I/O & Display

一段操纵外设的程序

int fd = open("/dev/xxx");
for (int i = 0; i < 10; i++) {
	write(fd, i, sizeof(int));
}
colse(fd);

• 操作系统为用户提供统一的接口：open、read、write、close……

• 不同的设备对应不同的设备文件（/dev/xxx），根据设备文件找到控制器的地址、内容格式等

向显示器输出

// linux/fs/read_write.c
// fd是找到file的索引
int sys_write(unsigned int fd, char *buf, int count)
{
	struct file* file;
	file = current->filp[fd];
	inode = file->f_inode;
	// ...
}

void main(void)
{
	if (!fork()) {
		init();
	}
}

void init(void)
{
	open("/dev/tty0", O_RDWR, 0); // 0
	dup(0); // 1 拷贝文件
	dup(0); // 2
	execve("/bin/sh", argv, envp);
}

// linux/fs/open.c
int sys_open(const char* filename, int flag)
{
	i = open_namei(filename, flag, &inode);  // inode是文件信息
	current->filp[fd] = f;  // 第一个空闲的fd
	f=f_mode = inode->i_mode;
	f->f_inode = inode;
	f->f_count = 1;
	return fd;
}

// linux/fs/read_write.c
int sys_write(unsigned int fd,char * buf,int count)
{
	// ...
	inode=file->f_inode;
	
	if (S_ISCHR(inode->i_mode))
		return rw_char(WRITE,inode->i_zone[0],buf,count,&file->f_pos);
	if (S_ISBLK(inode->i_mode))
		return block_write(inode->i_zone[0],&file->f_pos,buf,count);
	if (S_ISREG(inode->i_mode))
		return file_write(inode,file,buf,count);
	// ...
}

// linux/fs/char_dev.c

static int rw_ttyx(int rw,unsigned minor,char * buf,int count,off_t * pos)
{
	return ((rw==READ)?tty_read(minor,buf,count):
		tty_write(minor,buf,count));
}

static crw_ptr crw_table[]={
	NULL,		/* nodev */
	rw_memory,	/* /dev/mem etc */
	NULL,		/* /dev/fd */
	NULL,		/* /dev/hd */
	rw_ttyx,	/* /dev/ttyx */
	rw_tty,		/* /dev/tty */
	NULL,		/* /dev/lp */
	NULL};		/* unnamed pipes */

int rw_char(int rw,int dev, char * buf, int count, off_t * pos)
{
	crw_ptr call_addr;

	if (!(call_addr=crw_table[MAJOR(dev)]))
		return -ENODEV;
	return call_addr(rw,MINOR(dev),buf,count,pos);
}

// linux/kernel/tty_io.c
int tty_write(unsigned channel, char * buf, int nr)
{
	struct tty_struct * tty;
	char c, *b=buf;

	tty = channel + tty_table;
	while (nr>0) {
		sleep_if_full(&tty->write_q);
		// ...
		while (nr>0 && !FULL(tty->write_q)) {
			c=get_fs_byte(b);  // 从用户缓存区读
			// ...
			PUTCH(c,tty->write_q);
		}
		tty->write(tty);
	}
	// ...
}

// include/linux/tty.h
struct tty_struct {
	// ...
	void (*write)(struct tty_struct * tty);
	struct tty_queue read_q;
	struct tty_queue write_q;
	struct tty_queue secondary;
};

struct tty_struct tty_table[] = {
	con_write,
	{0, 0, 0, 0, ""},
	{0, 0, 0, 0, ""},
	{},
	// ...
}

// linux/kernel/chr_drv/console.c
void con_write(struct tty_struct * tty)
{
	// ...
	GETCH(tty->write_q,c);
	switch(state) {
		case 0:
			if (c>31 && c<127) {
				if (x>=video_num_columns) {
					x -= video_num_columns;
					pos -= video_size_row;
					lf();
				}
				__asm__("movb attr,%%ah\n\t"
					"movw %%ax,%1\n\t"
					::"a" (c),"m" (*(short *)pos)
					);
				pos += 2;
				x++;
			}
	// ...
}