CMOS - Free Knowledge Base- The DUCK Project: information for everyone

Complementary Metal-Oxide Semiconductor or CMOS is an on-board, battery powered semiconductor chip inside computers that stores time and date as well as hardware settings for your computer. The modern system clock used 14 bytes of RAM, leaving an additional 50 bytes for storing system settings. For more space to save more extensive system settings integrated NVRAM is utilized in the southbridge or Super I/O chips.

Other names for CMOS include Real-Time Clock (RTC), Non-Volatile RAM (NVRAM) or CMOS RAM. CMOS relates to the Basic Input/Output System and also known as the System BIOS or ROM BIOS. The BIOS uses information from CMOS RAM and other NVRAM EEPROM as mentioned above. A modern BIOS setup utility has a menu-based user interface accessed by pressing a certain key when the PC starts. Usually the key is advertised for short time during the early startup, for example "Press F1 to enter CMOS setup".

   Press F2 or Delete key to enter bios settings on Dell pc.
   Press F1 or F10 key to enter bios settings on HP pc.
   Press F1 or F10 key to enter bios settings on Compaq pc.
   Press F1, F2 or F3 key to enter bios settings on SonyVARO pc.
   Press F1 or F2 key to enter bios settings on Acer pc.
   Press F1 or F12 key to enter bios settings on Toshiba pc.
   Press F1 or F2 key to enter bios settings on Gateway pc.

Traditionally the BIOS chip on a motherboard is a ROM chip while the CMOS chip is a RAM chip, the PC having both. Because a RAM chip loses settings data without power, a CMOS battery is necessary. The CMOS battery on the motherboard supplies constant power to the CMOS chip. The program logic in the BIOS chip is retained even if the CMOS battery is removed and the system is without power for an extended duration while the CMOS settings are lost, leaving defaults supplied from the BIOS chip.

The BIOS program on the BIOS chip reads information from the CMOS chip when the computer is starting up, during the boot up process. On the most modern 64-bit computers an additional layer has been added to the duo called UEFI. This fulfills new requirements on modern systems that were not possible with the aging BIOS-CMOS design.