一.操作系統工作概述

1. 存儲程序計算機工作模型，計算機系統最最基礎性的邏輯結構；

2. 函數調用堆棧，高級語言得以執行的基礎；

3. 中斷。多道程序操作系統的基點。

二.代碼分析

在上一篇博文《搭建OS kernel環境方法》的基礎上進行時間片輪轉多道程序的小os.

主要對mypcb.h, mymain.c 和myinterrupt.c這三個文件進行分析。

<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 {//給任務定義一個eip和esp
    unsigned longip;
    unsigned longsp;
};
typedef struct PCB{
    int pid;//任務編號
    volatile long state;/* -1 unrunnable, 0 runnable, >0 stopped */
    char stack[KERNEL_STACK_SIZE];     //定義棧空間
    /* CPU-specific state of this task */
    struct Thread thread;       //定義進程的結構體thread, 當中有eip和esp
    unsigned longtask_entry;//任務的函數起始處, 也就是任務第一次運行的起始位置
    struct PCB *next;//一個任務鏈表, 指向下一個任務
}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"    //引入當中兩個結構體表示
tPCB task[MAX_TASK_NUM];//定義兩個數組
tPCB * my_current_task = NULL;
volatile int my_need_sched = 0;//定義是否調度, 1則調度, 0則不調度
void my_process(void);
void __init my_start_kernel(void)    //起始函數位置
{
    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號進程棧在最開始的位置</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));//復制0號進程的結構形式
        task[i].pid = i;
        task[i].state = -1;//初始的任務(除0號進程外)都設置成未運行
        task[i].thread.sp = (unsigned long)&task[i].stack[KERNEL_STACK_SIZE-1];
        task[i].next = task[i-1].next;<strong>//新fork的進程加到進程鏈表的尾部,  該新建任務的next指向上一個任務的next,也就是自己（最後一個）</strong>
        task[i-1].next = &task[i];  <strong>//配置上一個任務的next指向這時候新創建的任務</strong>
    }
    /* start process 0 by task[0] */
    pid = 0;
    my_current_task = &task[pid];//先讓0號進程先運行
  <strong>  asm volatile(
      "movl %1,%%esp\n\t" /* set task[pid].thread.sp to esp */
      "pushl %1\n\t"        /* push ebp ,當前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)//推斷是否調度。該值可有itnerrupt.c中的函數來配置
            {
                my_need_sched = 0;
                my_schedule(); //主動調動的機制
           }
           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)//時鐘中斷1000次的時候，調度一次, 配置調度值為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;">調度函數, 核心函數</span>
{
    tPCB * next;//定義兩個指針
    tPCB * prev;
    if(my_current_task == NULL //當前進程和下一進程為空, 即沒有任務, 返回
        || my_current_task->next == NULL)
    {
      return;
    }
    printk(KERN_NOTICE ">>>my_schedule<<<\n");
    <strong><span style="color:#ff0000;">/* 在調度函數中, next指向的是下一個將要被調度的任務, prev指向的是當前正在運行的任務*/</span></strong>
    /* schedule */
    next = my_current_task->next;//把當前進程的下一個進程賦值給next。當前進程賦值給prev
    prev = my_current_task;
    if(next->state == 0)/* -1 unrunnable, 0 runnable, >0 stopped */ 
    {   //<strong>假設下一個任務不是第一次被調度, 則運行，下一個進程<span style="color:#ff0000;">有進程上下文</span></strong>
      	/* switch to next process */
     	<span style="color:#ff0000;">asm volatile( 
        	"pushl %%ebp\n\t"       /* save 當前進程 ebp */
       		"movl %%esp,%0\n\t"     /* save 當前 esp 賦值到prev.thread.sp */
        	"movl %2,%%esp\n\t"     /* restore 下一個進程的sp到 esp */
        	"movl $1f,%1\n\t"       /*<strong> save 當前進程的 eip =[ 1：]處地址，即下一次從[ 1：]處開始繼續運行</strong> */
        
		/* 啟動下一個進程*/
		"pushl %3\n\t"          /*保存下一個進程eip保存到棧裏面*/
        	"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> //下一個進程為第一次運行時,<span style="color:#ff0000;">沒有進程上下文</span>, 則以以下這樣的方式來處理</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 當前進程的 eip =[ 1：]處地址，即下一次從[ 1：]處開始繼續運行</strong> */

		/* 啟動下一個進程*/
        	"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;
}