一、操作系统是如何工作的？
计算机的三个法宝分别是存储程序计算机、函数调用堆栈和中断机制，本章中主要
涉及到函数调用堆栈和中断机制
1.1中断
中断是指计算机运行过程中，出现某些意外情况需主机干预时，机器能自动停止正在运行的程序并转入处理新情况的程序，处理完毕后又返回原被暂停的程序继续运行。在
没有中断的机制之前，计算机只能批处理程序，而无法做到并发工作。
1.2堆栈
堆栈是c语言程序运行时必须使用的记录函数调用路径和参数存储的空间，其作用主要有：记录函数调用框架、传递函数参数、保存返回值的地址、提供函数内部局部变量
的存储空间等。
二、实验
实验要求：
完成一个简单的时间片轮转多道程序内核代码，完成一个简单的时间片轮转多道程序内核代码。
添加头文件mypcb.h
/*
* linux/mykernel/mypcb.h
*
* Kernel internal PCB types
*
* Copyright (C) 2013 Mengning
*
*/
#define MAX_TASK_NUM 4
#define KERNEL_STACK_SIZE 1024*2
/* CPU-specific state of this task */
struct Thread {
unsigned long ip;
unsigned long sp;
};
typedef struct PCB{
int pid;
volatile long state; /* -1 unrunnable, 0 runnable, >0 stopped */
unsigned long stack[KERNEL_STACK_SIZE];
/* CPU-specific state of this task */
struct Thread thread;
unsigned long task_entry;
struct PCB *next;
}tPCB;
void my_schedule(void);
修改mymain.c
/*
* linux/mykernel/mymain.c
*
* Kernel internal my_start_kernel
* Change IA32 to x86-64 arch, 2020/4/26
*
* Copyright (C) 2013, 2020 Mengning
*
*/
#include <linux/types.h>
#include <linux/string.h>
#include <linux/ctype.h>
#include <linux/tty.h>
#include <linux/vmalloc.h>
#include "mypcb.h"
tPCB task[MAX_TASK_NUM];
tPCB * my_current_task = NULL;
volatile int my_need_sched = 0;
void my_process(void);
void __init my_start_kernel(void)
{
int pid = 0;
int i;
/* Initialize process 0*/
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];
task[pid].next = &task[pid];
/*fork more process */
for(i=1;i<MAX_TASK_NUM;i++)
{
memcpy(&task[i],&task[0],sizeof(tPCB));
task[i].pid = i;
task[i].thread.sp = (unsigned long)(&task[i].stack[KERNEL_STACK_SIZE-1]);
task[i].next = task[i-1].next;
task[i-1].next = &task[i];
}
/* start process 0 by task[0] */
pid = 0;
my_current_task = &task[pid];
asm volatile(
"movl %1,%%esp
" /* set task[pid].thread.sp to esp */
"pushl %1
" /* push rbp */
"pushl %0
" /* push task[pid].thread.ip */
"ret
" /* pop task[pid].thread.ip to rip */
:
: "c" (task[pid].thread.ip),"d" (task[pid].thread.sp) /* input c or d mean %ecx/%edx*/
);
}
int i = 0;
void my_process(void)
{
while(1)
{
i++;
if(i%10000000 == 0)
{
printk(KERN_NOTICE "this is process %d -
",my_current_task->pid);
if(my_need_sched == 1)
{
my_need_sched = 0;
my_schedule();
}
printk(KERN_NOTICE "this is process %d +
",my_current_task->pid);
}
}
}
修改myinterrupt.c
/*
* linux/mykernel/myinterrupt.c
*
* Kernel internal my_timer_handler
* Change IA32 to x86-64 arch, 2020/4/26
*
* Copyright (C) 2013, 2020 Mengning
*
*/
#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(time_count%1000 == 0 && my_need_sched != 1)
{
printk(KERN_NOTICE ">>>my_timer_handler here<<<
");
my_need_sched = 1;
}
time_count ++ ;
return;
}
void my_schedule(void)
{
tPCB * next;
tPCB * prev;
if(my_current_task == NULL
|| my_current_task->next == NULL)
{
return;
}
printk(KERN_NOTICE ">>>my_schedule<<<
");
/* schedule */
next = my_current_task->next;
prev = my_current_task;
if(next->state == 0)/* -1 unrunnable, 0 runnable, >0 stopped */
{
my_current_task = next;
printk(KERN_NOTICE ">>>switch %d to %d<<<
",prev->pid,next->pid);
/* switch to next process */
asm volatile(
"pushl %%ebp
" /* save ebp of prev */
"movl %%esp,%0
" /* save esp of prev */
"movl %2,%%esp
" /* restore esp of next */
"movl $1f,%1
" /* save rip of prev */
"pushl %3
"
"ret
" /* restore rip of next */
"1: " /* next process start here */
"popq %%ebp
"
: "=m" (prev->thread.sp),"=m" (prev->thread.ip)
: "m" (next->thread.sp),"m" (next->thread.ip)
);
}
return;
}
运行结果：
四、遇到的问题及解决
刚开始从GitHub上下载的有关代码编译出现以下错误：
检查后发现是GitHub上代码与实验楼环境不兼容，将pushq改为pushl后该问题解决，后又出现以下问题：
这个问题不太清楚是怎么产生的，猜测是编译器中代码的格式问题，使用Gvim重新写入代码后问题消失，应该就是代码格式的问题。