本次移植u-boot-2010.09是基于S3C2440的FL440板子，板子自带NANDFLASH而没有NORFLASH，所以在U-BOOT启动的过程中必须实现从NANDFLASH到SDRAM的重定向。
其中最重要的就是在U-BOOT开始的start.S汇编代码,这段代码要完成工作：
1，异常中断向量表，复位后异常向量处理
2， 跳转到代码实际执行处start_code
3，关闭看门狗WATCHDOG
3，关闭所有中断INTERRUPT
4，设置时钟分频,主要设置寄存器CLKDVN,MPLLCON,UPLLCON
5，关闭MMU和CACHE，并调用lowlevel_init.S完成SDRAM和NANDFLASH的初始化，为代码的重定向做准备
6，设置堆栈，并且跳入第二阶段的C代码
7，异常向量处理代码
以下为start.S的分析：
1，异常中断向量表，复位后异常向量处理
//声明一个全局标量，在cpu/arm920t/u-boot.lds中有定义，即代码的入口地址，也是编译地址
_start: b start_codeldr pc, _undefined_instructionldr pc, _software_interruptldr pc, _prefetch_abortldr pc, _data_abortldr pc, _not_usedldr pc, _irqldr pc, _fiq_undefined_instruction: .word undefined_instruction //.word 定义一个32位的地址标识
_software_interrupt: .word software_interrupt
_prefetch_abort: .word prefetch_abort
_data_abort: .word data_abort
_not_used: .word not_used
_irq: .word irq
_fiq: .word fiq.balignl 16,0xdeadbeef//将地址偏移为16的整数倍，空余的内容填上0xdeadbeef，这个数即“Magic Number”,可用判断当前u-boot执行位置_TEXT_BASE:.word TEXT_BASE //在config.mk中有定义，即u-boot自启时flash从定向到sdram的地址
.globl _armboot_start //声明一个全局变量,之后要调用
_armboot_start:.word _start/** These are defined in the board-specific linker script.*/.globl _bss_start //连接脚本u-boot.lds中有定义
_bss_start:.word __bss_start.globl _bss_end //连接脚本u-boot.lds中有定义</span>
_bss_end:.word _end#ifdef CONFIG_USE_IRQ //堆栈设置
/* IRQ stack memory (calculated at run-time) */.globl IRQ_STACK_START
IRQ_STACK_START:.word 0x0badc0de/* IRQ stack memory (calculated at run-time) */.globl FIQ_STACK_START
FIQ_STACK_START:.word 0x0badc0de#endif2， 跳转到代码实际执行处（设置管理模式=>关闭看门狗=>关闭所有中断=>设置时钟分频）
/** the actual start code*/start_code:/** set the cpu to SVC32 mode *///设置管理模式 31 30 29 28 7 6 4 3 2 1 0 mrs r0, cpsr // CPSR N Z C V I F M4 M3 M2 M1 M0bic r0, r0, #0x1f // 1 0 0 1 1orr r0, r0, #0xd3 msr cpsr, r0 // CPSR为状态寄存器，用于设置系统运行状态，只能用MSR MRS指令#if defined(CONFIG_AT91RM9200DK) || defined(CONFIG_AT91RM9200EK) //系统中断重定向于RAM中，以便快速响应中断,搬运的代码为4*16bytes
/* relocate exception table */ldr r0, =_startldr r1, =0x0mov r2, #16
copyex: subs r2, r2, #1ldr r3, [r0], #4 str r3, [r1], #4 bne copyex
#endif//关闭看门狗
#if defined(CONFIG_S3C2400) || defined(CONFIG_S3C2410) || defined(CONFIG_S3C2440)/* turn off the watchdog */#if defined(CONFIG_S3C2400)
#define pWTCON 0x15300000
#define INTMSK 0x14400008 /* Interupt-Controller base addresses */
#define CLKDIVN 0x14800014 /* clock divisor register */
#else //查阅s3c2440的datesheet中指出寄存器地址
#define pWTCON 0x53000000
#define INTMSK 0x4A000008 /* Interupt-Controller base addresses */
#define INTSUBMSK 0x4A00001C
#define CLKDIVN 0x4C000014 /* clock divisor register */
