main函数源码

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/*
 * This is set up by the setup-routine at boot-time
 */
#define EXT_MEM_K (*(unsigned short *)0x90002) /*扩展内存数，单位为KB*/
#define DRIVE_INFO (*(struct drive_info *)0x90080)
#define ORIG_ROOT_DEV (*(unsigned short *)0x901FC)

void main(void)		/* This really IS void, no error here. */
{			/* The startup routine assumes (well, ...) this */
/*hexhe
 * 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);
	memory_end &= 0xfffff000;
	if (memory_end > 16*1024*1024)
		memory_end = 16*1024*1024;
	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;
	main_memory_start = buffer_memory_end;
#ifdef RAMDISK
	main_memory_start += rd_init(main_memory_start, RAMDISK*1024);
#endif
	mem_init(main_memory_start,memory_end);
	trap_init();
	blk_dev_init();
	chr_dev_init();
	tty_init();
	time_init();
	sched_init();
	buffer_init(buffer_memory_end);
	hd_init();
	floppy_init();
	sti();
	move_to_user_mode();
	if (!fork()) {		/* we count on this going ok */
		init();
	}
/*
 *   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();
}

下面我们来逐步分析。

main函数首先指定了ROOT_DEV(根文件系统设备号)的位置，在第几个硬盘的第几分区。之后驱动信息,文件的宏定义ORIG_ROOT_DEV

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#define DRIVE_INFO (*(struct drive_info *)0x90080)
#define ORIG_ROOT_DEV (*(unsigned short *)0x901FC)
void main(){
    ROOT_DEV = ORIG_ROOT_DEV;
	drive_info = DRIVE_INFO;
    /* .......*/
}

硬盘的逻辑设备号，也称为逻辑块地址（Logical Block Address，LBA），是硬盘上每个扇区的唯一标识符。它是一个从0开始的数字，表示扇区在硬盘上的位置。逻辑设备号由操作系统分配

img

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void main(void)		
{			
	/* ...... */
	memory_end = (1<<20) + (EXT_MEM_K<<10);
	memory_end &= 0xfffff000;
	if (memory_end > 16*1024*1024)
		memory_end = 16*1024*1024;
	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;
	main_memory_start = buffer_memory_end;
#ifdef RAMDISK
	main_memory_start += rd_init(main_memory_start, RAMDISK*1024);
#endif
	/* ...... */
}

linux0.11将内存划分为了以下三部分

image-20240228215112214

再划分好内存过后，则是进行各种初始化操作

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void main(void)		
{			
 	/*  ......  */
	mem_init(main_memory_start,memory_end);
	trap_init();
	blk_dev_init();
	chr_dev_init();
	tty_init();
	time_init();
	sched_init();
	buffer_init(buffer_memory_end);
	hd_init();
	floppy_init();
	sti();
	move_to_user_mode();
	if (!fork()) {		/* we count on this going ok */
		init();
	}
/*
 *   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();
}

内存初始化

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void mem_init(long start_mem, long end_mem)
{
	int i;

	HIGH_MEMORY = end_mem;
	for (i=0 ; i<PAGING_PAGES ; i++)
		mem_map[i] = USED;
	i = MAP_NR(start_mem);
	end_mem -= start_mem;
	end_mem >>= 12;
	while (end_mem-->0)
		mem_map[i++]=0;
}

在mem_init函数中使用mem_map数组对每个4K内存页(1页)进行管理。

中断初始化

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void trap_init(void)
{
	int i;
    /*设置0-16号中断*/
	set_trap_gate(0,&divide_error);
	set_trap_gate(1,&debug);
	set_trap_gate(2,&nmi);
	set_system_gate(3,&int3);	/* int3-5 can be called from all */
	set_system_gate(4,&overflow);
	set_system_gate(5,&bounds);
	set_trap_gate(6,&invalid_op);
	set_trap_gate(7,&device_not_available);
	set_trap_gate(8,&double_fault);
	set_trap_gate(9,&coprocessor_segment_overrun);
	set_trap_gate(10,&invalid_TSS);
	set_trap_gate(11,&segment_not_present);
	set_trap_gate(12,&stack_segment);
	set_trap_gate(13,&general_protection);
	set_trap_gate(14,&page_fault);
	set_trap_gate(15,&reserved);
	set_trap_gate(16,&coprocessor_error);
    
	for (i=17;i<48;i++)
		set_trap_gate(i,&reserved);
	set_trap_gate(45,&irq13);
	outb_p(inb_p(0x21)&0xfb,0x21);
	outb(inb_p(0xA1)&0xdf,0xA1);
	set_trap_gate(39,&parallel_interrupt);
}
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#define set_trap_gate(n,addr) \
	_set_gate(&idt[n],15,0,addr)
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#define _set_gate(gate_addr,type,dpl,addr) \
__asm__ ("movw %%dx,%%ax\n\t" \
	"movw %0,%%dx\n\t" \
	"movl %%eax,%1\n\t" \
	"movl %%edx,%2" \
	: \
	: "i" ((short) (0x8000+(dpl<<13)+(type<<8))), \
	"o" (*((char *) (gate_addr))), \
	"o" (*(4+(char *) (gate_addr))), \
	"d" ((char *) (addr)),"a" (0x00080000))

