u-boot启动过程之STEP1
1， 初始化中断向量表，正常情况下，直接跳转到reset
.globl _start
_start: b reset
ldr pc, _undefined_instruction
ldr pc, _software_interrupt
ldr pc, _prefetch_abort
ldr pc, _data_abort
ldr pc, _not_used
ldr pc, _irq
ldr pc, _fiq
_undefined_instruction: .word undefined_instruction
_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 16,0xdeadbeef2. 进入reset后，初始化CPU，使其进入SVreset:
/* * set the cpu to SVC32 mode */
mrsr0,cpsr
bicr0,r0,#0x1f
orrr0,r0,#0xd3
msrcpsr,r03. 关看门狗
# define pWTCON 0x53000000
# define INTMOD 0X4A000004
# define INTMSK 0x4A000008 /* Interupt-Controller base addresses */
# define INTSUBMSK 0x4A00001C
# define CLKDIVN 0x4C000014 /* clock divisor register */
ldr r0, =pWTCON
mov r1, #0x0
str r1, [r0]4. 屏蔽中断
mov r1, #0xffffffff
ldr r0, =INTMSK
str r1, [r0]
# if defined(CONFIG_S3C2410)
ldr r1, =0x3ff
ldr r0, =INTSUBMSK
str r1, [r0]
# endif5. 判断当前是否运行在SDRAM上，若没有运行在SDRAM上，则进行CPU底层的一些初始化：
5.1 获取_start的相对于PC的地址，赋值给r0，获取_start在编译时的链接地址赋值给r1。比较r0和r1。若不相等，则表示，当前程序运行在非SDRAM区，则调用cpu_init_crit函数。
#ifndef CONFIG_SKIP_LOWLEVEL_INIT
adr r0, _start /* r0 <- current position of code */
ldr r1, _TEXT_BASE /* test if we run from flash or RAM */
cmp r0, r1 /* don't reloc during debug */
blne cpu_init_crit
#endif5.2 使无效整个cache和TLB
Caches是CPU内部的一个2级缓存，它的作用是将常用的数据和指令放在CPU内部。Caches是通过CP15管理的，刚上电的时候，CPU还不能管理Caches。上电的时候指令Cache可关闭，也可不关闭，但数据Cache一定要关闭，否则可能导致刚开始的代码里面，去取数据的时候，从Cache里面取，而这时候RAM中数据还没有Cache过来，导致数据预取异常。
#ifndef CONFIG_SKIP_LOWLEVEL_INIT
cpu_init_crit:
/* * flush v4 I/D caches */
mov r0, #0
mcr p15, 0, r0, c7, c7, 0 /* flush v3/v4 cache，使无效整个cache */
mcr p15, 0, r0, c8, c7, 0 /* flush v4 TLB，使无效整个TLB */
/* * disable MMU stuff and caches * C1寄存器的格式可以参考 * [http://blog.csdn.net/skyflying2012/article/details/25823967](http://blog.csdn.net/skyflying2012/article/details/25823967) */
//读C1的内容到r0
mrc p15, 0, r0, c1, c0, 0
//清R,S,V，选择0x00000000-0x0000001c作为异常中断向量表的位置。
bic r0, r0, #0x00002300 @ clear bits 13, 9:8 (--V- --RS)
//禁止MMU，禁止数据cache，存储系统选择小端模式
bic r0, r0, #0x00000087 @ clear bits 7, 2:0 (B--- -CAM)
//开启内存访问时地址对其检查的功能
orr r0, r0, #0x00000002 @ set bit 2 (A) Align
//使能指令cache
orr r0, r0, #0x00001000 @ set bit 12 (I) I-Cache
mcr 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
bl lowlevel_init @调用lowlevel_init函数
mov lr, ip
mov pc, lr
#endif /* CONFIG_SKIP_LOWLEVEL_INIT */5.3 lowlevel_init函数，初始化ARM的BANK区
_TEXT_BASE:
.word TEXT_BASE
.globl lowlevel_init
lowlevel_init:
/* memory control configuration */
/* make r0 relative the current location so that it */
/* reads SMRDATA out of FLASH rather than memory ! */
ldr r0, =SMRDATA
ldr r1, _TEXT_BASE
sub r0, r0, r1
ldr r1, =BWSCON /* Bus Width Status Controller */
add r2, r0, #13*4
0:
ldr r3, [r0], #4
str r3, [r1], #4
cmp r2, r0
bne 0b
/* everything is fine now */
mov pc, lr
.ltorg
/* the literal pools origin */
SMRDATA:
.word (0+(B1_BWSCON<<4)+(B2_BWSCON<<8)+(B3_BWSCON<<12)+(B4_BWSCON<<16)+(B5_BWSCON<<20)+(B6_BWSCON<<24)+(B7_BWSCON<<28))
.word ((B0_Tacs<<13)+(B0_Tcos<<11)+(B0_Tacc<<8)+(B0_Tcoh<<6)+(B0_Tah<<4)+(B0_Tacp<<2)+(B0_PMC))
.word ((B1_Tacs<<13)+(B1_Tcos<<11)+(B1_Tacc<<8)+(B1_Tcoh<<6)+(B1_Tah<<4)+(B1_Tacp<<2)+(B1_PMC))
.word ((B2_Tacs<<13)+(B2_Tcos<<11)+(B2_Tacc<<8)+(B2_Tcoh<<6)+(B2_Tah<<4)+(B2_Tacp<<2)+(B2_PMC))
.word ((B3_Tacs<<13)+(B3_Tcos<<11)+(B3_Tacc<<8)+(B3_Tcoh<<6)+(B3_Tah<<4)+(B3_Tacp<<2)+(B3_PMC))
.word ((B4_Tacs<<13)+(B4_Tcos<<11)+(B4_Tacc<<8)+(B4_Tcoh<<6)+(B4_Tah<<4)+(B4_Tacp<<2)+(B4_PMC))