#endif#define CLK_CTL_BASE 0x4C000000 //添加时钟分频寄存器地址，用于时钟分频设置
#define MDIV_405 0x7f<<12
#define PSDIV_405 0x21
#define MDIV_200 0xa1<<12
#define PSDIV_200 0x31 </span>ldr r0, =pWTCON mov r1, #0x0str r1, [r0] /* mask all IRQs by setting all bits in the INTMR - default */mov r1, #0xffffffff //ARM920T有32个中断源，禁止所有中断，32位中断屏蔽寄存器置位 ldr r0, =INTMSK str r1, [r0]
#if defined(CONFIG_S3C2440)||defined(CONFIG_S3C2410) /* add by zhou */ ldr r1, =0x7ff<span style="white-space:pre"> //屏蔽所有的中断源，S3C2440中寄存器只有前15位有效，故0x7ff置位INTSUNMSK ldr r0, =INTSUBMSK str r1, [r0]
#endif
//设置时钟频率
#if defined(CONFIG_S3C2440)mov r1, #5str r1, [r0]mrc p15, 0, r1, c1, c0, 0 orr r1, r1, #0xc0000000mcr p15, 0, r1, c1, c0, 0 mov r1, #CLK_CTL_BASE //S3C2440系统主频为405MHZ，USB为48MHZ，要求MPLLCON = (0x7f<<12) | (0x02<<4) | (0x01) = 0x7f021mov r2, #MDIV_405add r2, r2, #PSDIV_405str r2, [r1, #0x04]
#else/* FCLK:HCLK:PCLK = 1:2:4 *//* default FCLK is 120 MHz ! */ldr r0, =CLKDIVNmov r1, #3str r1, [r0]mrc p15, 0, r1, c1, c0, 0orr r1, r1, #0xc0000000mcr p15, 0, r1, c1, c0, 0mov r1, #CLK_CTL_BASEmov r2, #MDIV_200add r2, r2,#PSDIV_200str r2, [r1,#0x04]#endif
#endif /* (CONFIG_S3C2400) || (CONFIG_S3C2410) || (CONFIG_S3C2440) */ /******************************************** we do sys-critical inits only at reboot,* not when booting from ram!******************************************/#ifndef CONFIG_SKIP_LOWLEVEL_INITbl cpu_init_crit //关闭MMU和CACHE，并调用lowlevel_init.S完成SDRAM和NANDFLASH的初始化
#endif
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/**************************************************************************** CPU_init_critical registers** setup important registers* setup memory timing***************************************************************************/#ifndef CONFIG_SKIP_LOWLEVEL_INIT
cpu_init_crit:/** flush v4 I/D caches*/mov r0, #0 //具体设置看下图,详细参考CP15指令：http://blog.csdn.net/gooogleman/article/details/3635238mcr p15, 0, r0, c7, c7, 0 //向c7写入0将使ICache与DCache 无效flush v3/v4 cache mcr p15, 0, r0, c8, c7, 0 //向c8写入0将使TLB失效 flush v4 TLB /** disable MMU stuff and caches <span style="color:#ff0000;"> //协处理器CP15的C1处理器可以设置MMU和caches,具体参考下图*/mrc p15, 0, r0, c1, c0, 0 bic r0, r0, #0x00002300 @ clear bits 13, 9:8 (--V- --RS)bic r0, r0, #0x00000087 @ clear bits 7, 2:0 (B--- -CAM)orr r0, r0, #0x00000002 @ set bit 2 (A) Alignorr r0, r0, #0x00001000 @ set bit 12 (I) I-Cachemcr p15, 0, r0, c1, c0, 0/** before relocating, we have to setup RAM timing* because memory timing is board-dependend, you will* find a lowlevel_init.S in your board directory.*/mov ip, lr //由于有两层调用，需要把lr保存到ip，以防止破坏bl lowlevel_init //调用c函数，初始化FLASH和SDRAM，为代码重定向做准备mov lr, ip mov pc, lr //返回
