main.c

一份精简的linux内核源代码

/*
 *  linux/init/main.c
 *
 *  (C) 1991  Linus Torvalds
 */

#define __LIBRARY__
#include <unistd.h>
#include <time.h>

/*
 * we need this inline - forking from kernel space will result
 * in NO COPY ON WRITE (!!!), until an execve is executed. This
 * is no problem, but for the stack. This is handled by not letting
 * main() use the stack at all after fork(). Thus, no function
 * calls - which means inline code for fork too, as otherwise we
 * would use the stack upon exit from 'fork()'.
 *
 * Actually only pause and fork are needed inline, so that there
 * won't be any messing with the stack from main(), but we define
 * some others too.
 */
static inline _syscall0(int,fork)
static inline _syscall0(int,pause)
static inline _syscall1(int,setup,void *,BIOS)
static inline _syscall0(int,sync)

#include <linux/tty.h>
#include <linux/sched.h>
#include <linux/head.h>
#include <asm/system.h>
#include <asm/io.h>

#include <stddef.h>
#include <stdarg.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>

#include <linux/fs.h>

static char printbuf[1024];   /*内核显示信息缓冲*/

extern int vsprintf();
extern void init(void);
extern void blk_dev_init(void);
extern void chr_dev_init(void);
extern void hd_init(void);
extern void floppy_init(void);
extern void mem_init(long start, long end);
extern long rd_init(long mem_start, int length);
extern long kernel_mktime(struct tm * tm);
extern long startup_time;

/*
 * This is set up by the setup-routine at boot-time
 */
#define EXT_MEM_K (*(unsigned short *)0x90002)    /*1mb以后的扩展内存大小（kb）*/
#define DRIVE_INFO (*(struct drive_info *)0x90080)/*硬盘参数表的32 字节内容*/
#define ORIG_ROOT_DEV (*(unsigned short *)0x901FC)/*根文件系统所在设备号*/

/*
 * Yeah, yeah, it's ugly, but I cannot find how to do this correctly
 * and this seems to work. I anybody has more info on the real-time
 * clock I'd be interested. Most of this was trial and error, and some
 * bios-listing reading. Urghh.
 */

#define CMOS_READ(addr) ({ /*读取CMOS实时时钟信息*/
outb_p(0x80|addr,0x70); \
inb_p(0x71); \
})

#define BCD_TO_BIN(val) ((val)=((val)&15) + ((val)>>4)*10) /*bcd码（以4bit表示一个10进制数）转成2进制数*/

static void time_init(void) /*读取CMOS实时时钟信息作为开机时间，并保存进startup_time中*/
{
	struct tm time;

	do {
		time.tm_sec = CMOS_READ(0); /*读取当前的秒（为bcd码）*/
		time.tm_min = CMOS_READ(2); /*读取当前的分钟（为bcd码）*/
		time.tm_hour = CMOS_READ(4);
		time.tm_mday = CMOS_READ(7);
		time.tm_mon = CMOS_READ(8);
		time.tm_year = CMOS_READ(9);
	} while (time.tm_sec != CMOS_READ(0));
	BCD_TO_BIN(time.tm_sec);/*bcd转2进制*/
	BCD_TO_BIN(time.tm_min);
	BCD_TO_BIN(time.tm_hour);
	BCD_TO_BIN(time.tm_mday);
	BCD_TO_BIN(time.tm_mon);
	BCD_TO_BIN(time.tm_year);
	time.tm_mon--;/*月份0－11*/
	startup_time = kernel_mktime(&time);/*计算从1970/1/1/0:00起至开机为止的时间（秒）*/
}

static long memory_end = 0;    /*物理内存大小*/
static long buffer_memory_end = 0;/*高速缓冲区末端地址*/
static long main_memory_start = 0;/*主内存开始位置*/

