一.操作系统工作概述
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；
函数调用堆栈，高级语言得以运行的基础；
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, 其中有eip和esp
unsigned longtask_entry;//任务的函数起始处, 也就是任务第一次执行的起始位置
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" //引入其中两个结构体表示
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号进程栈在最开始的位置</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号进程先执行
<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)//判断是否调度；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>如果下一个任务不是第一次被调度, 则执行，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：]处开始继续执行</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>, 则以下面这种方式来处理</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：]处开始继续执行</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;
}
author: 于凯
参考课程：《Linux内核分析》MOOC课程http://mooc.study.163.com/course/USTC-1000029000
原文：http://blog.csdn.net/kaiandshan/article/details/44278421