在裸機啟動流程里涉及到BL1，BL2為系統的加載啟動項，全稱為BootLoader。
Boot Loader 是在操作系統內核運行之前運行的一段小程序。通過這段小程序，我們可以初始化硬件設備、建立內存空間的映射圖，從而將系統的軟硬件環境帶到一個合適的狀態，以便為最終調用操作系統內核準備好正確的環境。通常，Boot Loader 是嚴重地依賴于硬件而實現的，特別是在嵌入式世界。因此，在嵌入式世界里建立一個通用的 Boot Loader 幾乎是不可能的，但是通過研究人員的努力，設計了一個通用的BootLoader，滿足大部分的開發板的需求。

（一）環境

• uboot_tiny4412-sdk1506.tar.bz2
• Ubuntu 16.04
• linux交叉編譯環境(我已經安裝過，沒有安裝的可以下載arm-linux-gcc-4.5.1-v6-vfp-20120301.tgz）
  獲取幫助的地方: uboot官方
  獲取uboot源代碼的地方：uboot源碼

（二）流程分析

1、 先將uboot的源代碼復制到Ubuntu下的目錄中（推薦在根目錄下創建driver目錄）

 這里以 uboot_tiny4412-sdk1506.tar.bz2的uboot為例操作，自己根據自己下載的uboot進行解壓

2、對uboot進行解壓操作

tar -xvf uboot_tiny4412-sdk1506.tar.bz2

3、進入uboot的目錄，分析uboot的目錄架構

在Linux文件中每一個目錄均有自己獨有的功能，一般情況下，目錄中都會有一個所謂README文件，該文件描述了對應目錄下文件的作用
閱讀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.
說明在uboot的makefile文件中配置的開發板可以直接使用該版本的uboot#目錄結構：
/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存放著和設置環境變量及設置配置參數相關的文件
/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系統中，一般的cfg或者config均為配置文件
boards.cfg：列舉了該版本的uboot所支持的所有開發板的名稱及相關信息

Target：目標名稱
ARCH：指的是架構名稱
CPU：處理器名
Board name：板子名稱       
Vendor：廠商名	        
SoC： 片上系統名        
Options：選項，一般無
如：
tiny4412   arm    armv7  tiny4412    samsung  exynos

每一個名字都對應了uboot的目錄結構下的某一個目錄，這些目錄中存放著對應開發版的先關資源文件，編譯的時候需要

從README可知，uboot可以引導多種平臺架構的開發板，如何選擇其引導自己的開發板？？

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：編譯可以加速
-j [N], --jobs[=N] 同時允許 N 個任務；無參數表明允許無限個任務。

（三）具體步驟

首先將SD卡插入度開啟并連接到Ubuntu系統
1、進行開發板和處理器選型

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

2、make

make 

3、將uboot燒寫到SD卡
為什么要燒寫到SD卡?
以后的嵌入式開發中，通常會先用SD卡或者TF卡進行測試，當功能調試成功再放到emmc。

cd sd_fuse
make
cd tiny4412/
./sd_fusing.sh  你的sd卡掛在點

好了做完上面的你就完成了uboot的制作，接下來把SD卡拔出來插入開發板使用串口工具可以進入uboot的交互模式進行控制。