CMOS
Jose Capellan
ITT-Tech

CMOS or complementary metal oxide semiconductor is a type of integrated circuit technology. The term is often used to refer to a battery-powered chip found in many personal computers that holds some basic information, including the date and time and system configuration settings, needed by BIOS, or basic input/output system to start the computer. The CMOS memory is usually located with the real-time clock in the motherboard chipset or in a separate real-time clock chip. The PC reads the time from the RTC when it boots up, after which the CPU keeps time – which is why system clocks are sometimes out of sync. Rebooting the PC causes the RTC to be reread, increasing their accuracy. In modern-day PCs the nonvolatile BIOS memory is generally an EEPROM or Flash memory chip. There is still a backup battery involved, but its role is not to maintain the data stored in the RAM, only to keep the RTC chip operational. This name is somewhat misleading, however, as most modern computers no longer use CMOS chips for this function, but instead depend on other forms of non-volatile memory. CMOS chips are still found in in many other electronic devices, including digital cameras. In a computer, the CMOS controls a variety of function, including POST or the Power on Self-Test. When the computer’s power supply fires up, CMOS runs a series of checks to make sure the system is functioning properly. One of these checks includes counting up RAM or random access memory. This delays boot time, so some people disable this feature in the CMOS settings, opting for a quick boot. If installing new RAM it is better to enable the feature until the RAM has been checked. CMOS memory size has remained relatively unchanged over the years - there was never any need to store more than 512 bytes in the memory as it holds the absolute basic boot settings for the system. The typical size is still 512 bytes currently The size of CMOS memory is the same today as it was when it was introduced with the 80286 processor in the late 80's, CMOS has since evolved from using a RAM to using EEPROM. It however uses it exclusively in its construction. EEPROM is an advanced feature and is yet to be embraced by the majority. The CMOS memory is actually a 64 or 128 byte battery-backed RAM memory module that is a part of the system clock chip. Some IBM PS/2 models have the capability for a 2k (2048 byte) CMOS ROM Extension. The extension was first used with clock-calendar cards for the IBM PC-XT, when the PC/AT (Advanced Technology) was introduced in 1985; A Motorola 146818 chip was the first RTC and CMOS RAM chip to be used in early IBM computers. The chip was capable of storing a total of 64 bytes of data. Since the system clock used 14 bytes of RAM, this left an additional 50 bytes of space that was available for IBM to store system setting the Motorola MC146818 became a part of the motherboard. Since the clock only uses fourteen of the RAM bytes, the rest are available for storing system configuration data. AMSTRAD 8086 machines were among the first to actively use the CMOS memory available and since they precede the AT, do not follow the AT standard. Originally, IBM only used a small portion of CMOS memory, in the IBM PC/AT Technical Reference Manual these were defined as bytes 10h, 12h, 14h-18h, 2Eh-33h. The balances was left indeterminate but were quickly taken by various BIOS manufacturers for user-selectable options like wait states, clock speeds, initial boot drive selection, and password storage. Later, as CMOS memory requirements grew newer clock chips with 128 bytes of RAM came into use. Once the AT standard was established, there was never any need to store any more than 512 bytes in the memory because it holds only the most basic of boot settings. CMOS
Bibliography
Tim Fisher. What is CMOS? Retrieved 11.06.2013 from http/www.pcsupport.about.com http://pcsupport.about.com/od/termsc/g/cmos.htm M. McGee, L. F. what is CMOS? Retrieved 11.06.2013 from http/www.wisegeek.orghttp://www.wisegeek.org/what-is-cmos.htm