BIOS is short for Basic Input Output System. Actually, it is a group of programs solidified to the ROM chip of computer main board. It saves the most important basic input and output programs of computer, system settings and the self-checking and self-triggered programs. The main function of it is to provide the basic and direct hardware settings and control.

Introduction
The abbreviation of plural Biography is Bios, reads /'baious/. BIOS setting program is saved in BIOS chip. Only when starting up it can be set. CMOS is mainly used for saving the parameter and data of BIOS. BIOS mainly manage and set the basic input and output system. It slao can exclude system faults or diagnosis system poblems. Somebody thinks that since BIOS is a program, it is software like Word or Excel. There are still some opposite opinions. BIOS is still very different from normal softwares. And it is similar to hardware. Actually BIOS is the bridge of software programs and hardware devices. It is incharge of the requirements of hardwares.
The BIOS chip on main board is the only labeled chip. Generally it is a 32-pin dual in-line type integrated circuit with BIOS mark. The BIOS before 586 are mostly rewritable EPROM chip. The label on it protects BIOS content (UV will cause the loss of EPROM content) and cannot be easily torn down. The BIOS after 586 mostly use EEPROM (Electrically Erasable Programmable Read - Only Memory). Through the jumper switch and the driver, it is possible to rewrite EEPROM to upgrade BIOS. Users will always come into the contact with BIOS while using computer. The functions of BIOS decide the performance of main board.
BIOS chip is a rectangular or square chip on the main board. BIOS mainly saves the following:
Self-checking program (power up self-checking): through reading the content identification hardware configuration of CMOS RAM, it self-checks and initializes.
CMOS setting: during guiding, it is started with special hot key. Settings will be saved to CMOS RAM.
System Bootstrap Loading Program: after the success of self-checking, the guiding program on the corresponding 0 to 1 sector of disc will be saved to RAM to install DOS system.
Major I/O devices drivers and interruption service (drviers based on external devices): as BIOS works directly with system hardware resource, so it is always aimed at a certain kind of hardware system, and the hardware systems are so different from each other, so there are various kinds of BIOS. With the development of hardware techniques, the same kind of BIOS has developed into different types. The new type has more power functions than old ones.

Differences between CMOS and BIOS
As CMOS and BIOS has a close relationship with computer system settings, so they are called CMOS setting and BIOS setting. CMOS is the special RAM chip on the main board. It is for saving system parameter. The system setting program of BIOS is the means to complete parameter settings. So you can say that set the CMOS parameter through BIOS setting program. CMOS setting and BIOS setting are just the simple saying. Actually, BIOS program is saved on the EEPROM Flash chip of main board. CMOS storage is for saving the data after BIOS setting, including hardware configuration and settings of certain parameter, such as traditional BIOS system password and devices starting order and so on.

Functions
Self-checking and initializing 　
This part is incharge of starting up the computer, including:
First part is used for the checking of hardware when plugging in. it is also called Power On Self Test, POST in short.
The second part is initializing, including create interrupt vector, set register, initialize and check the external devices.
The third part is guiding program, which is for guiding DOS or other operating systems.

Process service　
Process service is mainly for applications and operating system. Those services are related to input and output devices such as reading disc, transferring files to printer, etc.

Hardware Interrupt Handling
Hardware Interrupts Handling deals with the requirement of PC hardwares. The service function of BIOS is realized by calling intertupt servise routine. These services are devided into many groups and every group has a specialized interrupt. In addition, the improper BIOS settings will damage computer hardware or even burn the main board. It is strongly suggested that do noy modify the settings easily.

Setting Record
Users can change different settings through BIOS settings such as the size of internal storage of onboard display card.

Load Operating system
All the operating systems of users are delivered to guide sector by BIOS, and to each partition to activate corresponding operating systems.

Classification
The popular main board BIOS are Award BIOS, AMI BIOS, and Phonenix BIOS. Award BIOS is developed by Award Software and it is the most widely used. AMI BIOS is developed by AMI Company in the middle of 1980s. The early 286 and 386 mostly use AMI BIOS. It has great adaptability of various software and hardwares and can guarantee the stability of system performance. At the end of 1990s, green energy saving computer has been popularizing, but AMI has not released new versions for market in time. That makes Awars BIOS take the leading role.
Phoenix BIOS is developed by Phoenix. Phoenix is a kind of long-life bird of Egypt fairy tale. It has a meaning of perfect. Phoenix BIOS is mainly used at the high level 586 original brand PC and Notebook. The picture of it is simple and easy to operate.