struct drive_info { char dummy[32]; } drive_info;/*硬盘参数表信息*/

void main(void)		/* This really IS void, no error here. */
{			/* The startup routine assumes (well, ...) this */
/*
 * Interrupts are still disabled. Do necessary setups, then
 * enable them
 */
 	ROOT_DEV = ORIG_ROOT_DEV;
 	drive_info = DRIVE_INFO;
	memory_end = (1<<20) + (EXT_MEM_K<<10);/*物理内存大小＝1mb＋扩展内存（kb）×1024*/
	memory_end &= 0xfffff000;              /*忽略不到1页（4kb）的内存*/
	if (memory_end > 16*1024*1024)         /*物理内存大小>16mb，按16mb计算*/
		memory_end = 16*1024*1024;
	if (memory_end > 12*1024*1024)          /*物理内存大小>12mb，设缓冲区末端＝4mb*/
		buffer_memory_end = 4*1024*1024;
	else if (memory_end > 6*1024*1024)      /*物理内存大小>6mb，设缓冲区末端＝2mb*/
		buffer_memory_end = 2*1024*1024;
	else
		buffer_memory_end = 1*1024*1024;    /*否则，设缓冲区末端＝1mb*/
	main_memory_start = buffer_memory_end;  /*主内存起始位置＝缓冲区末端*/
#ifdef RAMDISK
	main_memory_start += rd_init(main_memory_start, RAMDISK*1024);/*若在makefile中定义了虚拟盘符号，则此时从主内存中分配*/
#endif
	mem_init(main_memory_start,memory_end);/*主内存初始化(memory.c)*/
	trap_init();                           /*陷阱门（硬件中断向量）初始化(traps.c)*/
	blk_dev_init();                      /*块设备初始化(ll_rw_blk.c)*/
	chr_dev_init();                      /*字符设备初始化(tty_io.c)*/
	tty_init();                      /*tty初始化(tty_io.c)*/
	time_init();                      /*设置开机时间（76行）*/
	sched_init();                      /*调度程序初始化（sched.c）*//*对任务0的运行环境进行了设置，包括人工预先设置好的任务0的PCB、在GDT中填入任务0的TSS和LDT，并分别把他们加载到任务寄存器tr和局部段描述符寄存器ldtr中*/	
	buffer_init(buffer_memory_end);                      /*缓冲管理初始化，建立内存链表等（buffer.c）*/
	hd_init();                      /*硬盘初始化(hd.c)*/
	floppy_init();                      /*软驱初始化(flopy.c)*/
	sti();                      /*所有初始化完成，开中断*/	
	move_to_user_mode();/*移动到任务0中执行，此时进程0出现*/
	if (!fork()) {/*首次调用fork()创建进程1*/		/* we count on this going ok */ 
		init();/*加载根文件系统*//*init()将在进程1中执行*/
	}
/*
 *   NOTE!!   For any other task 'pause()' would mean we have to get a
 * signal to awaken, but task0 is the sole exception (see 'schedule()')
 * as task 0 gets activated at every idle moment (when no other tasks
 * can run). For task0 'pause()' just means we go check if some other
 * task can run, and if not we return here.
 */
	for(;;) pause();
}

static int printf(const char *fmt, ...)/*产生格式化信息并输出到标准设备stdout(1)，即屏幕上的显示，该程序使用vsprintf()将格式化的字符串放入printbuf缓冲区，然后用write()将缓冲区的内容输出到标准设备stdout(1)*/
{
	va_list args;
	int i;

	va_start(args, fmt);
	write(1,printbuf,i=vsprintf(printbuf, fmt, args));
	va_end(args);
	return i;
}

static char * argv_rc[] = { "/bin/sh", NULL };/**/
static char * envp_rc[] = { "HOME=/", NULL };

static char * argv[] = { "-/bin/sh",NULL };
static char * envp[] = { "HOME=/usr/root", NULL };

void init(void)/*尚未阅读！*/
{
	int pid,i;

	setup((void *) &drive_info);/*hd.c，71*//*读取硬盘参数包括分区表信息并安装根文件系统*/
	(void) open("/dev/tty0",O_RDWR,0);/*tty0对应终端控制台*//*设置进程1的标准输入*/
	(void) dup(0);/*设置进程1的标准输出*/
	(void) dup(0);/*设置进程1的标准出错输出*/
	printf("%d buffers = %d bytes buffer space\n\r",NR_BUFFERS,
		NR_BUFFERS*BLOCK_SIZE);
	printf("Free mem: %d bytes\n\r",memory_end-main_memory_start);
	if (!(pid=fork())) {/*新建一子进程，首次执行shell*/
		close(0);
		if (open("/etc/rc",O_RDONLY,0))
			_exit(1);
		execve("/bin/sh",argv_rc,envp_rc);
		_exit(2);
	}
	if (pid>0)
		while (pid != wait(&i))/*父进程等待子进程结束*/
			/* nothing */;
	while (1) {/*死循环*/
		if ((pid=fork())<0) {/*创建一新进程并执行shell*/
			printf("Fork failed in init\r\n");
			continue;
		}
		if (!pid) {
			close(0);close(1);close(2);
			setsid();
			(void) open("/dev/tty0",O_RDWR,0);
			(void) dup(0);
			(void) dup(0);
			_exit(execve("/bin/sh",argv,envp));
		}
		while (1)
			if (pid == wait(&i))
				break;
		printf("\n\rchild %d died with code %04x\n\r",pid,i);
		sync();/*同步缓冲区*/
	}
	_exit(0);	/* NOTE! _exit, not exit() */
}