/*
* 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)    @@定义fork
--sys_fork在unistd.h中定义syscall0
static inline _syscall0(int,pause)
static inline _syscall0(int,setup)
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];

extern int vsprintf();
extern void init(void);
extern void hd_init(void);
extern long kernel_mktime(struct tm * tm);
extern long startup_time;

/*
* 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) ({ \
outb_p(0x80|addr,0x70); \
inb_p(0x71); \
})

#define BCD_TO_BIN(val) ((val)=((val)&15) + ((val)>>4)*10)

static void time_init(void)
{
struct tm time;

do {
  time.tm_sec = CMOS_READ(0);
  time.tm_min = CMOS_READ(2);
  time.tm_hour = CMOS_READ(4);
  time.tm_mday = CMOS_READ(7);
  time.tm_mon = CMOS_READ(8)-1;
  time.tm_year = CMOS_READ(9);
} while (time.tm_sec != CMOS_READ(0));
BCD_TO_BIN(time.tm_sec);
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);
startup_time = kernel_mktime(&time);
}

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
*/
time_init();
tty_init();
trap_init();
sched_init();
buffer_init();
hd_init();
sti();
move_to_user_mode();   @@切至用户模式???到哪去了,回来了!!!
if (!fork()) {  /* we count on this going ok */
  init();   @@fork在unistd.h有其宏扩展int 0x80　
              如何返回两次?应该说子进程如何激活?
}
/*
*   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();
  @@pause 同fork在unistd.h有其宏扩展int 0x80 然后调schedule()
}

static int printf(const char *fmt, ...)
{
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[] = { "-",NULL };
static char * envp[] = { "HOME=/usr/root", NULL };

void init(void)
{
int i,j;

setup();  @@系统0号调用
if (!fork())
  _exit(execve("/bin/update",NULL,NULL));
(void) open("/dev/tty0",O_RDWR,0);  @@打开控制台
(void) dup(0);  @@复制
(void) dup(0);
printf("%d buffers = %d bytes buffer space\n\r",NR_BUFFERS,
  NR_BUFFERS*BLOCK_SIZE);
printf(" Ok.\n\r");
if ((i=fork())<0)
  printf("Fork failed in init\r\n");
else if (!i) {  @@启动一root shell进程?
  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));
}
j=wait(&i);
printf("child %d died with code %04x\n",j,i);
sync();
_exit(0); /* NOTE! _exit, not exit() */
}
 
 

本文转自中文Linux论坛