Bootloader 它的终极使命就是启动内核，它的主要工作：在上电之后在硬件上进行一系列的初始化，为Linux内核启动铺路。具体的就是设置看门狗、设置系统时钟、初始化SDRAM与NANDflash控制器、代码重定位等，最后将内核从nandflash上拷贝到内存中，传递参数，跳转执行。

一、创建start.S文件

.text
.global _start
_start:

/* close the watchdog */
	ldr r0, =0x53000000
	mov r1, #0
	str r1, [r0]
/* close the watchdog */
@关闭看门狗
@数据手册：WTCON 0x53000000 R/W Watchdog timer control register

/*   set the clock    */
	ldr r0, =0x4c000014
	mov r1, #0x03;            @ FCLK:HCLK:PCLK=1:2:4, HDIVN=1,PDIVN=1
	str r1, [r0]

	mrc	p15, 0, r1, c1, c0, 0		/* read */ 
	orr	r1, r1, #0xc0000000			/* set asynchronous bus mode */
	mcr	p15, 0, r1, c1, c0, 0		/* write */

	ldr r0, =0x4c000004
	ldr r1, =((0x5c<<12)|(0x01<<4)|(0x02)) @MPLL:200MHz
	str r1, [r0]
/*   set the clock    */
@设置系统时钟
@如果HDIVN不是0，CPU的总线模式应该设置成asynchronous bus mode

/* enable the ICACHE  */
	mrc p15, 0, r0, c1, c0, 0	@ read control register
	orr r0, r0, #(1<<12)
	mcr	p15, 0, r0, c1, c0, 0   @ write back
/* enable the ICACHE  */
@使能高速缓存，为系统提速，此段可不要，但程序执行速度要慢

/*   init the SDRAM   */
	ldr r0, =0x48000000   @MEM_CTL_BASE
	adr r1, config     /* sdram config address */
	add r3, r0, #(52)       @13*4
1:
	ldr r2, [r1], #4
	str r2, [r0], #4
	cmp r0, r3
	bne 1b
/*   init the SDRAM   */
@初始化SDRAM
@根据数据手册对与SDRAM有关的13个寄存器进行配置

/*      relocate      */
	ldr sp, =0x34000000
	bl nand_init

	mov r0, #0
	ldr r1, =_start
	ldr r2, =__bss_start
	sub r2, r2, r1
	
	bl copy_code_to_sdram
	bl clear_bss
/*      relocate      */
@把bootloader本身的代码从nandflash复制到它的链接地址去	
	
/*     go to main     */
	ldr lr, =halt
	ldr pc, =main
halt:
	b halt
/*     go to main     */
@跳转到main函数执行

config:
	.long 0x22011110	 @BWSCON
	.long 0x00000700	 @BANKCON0
	.long 0x00000700	 @BANKCON1
	.long 0x00000700	 @BANKCON2
	.long 0x00000700	 @BANKCON3  
	.long 0x00000700	 @BANKCON4
	.long 0x00000700	 @BANKCON5
	.long 0x00018005	 @BANKCON6
	.long 0x00018005	 @BANKCON7
	.long 0x008C04F4	 @REFRESH
	.long 0x000000B1	 @BANKSIZE
	.long 0x00000030	 @MRSRB6
	.long 0x00000030	 @MRSRB7

/* nandflash controller */
#define NFCONF (*((volatile unsigned long *)0x4E000000))
#define NFCONT (*((volatile unsigned long *)0x4E000004))
#define NFCMMD (*((volatile unsigned char *)0x4E000008))
#define NFADDR (*((volatile unsigned char *)0x4E00000C))
#define NFDATA (*((volatile unsigned char *)0x4E000010))
#define NFSTAT (*((volatile unsigned char *)0x4E000020))

/* GPIO */
#define GPHCON              (*(volatile unsigned long *)0x56000070)
#define GPHUP               (*(volatile unsigned long *)0x56000078)