#endif /* CONFIG_SKIP_LOWLEVEL_INIT */
代码重定向基本思路:
1.内存运行与否，是则设置堆栈，跳入c函数阶段
2.若不在内存运行，判断是在norflash还在nandflash运行
//代码重定向部分
/***************CHECK_CODE_POSITION******************************/adr r0, _start /* r0 <- current position of code */ //检查代码是否在已经SDRAM中运行，是则设置堆栈，并跳入c代码部分ldr r1, _TEXT_BASE /* test if we run from flash or RAM */ //_start为u-boot的真正运行地址，_TEXT_BASE为FLASH加载到SDRAM的地址，在config.mk中定义为0x33f80000cmp r0, r1 /* don't reloc during debug */ //若相等，说明已经在SDRAM中运行，设置堆栈，并且调转到第二阶段的C函数beq stack_setup //若不相等，则要判断是从NORFLASF或NANDFLASH启动/***************CHECK_CODE_POSITION******************************//***************CHECK_BOOT_FLASH********************************/ldr r1, =((4<<28)|(3<<4)|(3<<2)) /*address in 4000003c*/mov r0, #0 //NANDFLASH的启动原理，启动时4K SRAM，即Stepping Stone,会映射到nGCS0,0x0000 0000地址，同时它还是会被映射到0x4000 0000地址 str r0,[r1] //而NORFLASH支持片上运行，并会被一直挂载到nGCS0,0x0000 0000，具体可以参照NANDFLASH启动原理mov r1, #0x3c /*address in 0x3c*/ //NANDFLASH启动时，因为地址为16倍数对齐，此时0x0000 003c 和 0x4000 003c都为唯一确定的0xdeadbeef，即"Magic Mumber"ldr r0, [r1] //当0x4000 003c清零，若0x0000 003c读出也是零，则u-boot代码从NANDFLASH启动，否则从NORFLASHcmp r0, #0bne relocate/*recover 0x4000003c */ldr r0, =(0xdeadbeef) //若在NANDFLASH启动，必须保证代码和前4K拷贝到SRAM一致，否则会进入死循环ldr r1, =((4<<28)|(3<<4)|(3<<2))str r0, [r1]/***************CHECK_BOOT_FLASH********************************//***************NAND_BOOT********************************/#ifdef CONFIG_S3C2440 //支持S3C2440的NANDFLASH
#define LENGTH_UBOOT 0x60000
#define NAND_CTL_BASE 0x4E000000/* Offset */
#define oNFCONF 0x00
#define oNFCONT 0x04
#define oNFCMD 0x08
#define oNFSTAT 0x20mov r1, #NAND_CTL_BASE //NAND Flash配置寄存器设置ldr r2, =( (7<<12)|(7<<8)|(7<<4)|(0<<0) )str r2, [r1, #oNFCONF]ldr r2, [r1, #oNFCONF]ldr r2, =( (1<<4)|(0<<1)|(1<<0) ) /* @ Active low CE Control */str r2, [r1, #oNFCONT] //NAND Flash控制寄存器设置ldr r2, [r1, #oNFCONT]ldr r2, =(0x6) /* @ RnB Clear */ //NAND Flash状态寄存器设置str r2, [r1, #oNFSTAT]ldr r2, [r1, #oNFSTAT]mov r2, #0xff //NAND Flash命令寄存器设置strb r2, [r1, #oNFCMD]mov r3, #0 nand1:add r3, r3, #0x1cmp r3, #0xablt nand1nand2:ldr r2, [r1, #oNFSTAT]tst r2, #0x4beq nand2 ldr r2, [r1, #oNFCONT]orr r2, r2, #0x2str r2, [r1, #oNFCONT] /* get read to call C functions (for nand_read()) */ldr sp, DW_STACK_STARTmov fp, #0/* copy U-Boot to RAM */ldr r0, =TEXT_BASE //汇编调用c函数nand_read_ll，第一个参数存于r0搬运到内存地址mov r1, #0x0 //第二个参数存于r1，NANDFLASH中u-boot地址mov r2, #LENGTH_UBOOT /第三个参数存于r2，u-boot的总大小bl nand_read_ll //在nand_read.c定义，支持不同NANDFLASH芯片代码拷贝tst r0, #0x0 //r0是存放函数返回的参数，返回值为0则正确拷贝，否则进入死循环 beq ok_nand_readbad_nand_read:
loop2:b loop2
