在裸机启动流程里涉及到BL1，BL2为系统的加载启动项，全称为BootLoader。
Boot Loader 是在操作系统内核运行之前运行的一段小程序。通过这段小程序，我们可以初始化硬件设备、建立内存空间的映射图，从而将系统的软硬件环境带到一个合适的状态，以便为最终调用操作系统内核准备好正确的环境。通常，Boot Loader 是严重地依赖于硬件而实现的，特别是在嵌入式世界。因此，在嵌入式世界里建立一个通用的 Boot Loader 几乎是不可能的，但是通过研究人员的努力，设计了一个通用的BootLoader，满足大部分的开发板的需求。

（一）环境

• uboot_tiny4412-sdk1506.tar.bz2
• Ubuntu 16.04
• linux交叉编译环境(我已经安装过，没有安装的可以下载arm-linux-gcc-4.5.1-v6-vfp-20120301.tgz）
  获取帮助的地方: uboot官方
  获取uboot源代码的地方：uboot源码

（二）流程分析

1、 先将uboot的源代码复制到Ubuntu下的目录中（推荐在根目录下创建driver目录）

 这里以 uboot_tiny4412-sdk1506.tar.bz2的uboot为例操作，自己根据自己下载的uboot进行解压

2、对uboot进行解压操作

tar -xvf uboot_tiny4412-sdk1506.tar.bz2

3、进入uboot的目录，分析uboot的目录架构

在Linux文件中每一个目录均有自己独有的功能，一般情况下，目录中都会有一个所谓README文件，该文件描述了对应目录下文件的作用
阅读README文件：

In general, all boards for which a configuration option exists in the Makefile have been
tested to some extent and can be considered  "working". In fact, many of them are used in
production systems.
说明在uboot的makefile文件中配置的开发板可以直接使用该版本的uboot#目录结构：
/api U-Boot machine/arch independent API for external apps
/arch			Architecture specific files/arm			Files generic to ARM architecture/cpu		CPU specific files/arm720t		Files specific to ARM 720 CPUs/arm920t		Files specific to ARM 920 CPUs/at91rm9200	Files specific to Atmel AT91RM9200 CPU/imx		Files specific to Freescale MC9328 i.MX CPUs/s3c24x0	Files specific to Samsung S3C24X0 CPUs/arm925t		Files specific to ARM 925 CPUs/arm926ejs	Files specific to ARM 926 CPUs/arm1136		Files specific to ARM 1136 CPUs/ixp		Files specific to Intel XScale IXP CPUs/pxa		Files specific to Intel XScale PXA CPUs/s3c44b0		Files specific to Samsung S3C44B0 CPUs/sa1100		Files specific to Intel StrongARM SA1100 CPUs/lib		Architecture specific library files/avr32		Files generic to AVR32 architecture/cpu		CPU specific files/lib		Architecture specific library files/blackfin		Files generic to Analog Devices Blackfin architecture/cpu		CPU specific files/lib		Architecture specific library files/i386			Files generic to i386 architecture/cpu		CPU specific files/lib		Architecture specific library files/m68k			Files generic to m68k architecture/cpu		CPU specific files/mcf52x2		Files specific to Freescale ColdFire MCF52x2 CPUs/mcf5227x		Files specific to Freescale ColdFire MCF5227x CPUs/mcf532x		Files specific to Freescale ColdFire MCF5329 CPUs/mcf5445x		Files specific to Freescale ColdFire MCF5445x CPUs/mcf547x_8x	Files specific to Freescale ColdFire MCF547x_8x CPUs/lib		Architecture specific library files/microblaze		Files generic to microblaze architecture/cpu		CPU specific files/lib		Architecture specific library files/mips			Files generic to MIPS architecture/cpu		CPU specific files/lib		Architecture specific library files/nios2		Files generic to Altera NIOS2 architecture/cpu		CPU specific files/lib		Architecture specific library files/powerpc		Files generic to PowerPC architecture/cpu		CPU specific files/74xx_7xx		Files specific to Freescale MPC74xx and 7xx CPUs/mpc5xx		Files specific to Freescale MPC5xx CPUs/mpc5xxx		Files specific to Freescale MPC5xxx CPUs/mpc8xx		Files specific to Freescale MPC8xx CPUs/mpc8220		Files specific to Freescale MPC8220 CPUs/mpc824x		Files specific to Freescale MPC824x CPUs/mpc8260		Files specific to Freescale MPC8260 CPUs/mpc85xx		Files specific to Freescale MPC85xx CPUs/ppc4xx		Files specific to AMCC PowerPC 4xx CPUs/lib		Architecture specific library files/sh			Files generic to SH architecture/cpu		CPU specific files/sh2		Files specific to sh2 CPUs/sh3		Files specific to sh3 CPUs/sh4		Files specific to sh4 CPUs/lib		Architecture specific library files/sparc		Files generic to SPARC architecture/cpu		CPU specific files/leon2		Files specific to Gaisler LEON2 SPARC CPU/leon3		Files specific to Gaisler LEON3 SPARC CPU/lib		Architecture specific library files
/api			Machine/arch independent API for external apps
/board			Board dependent files
/common			Misc architecture independent functions存放着和设置环境变量及设置配置参数相关的文件
/disk			Code for disk drive partition handling
/doc			Documentation (don't expect too much)
/drivers		Commonly used device drivers
/examples		Example code for standalone applications, etc.
/fs			Filesystem code (cramfs, ext2, jffs2, etc.)
/include		Header Files
/lib			Files generic to all architectures/libfdt		Library files to support flattened device trees/lzma			Library files to support LZMA decompression/lzo			Library files to support LZO decompression
/net			Networking code
/post			Power On Self Test
/rtc			Real Time Clock drivers
/tools			Tools to build S-Record or U-Boot images, etc.

在Linux系统中，一般的cfg或者config均为配置文件
boards.cfg：列举了该版本的uboot所支持的所有开发板的名称及相关信息

Target：目标名称
ARCH：指的是架构名称
CPU：处理器名
Board name：板子名称       
Vendor：厂商名	        
SoC： 片上系统名        
Options：选项，一般无
如：
tiny4412   arm    armv7  tiny4412    samsung  exynos

每一个名字都对应了uboot的目录结构下的某一个目录，这些目录中存放着对应开发版的先关资源文件，编译的时候需要

从README可知，uboot可以引导多种平台架构的开发板，如何选择其引导自己的开发板？？

Selection of Processor Architecture and Board Type:
For all supported boards there are ready-to-use default
configurations available; just type "make <board_name>_config".Example: For a TQM823L module type:cd u-bootmake TQM823L_config

make -j8：编译可以加速
-j [N], --jobs[=N] 同时允许 N 个任务；无参数表明允许无限个任务。

（三）具体步骤

首先将SD卡插入度开启并连接到Ubuntu系统
1、进行开发板和处理器选型

make <board_name>_config
//这里的board_name可以在board.cfg配置信息找到

2、make

make 

3、将uboot烧写到SD卡
为什么要烧写到SD卡?
以后的嵌入式开发中，通常会先用SD卡或者TF卡进行测试，当功能调试成功再放到emmc。

cd sd_fuse
make
cd tiny4412/
./sd_fusing.sh  你的sd卡挂在点

好了做完上面的你就完成了uboot的制作，接下来把SD卡拔出来插入开发板使用串口工具可以进入uboot的交互模式进行控制。