CMOS (complementary metal oxide semiconductor)

CMOS Battery

The information contained in the CMOS chip is maintained by a battery. If the battery runs low, the CMOS content will be lost and POST will display a "CMOS invalid" or "CMOS checksum invalid" message. The first symptom for a battery running low is time slowing down and eventually complete loss of date and time. In some cases you might get a boot device error because a boot device such as floppy disk or hard drive can not be located. Almost always the CMOS battery can be replaced very easily. Here is a picture of a common CMOS battery:



BIOS Upgrades

Most modern BIOSs are stored on flash memory, this enables you to upgrade the BIOS software when needed, in some cases this is necessary to support new hardware technologies/devices. Upgrading the BIOS is called flashing, before you do make sure you write down all settings and during the process pray the power won't be interrupted. The BIOS version ID is displayed during startup. Some mainboards allow replacing the BIOS ROM, although this is not done often.

COMMON CMOS SETTINGS Printer parallel port—Uni., bi-directional, disable/enable, ECP, EPP In the CMOS you are able to configure a parallel port to use EPP or ECP. Enhanced Parallel Port and Extended Capabilities Port are both bi-directional standards, operate in 8-bit, and allow data transfer speed of approximately 2 MB/s. Some of the main differences are that ECP supports Direct Memory Access (DMA) and data compression, which enables higher transfer rates. It is also possible to completely disable the parallel port in the BIOS. Most BIOSs allows you to set the DMA channel, when the port mode is set to ECP. COM/serial port Most personal computers have 2 serial ports. In the BIOS you can assign COM1/COM2/COM3/COM4 to serial port 1 or 2. Most BIOSs also allow you to set the I/O and IRQ but this is mostly done automatically. Floppy drive s The floppy drive(s) can be enabled/disabled in the BIOS (e.g. set to Not Installed). The BIOS also allows you to choose the capacity of the media. - 360 KB 5.25 inch - 1.2 MB 5.25 inch - 720 KB 3.5 inch - 1.44 MB 3.5 inch - 2.88 MB 3.5 inch Some BIOSs also allows you to swap A: and B: and disable seeking a floppy disk for a boot sector during startup.

Hard drives Most modern BIOSs allow automatically detection of disk parameters. These are some of the primary CMOS settings that apply to hard drives (and CD/DVD-ROM drives, etc.), the settings can be individually configured for the primary master and slave device and the secondary master and slave device. Type Common disk types are: - User, User-defined CHS values - Auto, automatically detects hard disks parameters at every startup. - 1-46, predefined combinations of CHS values. - CDROM, used fot atapi CD-ROM drives - ARMD, used fo atapi ZIP and LS 120 drives. - DVDROM Size Determines the capacity of the drive. CHS values - number of Cylinders - number of Heads - number of Sectors LBA (Large Block Addressing), technologie to overcome the 528 MB limit. Another common CMOS setting related to hard drives is Boot Sector Virus protection Enabled/Disabled, enabling this will make the BIOS issue a warning message/beep if a write to the boot sector or partition table of a hard disk is attempted. Memory Today's motherboards provide too many BIOS settings regarding to memory to discuss here and most are beyond the scope of the exam. CompTIA does mention parity, non-parity in the exam objectives. Parity adds an extra bit (odd or even) to the 8-bit data-string to ensure data integrity in memory modules. Its successor, ECC, provides even better ways to ensure the data integrity by adding information about individual bits. Boot sequence This setting is used to determine in which order devices (e.g. CD, floppy or hard disks) the computer should look for a boot sector. Date/Time The Date and Time is set in the BIOS, stored in CMOS, maintained by CMOS battery. Passwords In most cases a user (startup) password and a supervisor (setup) password can be set in the CMOS. When a Setup password is required the computer will prompt for it when you try to access the BIOS setup. When a Startup password is configured the computer will prompt for it at every startup. The CMOS password can be reset by shortening the "CMOS restore to factory defaults jumper" or by temporarily removing the CMOS battery. Plug & Play BIOS Today's BIOSs are PnP-aware. This means they are able to automatically assign resources such as IRQ and DMA to Plug and Play devices. Information about these PnP devices is stored in a separate area of non-volatile CMOS memory, called the ESCD (Extended System Configuration Database). The PnP BIOS and the operating system can both access this area so they can communicate with each other about resource settings assigned to PnP devices and also to non-plug and play devices. For example, when a fixed IRQ is manually assigned to a particular device using Device Manager, Windows will write this information to the ESCD on shutdown preventing the BIOS from assigning the same IRQ to a PnP device at startup. You can also reserve IRQ's for non-plug and play devices in the CMOS setup, this will prevent the BIOS from assigning these reserved resources to PnP devices, a common example is a legacy sound card that needs IRQ 5. Power Management Modern mainboards provide ACPI (Advanced Configuration and Power Management Interface) settings such as wake-up, power button function and standby/suspend timers, these are also configured in the CMOS Setup.