GT2440--U-Boot分析（二）
Step3：
U-Boot源码分析：
第一阶段：
硬件设备初始化
为加载Bootloader第二阶段代码准备RAM空间
复制Bootloader第二阶段代码到RAM准备好空间中
设置栈
跳转到第二阶段的C代码；
第二阶段:
初始化本阶段使用到的硬件设备；
检测系统内存映射；
将内核映像和根文件系统映像从Flash上读到RAM中；
为内核设置启动参数；
调用内核；
第一阶段由：start.s与lowlevel_init.s两个汇编文件完成，下面进行分析：
Start.s主要执行代码：
#include <config.h> #include <version.h> .globl _start _start: b reset reset: /* * set the cpu to SVC32 mode */ mrs r0,cpsr bic r0,r0,#0x1f orr r0,r0,#0xd3 msr cpsr,r0 #if defined(CONFIG_S3C2400) || defined(CONFIG_S3C2410) ldr r0, =pWTCON mov r1, #0x0 str r1, [r0] /* * mask all IRQs by setting all bits in the INTMR - default */ mov r1, #0xffffffff ldr r0, =INTMSK str r1, [r0] # if defined(CONFIG_S3C2410) ldr r1, =0x3ff ldr r0, =INTSUBMSK str r1, [r0] # endif #endif /* CONFIG_S3C2400 || CONFIG_S3C2410 */ #ifndef CONFIG_SKIP_LOWLEVEL_INIT bl cpu_init_crit #endif /* 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 */ bl clock_init /*系统时钟配置C函数 */ #ifndef CONFIG_SKIP_RELOCATE_UBOOT 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 */ #if 1 bl CopyCode2Ram /* r0: source, r1: dest, r2: size 这是一个C函数 */ #endif #endif /* CONFIG_SKIP_RELOCATE_UBOOT */ 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_l /* 转入第二阶段C函数入口 */ ldr pc, _start_armboot _start_armboot: .word start_armboot #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 */ mcr p15, 0, r0, c8, c7, 0 /* flush v4 TLB */ /* * disable MMU stuff and caches */ 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) Align 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 mov lr, ip mov pc, lr #endif /* CONFIG_SKIP_LOWLEVEL_INIT */
将start.s中未使用到的代码裁去后可得上述较为简易的代码，大部分代码均加有注释阅读也较为容易。
在start.s中通过bl lowlevel_init调用lowlevel_init.s。
lowlevel_init.s只要执行代码如下：
/*上面还有部分代码进行定义用于各个bank的设置用的用在此不列出*/
.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))
#if CONFIG_133MHZ_SDRAM
.word ((REFEN<<23)+(TREFMD<<22)+(Trp<<20)+(Tsrc<<18)+REFCNT)
.word 0xB2
.word 0xB0
.word 0xB0
#else
.word ((REFEN<<23)+(TREFMD<<22)+(Trp<<20)+(Trc<<18)+(Tchr<<16)+REFCNT)
.word 0x32
.word 0x30
.word 0x30
#endif
lowlevel_init.s主要功能为使能外接的SDRAM，代码也有较为详细的说明。