ok_nand_read: //前4K代码比较，即判定Stepping Stone中4k代码和c函数搬运的代码是否一致 mov r0, #0 ldr r1, =TEXT_BASE mov r2, #0x400 /* 4 bytes * 1024 = 4Kbytes */
go_next: ldr r3, [r0], #4 //r3存放NANDFLASH上u-boot的代码ldr r4, [r1], #4 //r4为在内存中的u-boot代码 teq r3, r4bne notmatch //不一致则进入死循环subs r2, r2, #4beq stack_setupbne go_nextnotmatch:
loop3:b loop3
#endif#ifdef CONFIG_S3C2410
/* Offset */
#define oNFCONF 0x00
#define oNFCMD 0x04
#define oNFSTAT 0x10mov r1, #NAND_CTL_BASEldr r2, =0xf830str r2, [r1, #oNFCONF]ldr r2, [r1, #oNFCONF]bic r2, r2, #0x800 /* enable chip */str r2, [r1, #oNFCONF]mov r2, #0xff /* @ RESET command + strb r2, [r1, #oNFCMD] */strb r2, [r1, #oNFCMD]mov r3, #0 nand1:add r3, r3, #0x1cmp r3, #0xablt nand1nand2:ldr r2, [r1, #oNFSTAT] /* @ wait ready */tst r2, #0x1beq nand2ldr r2, [r1, #oNFCONF]orr r2, r2, #0x800 /*@ disable chip */str r2, [r1, #oNFCONF]/* @ get read to call C functions (for nand_read()) */ldr sp, DW_STACK_START /* @ setup stack pointer */mov fp, #0 /* @ no previous frame, so fp=0 *//* @ copy U-Boot to RAM */ldr r0, =TEXT_BASEmov r1, #0x0mov r2, #LENGTH_UBOOTbl nand_read_lltst r0, #0x0beq ok_nand_read
bad_nand_read:
loop2:b loop2 /*@ infinite loop */ok_nand_read:
/* @ verify */mov r0, #0 ldr r1, =TEXT_BASEmov r2, #0x400 /* @ 4 bytes * 1024 = 4K-bytes */go_next:ldr r3, [r0], #4ldr r4, [r1], #4teq r3, r4bne notmatchsubs r2, r2, #4beq stack_setupbne go_nextnotmatch:
loop3:b loop3 /*@ infinite loop */
#endif/***************** NAND_BOOT ************************************************/</span>
3，设置堆栈，并且跳入第二阶段的C代码
/* Set up the stack */
stack_setup:ldr r0, _TEXT_BASE /* upper 128 KiB: relocated uboot */sub r0, r0, #CONFIG_SYS_MALLOC_LEN /* malloc area */sub r0, r0, #CONFIG_SYS_GBL_DATA_SIZE /* bdinfo */
#ifdef CONFIG_USE_IRQsub r0, r0, #(CONFIG_STACKSIZE_IRQ+CONFIG_STACKSIZE_FIQ)
#endifsub sp, r0, #12 /* leave 3 words for abort-stack */clear_bss: //将未初始化.bss段初始化0ldr r0, _bss_start /* find start of bss segment */ldr r1, _bss_end /* stop here */mov r2, #0x00000000 /* clear */clbss_l:str r2, [r0] /* clear loop... */add r0, r0, #4cmp r0, r1ble clbss_lldr pc, _start_armboot //跳到第二阶段c函数，进一步初始化板子硬件_start_armboot: .word start_armboot#define STACK_BASE 0x33f00000 //为nand_read_ll函数调用设置堆栈
#define STACK_SIZE 0x10000.align 2
DW_STACK_START: .word STACK_BASE+STACK_SIZE-4 </span>4，异常向量处理代码
/*************************************************************************** Interrupt handling***************************************************************************/
......详细可参考http://www.crifan.com/files/doc/docbook/uboot_starts_analysis/release/html/uboot_starts_analysis.html