/* UART registers*/
#define ULCON0              (*(volatile unsigned long *)0x50000000)
#define UCON0               (*(volatile unsigned long *)0x50000004)
#define UFCON0              (*(volatile unsigned long *)0x50000008)
#define UMCON0              (*(volatile unsigned long *)0x5000000c)
#define UTRSTAT0            (*(volatile unsigned long *)0x50000010)
#define UTXH0               (*(volatile unsigned char *)0x50000020)
#define URXH0               (*(volatile unsigned char *)0x50000024)
#define UBRDIV0             (*(volatile unsigned long *)0x50000028)

#define TXD0READY   (1<<2)

void nand_read(unsigned int addr, unsigned char *buf, unsigned int len);

void copy_code_to_sdram(unsigned char *src, unsigned char *dest, unsigned int len)
{	
	
		nand_read((unsigned int)src, dest, len);

}

void clear_bss(void)
{
	extern int __bss_start, __bss_end;
	
	int *p = &__bss_start;
	
	for (; p < &__bss_end; p++) *p = 0;
}

void nand_init(void)
{
#define TACLS   0
#define TWRPH0  1
#define TWRPH1  0
	/* sequence */
	NFCONF = (TACLS<<12)|(TWRPH0<<8)|(TWRPH1<<4);
	/* enable controller,init ECC,disable CS */
	NFCONT = (1<<4)|(1<<1)|(1<<0);	
}

void nand_select(void)
{
	NFCONT &= ~(1<<1);	
}

void nand_deselect(void)
{
	NFCONT |= (1<<1);	
}

void nand_cmd(unsigned char cmd)
{
	volatile int i;
	NFCMMD = cmd;
	for (i = 0; i < 10; i++);
}

void nand_addr(unsigned int addr)
{
	unsigned int col  = addr % 2048;
	unsigned int page = addr / 2048;
	volatile int i;

	NFADDR = col & 0xff;
	for (i = 0; i < 10; i++);
	NFADDR = (col >> 8) & 0xff;
	for (i = 0; i < 10; i++);
	
	NFADDR  = page & 0xff;
	for (i = 0; i < 10; i++);
	NFADDR  = (page >> 8) & 0xff;
	for (i = 0; i < 10; i++);
	NFADDR  = (page >> 16) & 0xff;
	for (i = 0; i < 10; i++);	
}

void nand_wait_ready(void)
{
	while (!(NFSTAT & 1));
}

unsigned char nand_data(void)
{
	return NFDATA;
}

void nand_read(unsigned int addr, unsigned char *buf, unsigned int len)
{
	int col = addr % 2048;
	int i = 0;
		
	/* select */
	nand_select();

	while (i < len)
	{
		/* read cmd 00h */
		nand_cmd(0x00);

		/* send addr */
		nand_addr(addr);

		/* read cmd 30h */
		nand_cmd(0x30);

		/* check */
		nand_wait_ready();

		/* read data */
		for (; (col < 2048) && (i < len); col++)
		{
			buf[i] = nand_data();
			i++;
			addr++;
		}
		
		col = 0;
	}

	/* deselect */		
	nand_deselect();
}

#define PCLK            50000000    // PCLK:50MHz
#define UART_CLK        PCLK        //  UART0:PCLK
#define UART_BAUD_RATE  115200      // baudrate
#define UART_BRD        ((UART_CLK  / (UART_BAUD_RATE * 16))-1)

/*115200,8N1,none*/
void uart0_init(void)
{
    GPHCON  |= 0xa0;    // GPH2,GPH3:TXD0,RXD0
    GPHUP   = 0x0c;     // GPH2,GPH3 pull up

    ULCON0  = 0x03;     
    UCON0   = 0x05;     
    UFCON0  = 0x00;     
    UMCON0  = 0x00;    
    UBRDIV0 = UART_BRD; // 115200
}


void putc(unsigned char c)
{
    
    while (!(UTRSTAT0 & TXD0READY));
    
