start.s

uboot 启动流程：

 

设置cpu--》关闭看门狗--》屏蔽中断--》设置时钟频率--》设置堆栈--》清除bss段--》

 

设置cpu为svc32模式：

 

mrs r0,cpsr    //mrs:将cpsr中的内容读进r0中 bic r0,r0,#0x1f   //bic:bit clear，位清除命令，将r0与0x1f的反码相与，然后将结果读进r0中 orr r0,r0,#0xd3  //逻辑或操作，我本人理解为置1指令 msr cpsr,r0

 

该段指令通过设置cpsr中的cpu工作模式位来设定其工作模式，svc32是超级保护模式

关闭看门狗，中断，设置时钟频率

 

#if defined(CONFIG_S3C2400) || defined(CONFIG_S3C2410)//注意一些符号:=，#，[] ldr     r0, =pWTCON mov     r1, #0x0                                        //mov：传送指令，将八位立即数或是寄存器中内容传进r1中 str     r1, [r0]                                             //将r1中的内容写到r0地址处 /*   * 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 /* FCLK:HCLK:PCLK = 1:2:4 */ /* default FCLK is 120 MHz ! */ ldr r0, =CLKDIVN mov r1, #3 str r1, [r0] #endif /* CONFIG_S3C2400 || CONFIG_S3C2410 */

 

如果是重启，执行下边部分，正常启动，则不执行

 

#ifndef CONFIG_SKIP_LOWLEVEL_INIT bl cpu_init_crit #endif 跳转到下边部分，执行:清零cache，关闭mmu及caches， #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 */

 

判断是否要重新加载uboot：判断r0与r1中的值是否相等，相等执行stack_setup，不等，执行下边的直到相等

 

#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     stack_setup ldr r2, _armboot_start ldr r3, _bss_start sub r2, r3, r2  /* r2 <- size of armboot            */ add r2, r0, r2  /* r2 <- source end address         */ copy_loop: ldmia r0!, {r3-r10}  /* copy from source address [r0]    */ stmia r1!, {r3-r10}  /* copy to   target address [r1]    */ cmp r0, r2   /* until source end addreee [r2]    */ ble copy_loop #endif /* CONFIG_SKIP_RELOCATE_UBOOT */

 

设置堆栈

/* 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    */ 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

 

进入step2：board.c

 

ldr pc, _start_armboot _start_armboot: .word start_armboot