task = &task[pid];//先让0号进程先运行
  <strong>  asm volatile(
      "movl %1,%%esp\n\t" /* set task[pid].thread.sp to esp */
      "pushl %1\n\t"        /* push ebp ,当前esp=ebp*/
      "pushl %0\n\t"        /* push task[pid].thread.ip */
      "ret\n\t"            /* pop task[pid].thread.ip to eip */
      "popl %%ebp\n\t"
      : 
      : "c" (task[pid].thread.ip),"d" (task[pid].thread.sp)/* input c or d mean %ecx/%edx*/
      );</strong>
}   
void my_process(void)
{
    int i = 0;
    while(1)
    {
        i++;
        if(i%10000000 == 0)
        {
            printk(KERN_NOTICE "this is process %d -\n",my_current_task->pid);
            if(my_need_sched == 1)//推断是否调度。该值可有itnerrupt.c中的函数来配置
            {
                my_need_sched = 0;
                my_schedule(); //主动调动的机制
           }
           printk(KERN_NOTICE "this is process %d +\n",my_current_task->pid);
        }     
    }
}

//myinterrupt.c
#include <linux/types.h>
#include <linux/string.h>
#include <linux/ctype.h>
#include <linux/tty.h>
#include <linux/vmalloc.h>
#include "mypcb.h"
extern tPCB task[MAX_TASK_NUM];
extern tPCB * my_current_task;
extern volatile int my_need_sched;
volatile int time_count = 0;
/*
 * Called by timer interrupt.
 * it runs in the name of current running process,
 * so it use kernel stack of current running process
 */
void my_timer_handler(void)
{
#if 1
    if(time_count%1000 == 0 && my_need_sched != 1)//时钟中断1000次的时候，调度一次, 配置调度值为1
    {
        printk(KERN_NOTICE ">>>my_timer_handler here<<<\n");
        my_need_sched = 1;
    } 
    time_count ++ ;  
#endif
    return;  
}
void my_schedule(void)     //<span style="color:#ff0000;">调度函数, 核心函数</span>
{
    tPCB * next;//定义两个指针
    tPCB * prev;
    if(my_current_task == NULL //当前进程和下一进程为空, 即没有任务, 返回
        || my_current_task->next == NULL)
    {
      return;
    }
    printk(KERN_NOTICE ">>>my_schedule<<<\n");
    <strong><span style="color:#ff0000;">/* 在调度函数中, next指向的是下一个将要被调度的任务, prev指向的是当前正在运行的任务*/</span></strong>
    /* schedule */
    next = my_current_task->next;//把当前进程的下一个进程赋值给next。当前进程赋值给prev
    prev = my_current_task;
    if(next->state == 0)/* -1 unrunnable, 0 runnable, >0 stopped */ 
    {   //<strong>假设下一个任务不是第一次被调度, 则运行，下一个进程<span style="color:#ff0000;">有进程上下文</span></strong>
      	/* switch to next process */
     	<span style="color:#ff0000;">asm volatile( 
        	"pushl %%ebp\n\t"       /* save 当前进程 ebp */
       		"movl %%esp,%0\n\t"     /* save 当前 esp 赋值到prev.thread.sp */
        	"movl %2,%%esp\n\t"     /* restore 下一个进程的sp到 esp */
        	"movl $1f,%1\n\t"       /*<strong> save 当前进程的 eip =[ 1：]处地址，即下一次从[ 1：]处開始继续运行</strong> */
        
		/* 启动下一个进程*/
		"pushl %3\n\t"          /*保存下一个进程eip保存到栈里面*/
        	"ret\n\t"               /* restore  eip */
        
		"1:\t"                  /* next process start here */
        	"popl %%ebp\n\t"
        	: "=m" (prev->thread.sp),"=m" (prev->thread.ip)
        	: "m" (next->thread.sp),"m" (next->thread.ip)
      ); </span>
      my_current_task = next; 
      printk(KERN_NOTICE ">>>switch %d to %d<<<\n",prev->pid,next->pid);   
    }
    else
    {  <strong> //下一个进程为第一次运行时,<span style="color:#ff0000;">没有进程上下文</span>, 则以以下这样的方式来处理</strong>
        next->state = 0;
        my_current_task = next;
        printk(KERN_NOTICE ">>>switch %d to %d<<<\n",prev->pid,next->pid);
        /* switch to new process */
    	<span style="color:#ff0000;">asm volatile( 
        	"pushl %%ebp\n\t"       /* save ebp */
       		"movl %%esp,%0\n\t"     /* save esp */x`
        	"movl %2,%%esp\n\t"     /* restore  esp */
        	"movl %2,%%ebp\n\t"     /* restore  ebp */
        	"movl $1f,%1\n\t"       /*<strong> save 当前进程的 eip =[ 1：]处地址，即下一次从[ 1：]处開始继续运行</strong> */

		/* 启动下一个进程*/
        	"pushl %3\n\t" 
        	"ret\n\t"               /* restore  eip */

        	: "=m" (prev->thread.sp),"=m" (prev->thread.ip)
        	: "m" (next->thread.sp),"m" (next->thread.ip)
          );          </span>
    }   
    return;
}