Most of the time when we start up our computers, the CPU or Central Processing Unit goes through a series of steps or procedures before the computer operating systems boots from the hard disk. These initial steps are all categorized in a system called CMOS which stands for Complementary Metal Oxide Semiconductor. This is the where the systems settings such as the system clock, hard disk status, optical drives and other settings like the CPU's clock speed, FSB and so on are all on this setting.
On the other hand, the chip that stores the settings is known as the BIOS chip also known as Basic Input Output System. This holds the most basic computer instructions that are required to boot the operating system like Windows XP or Linux based operating systems such as Ubuntu. Many people always get mistaken by calling is a CMOS chip, but since the BIOS and CMOS is so tightly intertwined, they can be both referred to as one (Difference between CMOS and BIOS, Dennis Faas). In other words, the CMOS chip stores the data needed for the BIOS so that the computer may boot us properly.
The battery used to store the data is a lithium button battery which is located just near the CMOS chip and the CMOS jumper. This battery hold provides power to the CMOS chip even when the power of the computer is shut off. This is necessary where the chip is volatile, means that it will lose data when there is no power unlike a hard disk which work on magnetism.
What is BIOS & CMOS
BIOS is commonly known as Basic Input/Output System. It is practically the lowest level processor that stands in between the hardware like the chipset and the processor with the operating system that is installed in a hard disk. The BIOS gives full access to the hardware components to allow a creation of high level operating systems such as Windows XP, Windows 7, Macintosh as well as Linux based operating systems. Moreover, the BIOS is also responsible over the behaviour of the hardware, such as the processor's clock speed, the Front Side Bus or known as FSB, system clock, and many other vital hardware settings which is required to load or configured before the operating system loads into the Random Access Memory or RAM (System BIOS, Charles M. Kozierok, 2001).
For a computer to operate, all the components must be given specific instructions on how and what they are suppose to do. Usually, that is where the BIOS come into play. The BIOS has a special software in which it runs preliminary codes or instructions which is preset into the ROM chip or Read Only Memory chip that is executed to load the operating system from the hard disk into the RAM for the computer to further execute more complicated commands and instructions so that it would be easier for the processor to communicate with the RAM in terms of instruction execution and access (The BIOS Program, Charles M. Kozierok, 2001).
One of the most basic operations of the BIOS is the POST (Power On Self Test). This part is done to make sure that all of the components in the computer such as the motherboard, hard disks, optical drives, graphics adapter card and other components are working and they are detected throughout the system.
Figure 1: BIOS that uses flash memory
CMOS or Complementary Metal Oxide Semiconductor is low in power consumption and has low heat production rate technology that is used in contemporary microchips and it is commonly known as a small battery powered configuration chip where the BIOS stores the time, date and the system configuration settings (What is CMOS?, 2010).
The main composition of this chip is made from silicon and germanium which in a way conducts electricity. These materials are then "doped" where impurities are added to transform it into a fully scaled transistor, either of extra negative charged (N-type) or positively charged (P-type). CMOS on the other hand uses both kinds of transistors in a complementary way to create a current gate where it makes an electrical control present. They practically use no power at all but this in another way, it makes it heat up fast where this sets a major limit to the speed where a microprocessor can operate (CMOS, 2005).
The CMOS has a software which is a setup that is included in the creation of the chip. This setup is very vital for system configuration such as the frequency and voltage control of the components, the system date and time and the POST settings as well as the boot priority settings for the operating system to load.
Figure 2: Examples of CMOS setup software interface
Types of BIOS & CMOS
Now we know that BIOS is mainly the system where as the CMOS is the chip that contains a software that runs the BIOS.
BIOSes were programmed initially into ROMs or PROMs (Programmable Read Only Memory) CMOS chips, where it only stored a very limited amount of data of the system configuration. Extensibility of the BIOS is limited to the capacity of the CMOS chip hence many years later BIOSes became more better when they introduce a new kind technology known as EPROM (Erasable Programmable Read Only Memory) and EEPROM (Electronically Erasable Programmable Read Only Memory). Though EPROMs are eraser in a very different manner where the chip has to be exposed to UV light which passes through a small glass window on the chip, in order to clear all the memory cells. The chip has a small window that allows the intake of UV light upon erasing the data instructions in the CMOS chip (How to erase and program an EPROM, 2003).
