2010年12月12日 星期日

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纠正基本错误:BIOS检测的基本顺序是CPU 、系统主板、基本的640KB 内存、1MB 以上的扩展内存、系统ROM BIOS 的测试;CMOS 中系统配置的校验;初始化视频控制器,测试视频内存、检验视频信号和同步信号,对CRT 接口进行测试;对键盘、软驱、硬盘及CD-ROM 子系统作检查;对并行口(打印机)和串行口(RS232)进行检查。当自检完成后,系统会自动转入BIOS 的下一步骤, 即从A 驱、C 驱或CD-ROM 以及网络服务器上寻找操作系统进行启动,然后将控制权交给操作系统。具体步骤如下:
  (1)当按下电源开关时,电源就开始向主板和其他设备供电。此时电压还不太稳定,主板上的控制芯片组会向CPU发出并保持一个Reset(重置)信号,让CPU 内部自动恢复到初始状态,但CPU在此刻不会马上执行指令。当芯片组检测到电源已经开始稳定供电后(当然从不稳定到稳定的过程只是一瞬间的事情),便撤去Reset信号(如果是手工按下计算机面板上的Reset按钮来重启机器,那么松开该按钮时芯片组就会撤去Reset信号),CPU 马上就从地址FFFF0H 处开始执行指令。无论是Award BIOS还是AMI BIOS,放在这里的只是一条跳转指令,跳到系统BIOS中真正的启动代码处。
  (2)BIOS的启动代码首先要做的事情就是执行POST 。POST的主要任务是检测系统中一些关键设备是否存在和能否正常工作,例如内存和显卡等设备。由于POST是最早进行的检测过程,此时显卡还没有初始化,如果系统BIOS在进行POST 的过程中发现了一些致命错误,例如没有找到内存或者内存有问题(此时只会检查640KB 常规内存),那么系统BIOS 就会直接控制喇叭发声来报告错误,声音的长短和次数代表了错误的类型。在正常情况下,POST过程进行得非常快,我们几乎无法感觉到它的存在,POST结束之后就会调用其他代码来进行更完整的硬件检测。
  (3)接下来主板BIOS 将查找显卡的BIOS,找到显卡BIOS 之后就调用它的初始化代码,由显卡BIOS 来初始化显卡,此时多数显卡都会在屏幕上显示出一些初始化信息,介绍生产厂商、图形芯片类型等内容,不过这个画面几乎是一闪而过。主板BIOS 接着会查找其他设备的BIOS 程序,找到之后同样要调用这些BIOS内部的初始化代码来初始化相关的设备。
  (4)查找完所有其他设备的BIOS之后,系统BIOS将显示出它自己的启动画面,其中包括有系统BIOS 的类型、序列号和版本号等内容。
  (5)接着主板BIOS将检测和显示CPU的类型和工作频率,然后开始测试所有的RAM,并同时在屏幕上显示内存测试的进度,我们可以在CMOS设置中自行决定使用简单耗时少或者详细耗时多的测试方式(测试1 次或3 次)。
  (6)内存测试通过之后,主板BIOS将开始检测系统中安装的一些标准硬件设备,包括硬盘、CD-ROM 、串口、并口、软驱等设备,另外绝大多数较新版本的BIOS 在这一过程中还要自动检测和设置内存的定时参数、硬盘参数和访问模式等。标准设备检测完毕后,系统BIOS内部支持即插即用的代码将开始检测和配置系统中安装的即插即用设备,每找到一个设备之后,系统BIOS都会在屏幕上显示出设备的名称和型号等信息,同时为该设备分配中断、DMA 通道和I/O 端口等资源。
  (7)到这一步为止,所有硬件都已经检测配置完毕了,主板BIOS 会重新清屏并在屏幕上方显示出一个表格,其中概略地列出了系统中安装的各种标准硬件设备,以及它们使用的资源和一些相关工作参数。接下来主板BIOS 将更新ESCD(Extended System Configuration Data,扩展系统配置数据)。ESCD 是主板BIOS 用来与操作系统交换硬件配置信息的一种手段,这些数据被存放在CMOS(一小块特殊的RAM,由主板上的电池来供电)之中。 通常ESCD 数据只在系统硬件配置发生改变后才会更新,所以不是每次启动机器时我们都能够看到 “Update ESCD ……Success ”这样的信息。不过,某些主板的系统BIOS在保存ESCD 数据时使用了与 Windows 9x 不相同的数据格式,于是Windows 9x 在它自己的启动过程中会把ESCD 数据修改成自己的格式,但在下一次启动机器时,即使硬件配置没有发生改变,系统BIOS 也会把ESCD 的数据格式改回来, 如此循环,将会导致在每次启动机器时,系统BIOS 都要更新一遍ESCD,这就是为什么有些机器在每次启动时都会显示出相关信息的原因。
  (8)ESCD更新完毕后,系统BIOS的启动代码将进行它的最后一项工作,即根据用户指定的启动顺序启动。以从C盘启动为例,系统BIOS 将读取并执行硬盘上的主引导记录,主引导记录接着从分区表中找到第一个活动分区,然后读取并执行这个活动分区的分区引导记录,而分区引导记录将负责读取并执行Io.sys,这是DOS 和Windows 9x 最基本的系统文件。Windows 9x的Io.sys首先要初始化一些重要的系统数据,然后就显示出我们熟悉的蓝天白云,Windows 将继续进行DOS部分和GUI(图形用户界面)部分的引导和初始化工作。如果系统之中安装有引导多种操作系统的工具软件,通常主引导记录将被替换成该软件的引导代码,这些代码将允许用户选择一种操作系统,然后读取并执行该操作系统的 基本引导代码(DOS 和Windows 的基本引导代码就是分区引导记录).

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