    UTXH0 = c;
}

void puts(char *str)
{
	int i = 0;
	while (str[i])
	{
		putc(str[i]);
		i++;
	}
}

#include "setup.h"

extern void uart0_init(void);
extern void nand_read(unsigned int addr, unsigned char *buf, unsigned int len);
extern void puts(char *str);
extern void puthex(unsigned int val);

static struct tag *params;

void setup_start_tag(void)
{
	params = (struct tag *)0x30000100;

	params->hdr.tag = ATAG_CORE;
	params->hdr.size = tag_size (tag_core);

	params->u.core.flags = 0;
	params->u.core.pagesize = 0;
	params->u.core.rootdev = 0;

	params = tag_next (params);
}

void setup_memory_tags(void)
{
	params->hdr.tag = ATAG_MEM;
	params->hdr.size = tag_size (tag_mem32);
	
	params->u.mem.start = 0x30000000;
	params->u.mem.size  = 64*1024*1024;
	
	params = tag_next (params);
}

int strlen(char *str)
{
	int i = 0;
	while (str[i])
	{
		i++;
	}
	return i;
}

void strcpy(char *dest, char *src)
{
	while ((*dest++ = *src++) != '\0');
}

void setup_commandline_tag(char *cmdline)
{
	int len = strlen(cmdline) + 1;
	
	params->hdr.tag  = ATAG_CMDLINE;
	params->hdr.size = (sizeof (struct tag_header) + len + 3) >> 2;

	strcpy (params->u.cmdline.cmdline, cmdline);

	params = tag_next (params);
}

void setup_end_tag(void)
{
	params->hdr.tag = ATAG_NONE;
	params->hdr.size = 0;
}


int main(void)
{
	void (*theKernel)(int zero, int arch, unsigned int params);

	uart0_init();
	
	/* copy kernel to memory from nandflash */
	puts("MINI2440 simple bootloader\n\r");
	puts("\n\r");
	puts("start ...\n\r");
	puts("\n\r");
	puts("copy kernel to memory from nandflash ...\n\r");
	nand_read(0x60000, (unsigned char *)0x30008000, 0x500000);
	@起始地址0x60000，长度0x500000，内存地址30008000
	puts("\n\r");
	puts("\n\r");

	/* set params */
	puts("Setting params ...\n\r");
	setup_start_tag();
	setup_memory_tags();
	setup_commandline_tag("noinitrd root=/dev/mtdblock3 init=/linuxrc console=ttySAC0");
	setup_end_tag();
	puts("\n\r");
	/* boot */
	puts("Boot ...\n\r");
	theKernel = (void (*)(int, int, unsigned int))0x30008000;
	theKernel(0, 1999, 0x30000100);  
	@注意，mini2440机器码1999
	/* 
	 *  mov r0, #0
	 *  ldr r1, =1999
	 *  ldr r2, =0x30000100
	 *  mov pc, #0x30008000 
	 */

	puts("Error!\n\r");
	/* error */

	return -1;
}

/*
 *  linux/include/asm/setup.h
 *
 *  Copyright (C) 1997-1999 Russell King
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation.
 *
 *  Structure passed to kernel to tell it about the
 *  hardware it's running on.  See linux/Documentation/arm/Setup
 *  for more info.
 *
 * NOTE:
 *  This file contains two ways to pass information from the boot
 *  loader to the kernel. The old struct param_struct is deprecated,
 *  but it will be kept in the kernel for 5 years from now
 *  (2001). This will allow boot loaders to convert to the new struct
 *  tag way.
 */
#ifndef __ASMARM_SETUP_H
#define __ASMARM_SETUP_H

#define u8  unsigned char
#define u16 unsigned short
#define u32 unsigned long

/*
 * Usage:
 *  - do not go blindly adding fields, add them at the end
 *  - when adding fields, don't rely on the address until
 *    a patch from me has been released
 *  - unused fields should be zero (for future expansion)
 *  - this structure is relatively short-lived - only
 *    guaranteed to contain useful data in setup_arch()
 */
#define COMMAND_LINE_SIZE 1024

