BIOS是开机后最先运行的一组程序，主要用来控制系统最重要的基本输入输出程序，系统信息设置程序，开机上电自检程序。系统启动自检程序以及一些控制基本输入输出设备的中断服务例程等。主要是初始系统中的硬件设备，然后从指定的设备上载入操作系统。
就像系统硬件与其他软件间的一个接口程序，让使用者不必更换硬件或操作系统的情况下而更换相应的操作系统或软件。
BISO程序与其他程序的区别
1 读取的先后顺序不同；
2 所属类型不同，既是软件，又属于硬件，所以称之为固件。其本身就是一组程序，比OS更底层；
BIOS是以烧录的形式固化在主板的ROM芯片上，每次加电或重启时被触发并自动运行的一组程序（启动程序（引导程序），驱动程序。CMOS是BIOS设定系统参数的存放场所。
驱动程序是对BIOS不能支持的各种硬件设备进行解释；CPU，内存、键盘、显示器等标准硬件是BIOS能直接支持的硬件，而显示、声卡、网卡等则一定要安装驱动。
The BIOS is the third type of software.It runs below the OS,and acts as interface between the major hardware components of your computer and the operating system.BIOS is usually stored on a Flash memory chip on the motherboard.It is the first software your computer runs after power up.
The BIOS has a number of different roles,but its most important role is to load the operating system.When you turn on your computer and the microprocessor tries to execute its first instruction,it cannot get it from the operating system because the operating system is located on a hard disk,and the microprocessor cannot get to it without some instructions that tell it how.The BIOS provides those instructions.
1 check the CMOS Setup for custom settings;
2 Load the interrupt handlers and device drivers;
3 Initialize registers and power management;
4 Perform the power-on self test;
5 Display system settings;
6 Determine which devices are bootable;
7 Initialize the boostrap sequence;
BIOS also activates other BIOS chips on different cards installed in the computer.For example,SCSI and Graphics cards often have their own BIOS chips;
the BIOSchecks to see if this is a cold boot or a reboot.If it is a reboot,the BIOS skips the rest of POST.If it is a cold boot,the BIOS verifies RAM by performing a write-read test of each memory address.It checks the PS/2 ports or USB ports for a keyboard and a mouse.It looks for a Peripheral component interconnect(PCI) bus and,if it finds one,checks all the PCI cards.If the BIOS finds any errors during the POST,it will notify you by a series of beeps or a text message displayed on the screen.
the BIOS then displyas some details about your systme:
1 the processor
2 the floppy drive and hard driver;
3 memory;
4 BIOS revision and date;
5 display
6 special drivers,such as the SCSI adapters.
the BIOS then looks at the sequence of storage devices identified as boot devices in the CMOS Setup.Boot refers to the process of launching the operating system.The BIOS will try to initiate the boot sequence from the first device.If the BIOS does not find a device,it will try the next device in the list.If it does not find the proper files on a device,the startup process will halt.
Voltage sent over the pins can be in one of two states,ON or Off.ON(binary value"1")menas that the pin is transmitting a signal between -3 and -25 Volt,while OFF(binary value"0")means that it is transmitting a signal between +3 and +255 Volt.