.word ((B5_Tacs<<13)+(B5_Tcos<<11)+(B5_Tacc<<8)+(B5_Tcoh<<6)+(B5_Tah<<4)+(B5_Tacp<<2)+(B5_PMC))
.word ((B6_MT<<15)+(B6_Trcd<<2)+(B6_SCAN))
.word ((B7_MT<<15)+(B7_Trcd<<2)+(B7_SCAN))
.word ((REFEN<<23)+(TREFMD<<22)+(Trp<<20)+(Trc<<18)+(Tchr<<16)+REFCNT)
.word 0xb1
.word 0x30
.word 0x306. 初始化栈，初始化后，SDRAM中的一部分内存分配是：
_TEXT_BASE –> MALLOC –> GBL_DATA –> Stack of IRQ and FIQ –> abord stack –> sp
/* Set up the stack */
stack_setup:
ldr r0, _TEXT_BASE /* upper 128 KiB: relocated uboot */
sub r0, r0, #CFG_MALLOC_LEN /* malloc area */
sub r0, r0, #CFG_GBL_DATA_SIZE /* bdinfo */
#ifdef CONFIG_USE_IRQ
sub r0, r0, #(CONFIG_STACKSIZE_IRQ+CONFIG_STACKSIZE_FIQ)
#endif
sub sp, r0, #12 /* leave 3 words for abort-stack */7. 初始化时钟,提高时钟频率，加快程序的执行速度。
#ifndef CONFIG_SKIP_LOWLEVEL_INIT
bl clock_init
#endif void clock_init(void)
{
S3C24X0_CLOCK_POWER *clk_power = (S3C24X0_CLOCK_POWER *)0x4C000000;
/* FCLK:HCLK:PCLK = 1:4:8 */
clk_power->CLKDIVN = S3C2440_CLKDIV;
/* change to asynchronous bus mod */
__asm__( "mrc p15, 0, r1, c1, c0, 0\n" /* read ctrl register */
"orr r1, r1, #0xc0000000\n" /* Asynchronous */
"mcr p15, 0, r1, c1, c0, 0\n" /* write ctrl register */
:::"r1"
);
/* to reduce PLL lock time, adjust the LOCKTIME register */
clk_power->LOCKTIME = 0xFFFFFFFF;
/* configure UPLL */
clk_power->UPLLCON = S3C2440_UPLL_48MHZ;
/* some delay between MPLL and UPLL */
delay (4000);
/* configure MPLL */
clk_power->MPLLCON = S3C2440_MPLL_400MHZ;
/* some delay between MPLL and UPLL */
delay (8000);
}8. 将u-boot的源码从FLASH区复制到SDRAM
relocate: /* relocate U-Boot to RAM */
adr r0, _start /* r0 <- current position of code */
ldr r1, _TEXT_BASE /* test if we run from flash or RAM */
cmp r0, r1 /* don't reloc during debug */
beq clear_bss
ldr r2, _armboot_start
ldr r3, _bss_start
sub r2, r3, r2 /* r2 <- size of armboot */
bl CopyCode2Ram /* r0: source, r1: dest, r2: size *///根据ATPCS,
//start_addr是当前运行时_start的地址
//*buf指向SDRAM上代码段的起始地址
//size是u-boot程序的大小
int CopyCode2Ram(unsigned long start_addr, unsigned char *buf, int size)
{
unsigned int *pdwDest;
unsigned int *pdwSrc;
int i;
if (bBootFrmNORFlash())
{
pdwDest = (unsigned int *)buf;
pdwSrc = (unsigned int *)start_addr;
/* 从 NOR Flash启动 */
for (i = 0; i < size / 4; i++)
{
pdwDest[i] = pdwSrc[i];
}
return 0;
}
else
{
/* 初始化NAND Flash */
nand_init_ll();
/* 从 NAND Flash启动 */
nand_read_ll_lp(buf, start_addr, (size + NAND_BLOCK_MASK_LP)&~(NAND_BLOCK_MASK_LP));
return 0;
}
}int bBootFrmNORFlash(void)
{
volatile unsigned int *pdw = (volatile unsigned int *)0;
unsigned int dwVal;
/* * 无论是从NOR Flash还是从NAND Flash启动， * 地址0处为指令"b Reset", 机器码为0xEA00000B， * 对于从NAND Flash启动的情况，其开始4KB的代码会复制到CPU内部4K内存中， * 对于从NOR Flash启动的情况，NOR Flash的开始地址即为0。 * 对于NOR Flash，必须通过一定的命令序列才能写数据， * 所以可以根据这点差别来分辨是从NAND Flash还是NOR Flash启动: * 向地址0写入一个数据，然后读出来，如果没有改变的话就是NOR Flash */
dwVal = *pdw;
*pdw = 0x12345678;
if (*pdw != 0x12345678)
{
return 1;
}
else
{
*pdw = dwVal;
return 0;
}
}9. 将BSS段清零.
BSS段包含了程序中未初始化全局变量，在内存中 bss段全部置零。BSS是block started by symbol的缩写。因为未初始化的变量没有对应的值,所以并不需要存储在可执行对象中。但是因为C标准强制规定未初始化的全局变量要被赋予特殊的默认值(基本上是0值)，所以内核要从可执行代码装入变量(未赋值的)到内存中，然后将零页映射到该片内存上，于是这些未初始化变量就被赋予了0值。这样做避免了在目标文件中进行显式地初始化，减少空间浪费。
clear_bss:
ldr 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, #4
cmp r0, r1
ble clbss_l10, 跳转到start_armboot中，开始执行STEP2部分。
ldr pc, _start_armboot
_start_armboot: .word start_armboot