CMOS User Interface BIOS IC Chip
Your PC has to keep certain settings when it's turned off and its power cord is unplugged.
These settings include the following:
Date, Time, Hard drive configuration, Memory, Integrated ports, Boot sequence, Power management
Your PC keeps these settings in a special memory chip called the complementary metal oxide semiconductor (CMOS) memory chip. Actually, CMOS (usually pronounced see-moss) is a manufacturing technology for integrated circuits. The first commonly used chip made from CMOS technology was a type of memory chip, the memory for the BIOS. As a result, the term CMOS is the accepted name for this memory chip.
Computer power up is orchestrated by two parts. The BIOS ROM (Read Only Memory) and a read/write RAM generally known as CMOS (named after the technology in use). The CMOS is also a separate chip on the motherboard, but more specifically, it is a RAM chip.This is a type of memory chip which stores information about the computer components, as well as settings for those components. However, normal RAM chips lose the information stored in them when power is no longer supplied to them.
To retain the information in the CMOS chip, a CMOS battery on the motherboard supplies constant power to that CMOS chip. If the battery is removed from the motherboard or runs out of charge, the CMOS would lose the information stored in it. Any settings you made in the CMOS setup would be lost, and you would need to make those settings changes again after a new CMOS battery was put on the motherboard. For example, with a dead CMOS battery the time and date will reset back to the manufactured date if it has been off for a long period of time.
The BIOS program on the BIOS chip reads information from the CMOS chip when the computer is starting up, during the boot up process. The BIOS starts with its own default information and then reads information from the CMOS, such as which hard drive types are configured for this computer to use, which drive(s) it should search for boot sectors, and so on. Any overlapping information read from the CMOS overrides the default information from the BIOS. A lack of corresponding information in the CMOS does not delete information that the BIOS knows natively. This process is a merge, not a write-over. CMOS memory is usually not upgradeable in terms of its capacity and might be integrated into the BIOS chip or some other chip.
One of the most common problems in upgrading to a larger hard disk is the BIOS's inability to support the larger disk size. In the original IDE specification, the size limit was 540MB. This limitation was increased to 8GB with the introduction of Logical Block Addressing, or LBA, in 1996, which the BIOS must support. A BIOS update may be available for the motherboard to enable LBA if needed.
The 8GB limitation can be broken if the BIOS supports Enhanced BIOS Services for Disk Drives, a 1998 update. Interrupt 13h extensions to the BIOS allow for drives as large as 137GB. Large LBA is a solution for drives that exceed 137GB. Again, a BIOS update for the motherboard may enable this support if it's lacking.
Most BIOS Setup programs today support Plug and Play, so they detect the new drive automatically at startup. If this doesn't work, the drive may not be installed correctly, the jumper settings may be wrong, or the BIOS Setup may have the IDE interface set to None or Disable rather than Auto. In BIOS Setup for the drive, you might have the option of selecting a DMA or programmed input/output, known as PIO, setting for the drive. Both are methods for improving drive performance by allowing the drive to write directly to RAM, bypassing the CPU when possible. For modern drives that support UltraDMA, neither of these settings is necessary or desirable.
Use the Reset CMOS jumper to clear the CMOS memory when an unknown password, for example, is keeping you out of the BIOS configuration utility.