U-boot学习.docxVIP

U-boot学习笔记本u-boot分析基于u-boot官网u-boot2010-12文件的goni开发板。1:start.S:第一步跳转到reset执行代码。reset：1:设置CPU为特权模式mrsr0, cpsrbic r0, r0, #0x1forr r0, r0, #0xd3msrcpsr,r0操作cpsr寄存器，更改模式和禁止中断也就是把低五位置0b10011把第6,7位写1禁止中断。#if (CONFIG_OMAP34XX)/* Copy vectors to mask ROM indirect addr */adrr0, _start@ r0 - current position of codeaddr0, r0, #4@ skip reset vectormovr2, #64@ r2 - size to copyaddr2, r0, r2@ r2 - source end addressmovr1, #SRAM_OFFSET0@ build vect addrmovr3, #SRAM_OFFSET1addr1, r1, r3movr3, #SRAM_OFFSET2addr1, r1, r3next:ldmiar0!, {r3 - r10}@ copy from source address [r0]stmiar1!, {r3 - r10}@ copy to target address [r1]cmpr0, r2@ until source end address [r2]bnenext@ loop until equal */#if!defined(CONFIG_SYS_NAND_BOOT) !defined(CONFIG_SYS_ONENAND_BOOT)/* No need to copy/exec the clock code - DPLL adjust already done * in NAND/oneNAND Boot. */blcpy_clk_code@ put dpll adjust code behind vectors#endif /* NAND Boot */#endif上面这段代码使用了宏编译，查找了包含的头文件以及本文件，并没有该宏的定义，所以该段程序不参与编译。#ifndef CONFIG_SKIP_LOWLEVEL_INITblcpu_init_crit#endif这段代码宏也没有定义，故参与程序的编译。分析可知程序带返回跳到cpu_init_crit汇编函数执行。下面我们分析cpu_init_crit函数。cpu_init_crit:/* * Invalidate L1 I/D */movr0, #0@ set up for MCRmcrp15, 0, r0, c8, c7, 0@ invalidate TLBsmcrp15, 0, r0, c7, c5, 0@ invalidate icache/* * disable MMU stuff and caches */mrcp15, 0, r0, c1, c0, 0bicr0, r0, #0 clear bits 13 (--V-)bic r0, r0, #0 clear bits 2:0 (-CAM)orr r0, r0, #0 set bit 1 (--A-) Alignorr r0, r0, #0 set bit 12 (Z---) BTBmcrp15, 0, r0, c1, c0, 0/* * Jump to board specific initialization... * The Mask ROM will have already initialized * basic memory. Go here to bump up clock rate and handle * wake up conditions. */movip, lr@ persevere link reg across callbl lowlevel_init@ go setup pll,mux,memorymovlr, ip@ restore linkmovpc, lr@ back to my caller在第一段中操作了cp15寄存器，关掉了TLB和指令缓存icache。其中TLB是MMU内存映射时候需要从虚拟地址查表转换成物理地址，页表位于内存中，会影响查表速度，于是在CPU内部开辟了寄存器来存一小部分的页表，称为TLB。CP15中寄存器C8和C10控制TLB的相关操作。mcrp15, 0, r0, c8, c7, 0可知这句是失效TLBC7用于控制cac