/* This is the old deprecated way to pass parameters to the kernel */
struct param_struct {
    union {
	struct {
	    unsigned long page_size;		/*  0 */
	    unsigned long nr_pages;		/*  4 */
	    unsigned long ramdisk_size;		/*  8 */
	    unsigned long flags;		/* 12 */
#define FLAG_READONLY	1
#define FLAG_RDLOAD	4
#define FLAG_RDPROMPT	8
	    unsigned long rootdev;		/* 16 */
	    unsigned long video_num_cols;	/* 20 */
	    unsigned long video_num_rows;	/* 24 */
	    unsigned long video_x;		/* 28 */
	    unsigned long video_y;		/* 32 */
	    unsigned long memc_control_reg;	/* 36 */
	    unsigned char sounddefault;		/* 40 */
	    unsigned char adfsdrives;		/* 41 */
	    unsigned char bytes_per_char_h;	/* 42 */
	    unsigned char bytes_per_char_v;	/* 43 */
	    unsigned long pages_in_bank[4];	/* 44 */
	    unsigned long pages_in_vram;	/* 60 */
	    unsigned long initrd_start;		/* 64 */
	    unsigned long initrd_size;		/* 68 */
	    unsigned long rd_start;		/* 72 */
	    unsigned long system_rev;		/* 76 */
	    unsigned long system_serial_low;	/* 80 */
	    unsigned long system_serial_high;	/* 84 */
	    unsigned long mem_fclk_21285;       /* 88 */
	} s;
	char unused[256];
    } u1;
    union {
	char paths[8][128];
	struct {
	    unsigned long magic;
	    char n[1024 - sizeof(unsigned long)];
	} s;
    } u2;
    char commandline[COMMAND_LINE_SIZE];
};


/*
 * The new way of passing information: a list of tagged entries
 */

/* The list ends with an ATAG_NONE node. */
#define ATAG_NONE	0x00000000

struct tag_header {
	u32 size;
	u32 tag;
};

/* The list must start with an ATAG_CORE node */
#define ATAG_CORE	0x54410001

struct tag_core {
	u32 flags;		/* bit 0 = read-only */
	u32 pagesize;
	u32 rootdev;
};

/* it is allowed to have multiple ATAG_MEM nodes */
#define ATAG_MEM	0x54410002

struct tag_mem32 {
	u32	size;
	u32	start;	/* physical start address */
};

/* VGA text type displays */
#define ATAG_VIDEOTEXT	0x54410003

struct tag_videotext {
	u8		x;
	u8		y;
	u16		video_page;
	u8		video_mode;
	u8		video_cols;
	u16		video_ega_bx;
	u8		video_lines;
	u8		video_isvga;
	u16		video_points;
};

/* describes how the ramdisk will be used in kernel */
#define ATAG_RAMDISK	0x54410004

struct tag_ramdisk {
	u32 flags;	/* bit 0 = load, bit 1 = prompt */
	u32 size;	/* decompressed ramdisk size in _kilo_ bytes */
	u32 start;	/* starting block of floppy-based RAM disk image */
};

/* describes where the compressed ramdisk image lives (virtual address) */
/*
 * this one accidentally used virtual addresses - as such,
 * its depreciated.
 */
#define ATAG_INITRD	0x54410005

/* describes where the compressed ramdisk image lives (physical address) */
#define ATAG_INITRD2	0x54420005

struct tag_initrd {
	u32 start;	/* physical start address */
	u32 size;	/* size of compressed ramdisk image in bytes */
};

/* board serial number. "64 bits should be enough for everybody" */
#define ATAG_SERIAL	0x54410006

struct tag_serialnr {
	u32 low;
	u32 high;
};