中断向量表中添加了一些段描述符

块设备初始化

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struct request request[NR_REQUEST];
void blk_dev_init(void)
{
	int i;

	for (i=0 ; i<NR_REQUEST ; i++) {
		request[i].dev = -1;
		request[i].next = NULL;
	}
}
struct blk_dev_struct {
	void (*request_fn)(void);
	struct request * current_request;
};

将request结构体的dev设置为-1，表示没有块请求，next设置为NULL

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/*
 * Ok, this is an expanded form so that we can use the same
 * request for paging requests when that is implemented. In
 * paging, 'bh' is NULL, and 'waiting' is used to wait for
 * read/write completion.
 */
struct request {
	int dev;		/* -1 if no request */
	int cmd;		/* READ or WRITE */
	int errors;		/*操作产生的错误数*/
	unsigned long sector; 		/*起始扇区*/
	unsigned long nr_sectors;	/*扇区数*/
	char * buffer;				/*缓冲区*/
	struct task_struct * waiting;	/*任务*/
	struct buffer_head * bh;
	struct request * next;
};

如果我们要访问硬盘的数据，我们需要告诉CPU我们访问硬盘的起始扇区和扇区数，是写入还是读取数据。

每一次访问都是一次请求。通过链表来维护访问。

TTY初始化

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#define ORIG_X			(*(unsigned char *)0x90000) /*光标位置*/
#define ORIG_Y			(*(unsigned char *)0x90001)
#define ORIG_VIDEO_PAGE		(*(unsigned short *)0x90004)	/*当前视频页面*/
#define ORIG_VIDEO_MODE		((*(unsigned short *)0x90006) & 0xff)	/*显示模式*/
#define ORIG_VIDEO_COLS 	(((*(unsigned short *)0x90006) & 0xff00) >> 8)
#define ORIG_VIDEO_LINES	(25)
#define ORIG_VIDEO_EGA_AX	(*(unsigned short *)0x90008)
#define ORIG_VIDEO_EGA_BX	(*(unsigned short *)0x9000a)
#define ORIG_VIDEO_EGA_CX	(*(unsigned short *)0x9000c)

#define VIDEO_TYPE_MDA		0x10	/* Monochrome Text Display	*/
#define VIDEO_TYPE_CGA		0x11	/* CGA Display 			*/
#define VIDEO_TYPE_EGAM		0x20	/* EGA/VGA in Monochrome Mode	*/
#define VIDEO_TYPE_EGAC		0x21	/* EGA/VGA in Color Mode	*/

img

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void tty_init(void)
{
	rs_init();
	con_init();
}
void rs_init(void)
{
	set_intr_gate(0x24,rs1_interrupt);
	set_intr_gate(0x23,rs2_interrupt);
	init(tty_table[1].read_q.data);
	init(tty_table[2].read_q.data);
	outb(inb_p(0x21)&0xE7,0x21);
}
/*
 *  void con_init(void);
 *
 * This routine initalizes console interrupts, and does nothing
 * else. If you want the screen to clear, call tty_write with
 * the appropriate escape-sequece.
 *
 * Reads the information preserved by setup.s to determine the current display
 * type and sets everything accordingly.
 */
void con_init(void)
{
	register unsigned char a;
	char *display_desc = "????";
	char *display_ptr;

	video_num_columns = ORIG_VIDEO_COLS;
	video_size_row = video_num_columns * 2;
	video_num_lines = ORIG_VIDEO_LINES;
	video_page = ORIG_VIDEO_PAGE;
	video_erase_char = 0x0720;
	
	if (ORIG_VIDEO_MODE == 7)			/* Is this a monochrome display? */
	{
		video_mem_start = 0xb0000;
		video_port_reg = 0x3b4;
		video_port_val = 0x3b5;
		if ((ORIG_VIDEO_EGA_BX & 0xff) != 0x10)
		{
			video_type = VIDEO_TYPE_EGAM;
			video_mem_end = 0xb8000;
			display_desc = "EGAm";
		}
		else
		{
			video_type = VIDEO_TYPE_MDA;
			video_mem_end	= 0xb2000;
			display_desc = "*MDA";
		}
	}
	else								/* If not, it is color. */
	{
		video_mem_start = 0xb8000;
		video_port_reg	= 0x3d4;
		video_port_val	= 0x3d5;
		if ((ORIG_VIDEO_EGA_BX & 0xff) != 0x10)
		{
			video_type = VIDEO_TYPE_EGAC;
			video_mem_end = 0xbc000;
			display_desc = "EGAc";
		}
		else
		{
			video_type = VIDEO_TYPE_CGA;
			video_mem_end = 0xba000;
			display_desc = "*CGA";
		}
	}

	/* Let the user known what kind of display driver we are using */
	
	display_ptr = ((char *)video_mem_start) + video_size_row - 8;
	while (*display_desc)
	{
		*display_ptr++ = *display_desc++;
		display_ptr++;
	}
	
	/* Initialize the variables used for scrolling (mostly EGA/VGA)	*/
	
	origin	= video_mem_start;
	scr_end	= video_mem_start + video_num_lines * video_size_row;
	top	= 0;
	bottom	= video_num_lines;

	gotoxy(ORIG_X,ORIG_Y);
	set_trap_gate(0x21,&keyboard_interrupt);
	outb_p(inb_p(0x21)&0xfd,0x21);
	a=inb_p(0x61);
	outb_p(a|0x80,0x61);
	outb(a,0x61);
}

时间初始化

sched初始化

缓存区初始化