static inline _syscall0(int,fork) 
static inline _syscall0(int,pause)

int fun(int a,int b);
int m=10;
int main(){
int i=4;
int j=5;
m=fun(i,j);
return 0;
}
int fun(int a,int b){
int c=0;
c=a+b;
return c;
}

void init(void)
{
	int pid,i;

	setup((void *) &drive_info);
	(void) open("/dev/tty0",O_RDWR,0);//创建标准输入设备
	(void) dup(0);//创建标准输出设备
	(void) dup(0);
	...
}

int open(const char * filename, int flag, ...)
{
	register int res;
	va_list arg;

	va_start(arg,flag);
	__asm__("int $0x80"
		:"=a" (res)
		:"0" (__NR_open),"b" (filename),"c" (flag),
		"d" (va_arg(arg,int)));
	if (res>=0)
		return res;
	errno = -res;
	return -1;
}

int sys_open(const char * filename,int flag,int mode) //filename "/dev/tty0"
{
	struct m_inode * inode;
	struct file * f;
	int i,fd;

	mode &= 0777 & ~current->umask;
	//打开文件的进程是当前进程，在filep里面找一个空闲项
	for(fd=0 ; fd<NR_OPEN ; fd++) // #define NR_OPEN 20
		if (!current->filp[fd])
			break;
	if (fd>=NR_OPEN)
		return -EINVAL;
	current->close_on_exec &= ~(1<<fd);
	f=0+file_table;
	//找完找file table，引用计数为0的空闲项
	for (i=0 ; i<NR_FILE ; i++,f++)//#define NR_FILE 64
		if (!f->f_count) break;
	if (i>=NR_FILE)
		return -EINVAL;
	(current->filp[fd]=f)->f_count++;//当前进程的空闲的filep的哪一项指向空闲的file table里面的项，引用计数++
	//读i结点，filename：sysopen的参数const char*类型，返回给inode
	//如果open_namei成功返回0，失败返回1
	if ((i=open_namei(filename,flag,mode,&inode))<0) {
		current->filp[fd]=NULL;
		f->f_count=0;
		return i;
	}
/* ttys are somewhat special (ttyxx major==4, tty major==5) */
	if (S_ISCHR(inode->i_mode))//tty0是设备文件
        //设备文件的设备号放在i_zone[0]里面
		if (MAJOR(inode->i_zone[0])==4) {
			if (current->leader && current->tty<0) {
				current->tty = MINOR(inode->i_zone[0]);
				tty_table[current->tty].pgrp = current->pgrp;
			}
		} else if (MAJOR(inode->i_zone[0])==5)
			if (current->tty<0) {
				iput(inode);
				current->filp[fd]=NULL;
				f->f_count=0;
				return -EPERM;
			}
/* Likewise with block-devices: check for floppy_change */
	if (S_ISBLK(inode->i_mode))
		check_disk_change(inode->i_zone[0]);
	f->f_mode = inode->i_mode;
	f->f_flags = flag;
	f->f_count = 1;
	f->f_inode = inode;
	f->f_pos = 0;
	return (fd);
}

void init(void)
{
	...
	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())) {
        //  进程2：
		close(0);//关闭设备标准输入
		if (open("/etc/rc",O_RDONLY,0))
			_exit(1);
		execve("/bin/sh",argv_rc,envp_rc); //走syscall3，需要打开的执行文件和环境变量
		_exit(2);
	}
	if (pid>0)
        //进程1等待子进程退出
		while (pid != wait(&i))
			/* nothing */;
	while (1) {
		if ((pid=fork())<0) {
			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));
		}
		...
		}

static inline _syscall0(int,fork) //定义了fork函数
    
void main(void){
...
if(!fork())
init();
...
}

#define _syscall0(type,name) \
type name(void) \
{ \
long __res; \
__asm__ volatile ("int $0x80" \
	: "=a" (__res) \
	: "0" (__NR_##name)); \
if (__res >= 0) \
	return (type) __res; \
errno = -__res; \
return -1; \
}

//之前把0x9000-0x901F部分用来存储机器系统信息
#define EXT_MEM_K (*(unsigned short *)0x90002)
#define DRIVE_INFO (*(struct drive_info *)0x90080)//90080是硬盘参数表
#define ORIG_ROOT_DEV (*(unsigned short *)0x901FC)//901fc是跟设备号

 	ROOT_DEV = ORIG_ROOT_DEV;
 	drive_info = DRIVE_INFO;
	//内存大小=1M+扩展内存
	memory_end = (1<<20) + (EXT_MEM_K<<10);//左移20位：1M EXT_MEM_K机器设备信息，0x90002扩展内存kb数 左移10位kb->mb 
	//忽略不到4KB（一页）的内存页
	memory_end &= 0xfffff000;
	if (memory_end > 16*1024*1024)
		memory_end = 16*1024*1024;//如果内存大于16M按照16M计算
	if (memory_end > 12*1024*1024) 
		buffer_memory_end = 4*1024*1024;
	else if (memory_end > 6*1024*1024)
		buffer_memory_end = 2*1024*1024;
	else
		buffer_memory_end = 1*1024*1024;//设置缓冲区
	main_memory_start = buffer_memory_end;
//如果在Makefile里面指定了RAMDISK则建立虚拟盘
#ifdef RAMDISK 
	main_memory_start += rd_init(main_memory_start, RAMDISK*1024);
	//虚拟盘设置

int iam(const char * name);