Figure 3: DIP type ROM CMOS chip
Figure 4: PLCC type PROM CMOS chip
The CMOS chip on the other hand has 2 commonly used types, categorized by their packages, PLCC and DIP. PLCC stands for Plastic Leaded Chip Carrier which is a four sided chip carrier with a 'J' lead and pin spacings of 1.27mm. The lead count or legs usually range from 20 to 84 (Minges, Merrill L. (1989). Electronic Materials Handbook). DIP stands for Dual Inline Package which consists of two parallel sided electrical connecting pins connected to a rectangular housing. Both types of packages for the CMOS chip is fixed at 32 pins, since the CMOS settings only need to transfer about 32 bits across the board.
Like mentioned in the introduction, the BIOS or the CMOS chip is powered by a battery that is used to store the settings and instructions intact even after the power is turned off. Below are the several types of batteries the CMOS chip uses (CMOS Battery, 2010):
Figure 5: 3V Lithium Button Cell Battery
Figure 6: 4.5V Alkaline Battery
Figure 7: 3.6V Nickel-Cadmium Battery (Solder Connectors)
Current Technologies and Models for BIOS & CMOS
In today's market, the current technology used in BIOS and CMOS is the PLCC type, EEPROM CMOS chip and powered by a 3V Lithium Battery Cell. The CMOS setup for the BIOS include two major companies which are American Megatrends and Phoenix BIOS. There are also other type's setups which are configured by the computer manufacturer such as Dell, Acer, HP Compaq and many more.
The BIOS on the other hand provides extra facilities in terms of the configurability of the onboard devices like the embedded audio, the processor's voltage, FSB, Multiplier, RAM voltage, hard disk boot sequence and many more.
Below are some examples of the CMOS setup screens in configuration mode for different types of setup:
Figure 8: CMOS setup main page for AWARD Software.
Figure 9: CPU Configuration settings
Figure 10: Boot Sequence settings
BIOS & CMOS Failures and Solutions
Like the RAMs and hard disks, the BIOS and CMOS chip can also have error and failures. There are three types of virus invasion that can affect the BIOS system:
Black Hat 2006
Persistent BIOS Infection
CIH was the first virus that was able to erase the content on a Flash ROM BIOS. It was also given the nickname as "Chernobyl Virus" The extent of the virus lead to the ejection of the CMOS chip from the motherboard and sent for reprogramming for it has also distorted its basic functionality.
Black Hat 2006 is a virus that is capable to elevate privileges and read physical memory, using malicious procedures that replaced normal ACPI functions stored in flash memory.
Persistent BIOS Infection work as an insertion of malicious code into the decompression routines in the BIOS, allowing for nearly full control of the PC at every start-up, even before the operating system is booted. The proof of the concept does not exploit a flaw in the BIOS implementation, but only involves the normal BIOS flashing procedures. Thus, it requires physical access to the machine or for the user on the operating system to be root (Researchers unveil persistent BIOS attack methods, 2009).
Solutions to overcome these kinds of virus usually required either reprogramming of the CMOS chip or just an upgrade of the CMOS firmware.
To reset all the data in the BIOS via the CMOS chip are usually done as follows:
Turn off all power and disconnect all wires from the PC
Open the CPU casing to reveal the motherboard
Find the jumper pins that is used to clear the CMOS settings or a small button near the CMOS chip
Short the jumper pins by moving the connector from "1-2" to "2-3" position to clear the settings and remove the system battery. Else, press the button for 8 seconds.
After this is done, move the jumper connector back to its original position "1-2" and then place the system battery back in its place.
Place the cover back and connect all the wires and turn on the power
Once the computer loads and pass the POST test, the BIOS will prompt to enter CMOS settings to set it back again.
After the settings are set, save the settings and exit the menu so the computer can restart and load the operating system.
In conclusion, the BIOS is the most system for a computer to boot and load settings for the all the hardware components. The CMOS chip is a requirement for it is the component that stores the instructions of the BIOS into the setup. Without this, the computer would not know how to load the operating system or identify the existence of any hardware components.