【转】u-boot代码lowlevel.S分析

上面说到了start.S这个文件，现在说说lowlevel.S这个代码的作用以及代码的相关注释！

/*
 * Memory Setup stuff - taken from blob memsetup.S
 *
 * Copyright (C) 1999 2000 2001 Erik Mouw (J.A.K.Mouw@its.tudelft.nl) and
 *                     Jan-Derk Bakker (J.D.Bakker@its.tudelft.nl)
 *
 * Modified for the Samsung SMDK2410 by
 * (C) Copyright 2002
 * David Mueller, ELSOFT AG, <d.mueller@elsoft.ch>
 *
 * See file CREDITS for list of people who contributed to this
 * project.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation; either version 2 of
 * the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston,
 * MA 02111-1307 USA
 */

#include <config.h>
#include <version.h>

#include <s3c2440.h>
#include "smdk2440_val.h"

_TEXT_BASE:
 .word TEXT_BASE

 .globl lowlevel_init          /* lowlevel_init的入口 */
lowlevel_init:
 mov r12, lr                 

 /* init system clock */
 bl system_clock_init          /* 跳转到system_clock_init */

 /* for UART */
 bl uart_asm_init              /* 跳转到uart_asm_init */

 /* simple init for NAND */
 bl nand_asm_init              /* 跳转到nand_asm_init */

 /* when we already run in ram, we don't need to relocate U-Boot.
  * and actually, memory controller must be configured before U-Boot
  * is running in ram.
  */
 ldr r0, =0xf0000fff          /* r0=0xf0000fff                   */
 bic r1, pc, r0              /* r0 <- current base addr of code */
 ldr r2, _TEXT_BASE          /* r1 <- original base addr in ram */
 bic r2, r2, r0              /* r0 <- current base addr of code */
 cmp     r1, r2             /* compare r0, r1                  */
 beq     1f                 /* r0 == r1 then skip sdram init   */

 adrl    r0, mem_cfg_val      /* 取得mem_cfg_val子程序的入口 */
 bl mem_con_init             /* 跳转到mem_con_init，文件在\cpu\s3c24xx\s3c2440\cpu_init.S下 */

 ldr r0, =ELFIN_UART_BASE     /* 加载ELFIN_UART_BASE地址到r0 ，ELFIN_UART_BASE是UART的初始地址 */
 ldr r1, =0x4b4b4b4b          /* 加载立即数0x4b4b4b4b到r1    */
 str r1, [r0, #0x20]          /* 地址（ELFIN_UART_BASE+0x20）=0x4b4b4b4b，用来设置串口传输缓冲寄存器，输出K字母 */

1: mov lr, r12
 mov pc, lr                   /* 子程序返回 */

/*
 * system_clock_init: Initialize core clock and bus clock.
 * void system_clock_init(void)
 */
system_clock_init:
 /* Disable Watchdog */
 ldr r0, =ELFIN_WATCHDOG_BASE            /* 加载ELFIN_WATCHDOG_BASE地址到r0    */
 mov r1, #0                              /* r1=0x0                             */
 str r1, [r0]                            /* ELFIN_WATCHDOG_BASE=0，关闭看门狗  */
 
        
 /* mask all IRQs by setting all bits in the INTMR - default */
 ldr r0, =ELFIN_INTERRUPT_BASE           /* 加载ELFIN_INTERRUPT_BASE地址到r0，ELFIN_INTERRUPT_BASE是SRCPND寄存器地址 */
 mov r1, #0xffffffff                     /* r1=0xffffffff */
 str r1, [r0, #INTMSK_OFFSET]            /* (ELFIN_INTERRUPT_BASE+INTMSK_OFFSET)=0xffffffff */
  
 /***************************************************************************************************
  *NOTE:INTERRUPT，中断掩码寄存器，决定那个中断源被屏蔽，某位为1则屏蔽中断源，初始值为0xFFFFFFFF，屏蔽所有中断 *
   ***************************************************************************************************/
 ldr r1, =0x00001fff                     /* 加载立即数0x000007ff*/    /*modify by chenpx 7ff */
 str r1, [r0, #INTSUBMSK_OFFSET]         /* (ELFIN_INTERRUPT_BASE+INTSUBMSK_OFFSET)=0x000001fff */
 
 /*********************************************************************************************
  * NOTE:INTSUBMSK，中断子掩码寄存器，该寄存器只能屏蔽13个中断源，因此其仅低11位有效，初始值为0x7FF      *
   *********************************************************************************************/
    
 /* Initialize System Clock */
 ldr r0, =ELFIN_CLOCK_POWER_BASE           /* 加载ELFIN_CLOCK_POWER_BASE地址到r0中 */
        ldr r1, =0x00ffffff                 /* 加载立即数0x00ffffff到r1中 */
        str r1, [r0, #LOCKTIME_OFFSET]     /* Set Clock Divider */
#ifndef S3C2440_PLL_OFF
        /* FCLK:HCLK:PCLK */                /* FCLK是帧同步时钟，HCLK是行同步时钟，PCLK是LCD的扫描时钟 */
 mov r1, #0x00000000                      /* r1=0x00000000      */
 str r1, [r0, #CAMDIVN_OFFSET]            /* CAMDIVN=0x00000000,初始化照相机时钟分配寄存器 */
        ldr     r1, =CLKDIVN_VAL            /* 加载CLKDIVN_VAL地址到r1中 */
        str     r1, [r0, #CLKDIVN_OFFSET]   /* CLKDIVN=CLKDIVN_VAL，设置时钟分配控制寄存器 */
   
    /*********************************************************************
     * NOTE:设置时钟寄存器，CLKDIVN第0位为PDIVN，为0则PCLK=HCLK，        *
     * 为1则PCLK=HCLK/2 ;第1位为HDIVN，为0则HCLK=FCLK,为1则HCLK=FCLK/2   *
     *********************************************************************/