/* board revision */
#define ATAG_REVISION	0x54410007

struct tag_revision {
	u32 rev;
};

/* initial values for vesafb-type framebuffers. see struct screen_info
 * in include/linux/tty.h
 */
#define ATAG_VIDEOLFB	0x54410008

struct tag_videolfb {
	u16		lfb_width;
	u16		lfb_height;
	u16		lfb_depth;
	u16		lfb_linelength;
	u32		lfb_base;
	u32		lfb_size;
	u8		red_size;
	u8		red_pos;
	u8		green_size;
	u8		green_pos;
	u8		blue_size;
	u8		blue_pos;
	u8		rsvd_size;
	u8		rsvd_pos;
};

/* command line: \0 terminated string */
#define ATAG_CMDLINE	0x54410009

struct tag_cmdline {
	char	cmdline[1];	/* this is the minimum size */
};

/* acorn RiscPC specific information */
#define ATAG_ACORN	0x41000101

struct tag_acorn {
	u32 memc_control_reg;
	u32 vram_pages;
	u8 sounddefault;
	u8 adfsdrives;
};

/* footbridge memory clock, see arch/arm/mach-footbridge/arch.c */
#define ATAG_MEMCLK	0x41000402

struct tag_memclk {
	u32 fmemclk;
};

struct tag {
	struct tag_header hdr;
	union {
		struct tag_core		core;
		struct tag_mem32	mem;
		struct tag_videotext	videotext;
		struct tag_ramdisk	ramdisk;
		struct tag_initrd	initrd;
		struct tag_serialnr	serialnr;
		struct tag_revision	revision;
		struct tag_videolfb	videolfb;
		struct tag_cmdline	cmdline;

		/*
		 * Acorn specific
		 */
		struct tag_acorn	acorn;

		/*
		 * DC21285 specific
		 */
		struct tag_memclk	memclk;
	} u;
};

struct tagtable {
	u32 tag;
	int (*parse)(const struct tag *);
};


#define tag_member_present(tag,member)					((unsigned long)(&((struct tag *)0L)->member + 1)			<= (tag)->hdr.size * 4)

#define tag_next(t)	((struct tag *)((u32 *)(t) + (t)->hdr.size))
#define tag_size(type)	((sizeof(struct tag_header) + sizeof(struct type)) >> 2)

#define for_each_tag(t,base)			for (t = base; t->hdr.size; t = tag_next(t))

/*
 * Memory map description
 */
#define NR_BANKS 8

struct meminfo {
	int nr_banks;
	unsigned long end;
	struct {
		unsigned long start;
		unsigned long size;
		int           node;
	} bank[NR_BANKS];
};

extern struct meminfo meminfo;

#endif

SECTIONS {
    . = 0x33f80000;
    .text : { *(.text) }
    
    . = ALIGN(4);
    .rodata : {*(.rodata*)} 
    
    . = ALIGN(4);
    .data : { *(.data) }
    
    . = ALIGN(4);
    __bss_start = .;
    .bss : { *(.bss)  *(COMMON) }
    __bss_end = .;
}

CC      = arm-linux-gcc
LD      = arm-linux-ld
AR      = arm-linux-ar
OBJCOPY = arm-linux-objcopy
OBJDUMP = arm-linux-objdump

CFLAGS 		:= -Wall -O2
CPPFLAGS   	:= -nostdinc -nostdlib -fno-builtin

objs := start.o init.o boot.o

boot.bin: $(objs)
	${LD} -Tboot.lds -o boot.elf $^
	${OBJCOPY} -O binary -S boot.elf $@
	${OBJDUMP} -D -m arm boot.elf > boot.dis
	
%.o:%.c
	${CC} $(CPPFLAGS) $(CFLAGS) -c -o $@ $<

%.o:%.S
	${CC} $(CPPFLAGS) $(CFLAGS) -c -o $@ $<

clean:
	rm -f *.o *.bin *.elf *.dis