CMOS

The system’s configuration options, to store the system’s setup parameters in a small, low-power RAM IC called the CMOS RAM. CMOS RAM is typically combined on the same IC with the real-time clock.
Computer starts, its system attributes—stored in the CMOS RAM—are read by the BIOS. BIOS then uses those attributes during normal system operation. It is vitally important that the correct settings be used when configuring a system.

BIOS and CMOS RAM are not the same thing, although the two are intimately related. BIOS refers to the firmware instructions located on the BIOS ROM, and CMOS refers to the low-power RAM that is holding the system’s setup parameters. BIOS reads the CMOS RAM into memory at start time, and provides the “setup” routine that allows you to change the contents of CMOS, but the CMOS RAM/RTC device is a totally different IC.
ENTERING CMOS SETUP
The configuring of CMOS setup is to launch the setup utility. BIOS manufacturers are rarely consistent when it comes to accessing the setup utility. we launch Setup in the first few moments after the system boots—a note on the display will usually indicate the correct key or key combination such as:
Press <F1> to enter Setup...
BIOS/SYSTEM                                KEY OR KEY SEQUENCE
AMI BIOS                                       <Del> key during the POST
Award BIOS                                    <Ctrl>+<Alt>+<Esc>
DTK BIOS                                      <Esc> key during the POST
IBM PS/2 BIOS                              <Ctrl>+<Alt>+<Ins> after <Ctrl>+<Alt>+<Del>
Phoenix BIOS                                  <Ctrl>+<Alt>+<Esc> or <Ctrl>+<Alt>+<S>
ALR PC                                          <F2> (for PCI systems)
                                                        or <Ctrl>+<Alt>+<Esc> (for non-PCI systems)
Compaq PCs                                   <F10>
Gateway 2000 PC                           <F1>
Sony PC                                          <F3> while the PC is starting (you see the Sony logo), then hit <F1>


THE CMOS MAP
standard 128-byte ISAcompatible CMOS is divided into four fairly distinct sections: 16 bytes of real-time clock data (00h–0Fh), 32 bytes of ISA configuration data (10h–2Fh), 16 bytes of BIOS-specific configuration data (30h–3Fh), and 64 bytes of extended CMOS data (40h–7Fh). Additional CMOS
RAM is typically used as the ESCD.

BIOS ERROR MESSAGES

• BIOS ROM checksum error—system halted The checksum of the BIOS code in the BIOS chip is incorrect—this is a fatal problem, indicating the BIOS code might have become corrupt. They will need to replace the motherboard BIOS before the system will initialize.

• CMOS battery failed The CMOS battery is no longer functional. Need to replace the CMOS battery.

• DMA bus time-out A device has driven the bus signal for more than 7.8 microseconds. This might be a random fault, but chances are that a device in the PC has failed. Try removing expansion devices first. Otherwise, replace the motherboard.

• Cache memory bad, do not enable cache POST has determined that your cache memory is defective. Do not attempt to enable the cache in your system.

• DMA  error A serious fault has occurred in the DMA-controller system of your motherboard. In virtually all cases, the motherboard will have to be replaced.

• INTR #1 (or INTR #2) error A serious fault has occurred with your interrupt controller on the motherboard. In virtually all cases, the motherboard will have to be replaced entirely.

• Primary input device not found: The primary input device, such as the keyboard or mouse or other device if input is redirected, could not be found. Check the installation and configuration of all your input devices. Be sure that the input devices are also enabled in CMOS setup.

• PCI IRQ conflict: Two devices requested the same IRQ, resulting in a conflict. Try freeing the IRQs needed to allow both devices to be configured properly.

BIOS Features

PC is constantly advancing in CPU’s, chipsets, memory, video, drives, etc. The key features that are included in a BIOS.

• CPU support BIOS should support a rich range of CPUs, preferably from such CPU makers as Intel and, AMD.
• Chipset support The BIOS should support the latest chipset families. Chipset support is crucial because it allows motherboard designers to implement other features, such as USB and advanced memory.
• Memory support The BIOS should be able to auto-size and support the most modern forms of memory. Memory error checking (parity and ECC) should also be supported.
• Drive support The BIOS must support large SATA/EIDE/Ultra-ATA hard drives  with very fast data-transfer modes, such as PIO Modes and Ultra DMA.
• ACPI/APM support The BIOS should be fully compliant with the Advanced Configuration and Power Interface (ACPI) specification , and supports APM BIOS specifications through version 1.2. Power management is important for mobile systems, and is widely used in desktop/tower systems to reduce energy waste.
• Boot versatility support The BIOS should be able to boot from a number of different drives, and include the BIOS Boot Specification for Initial Program Load (IPL) devices. This currently supports booting from up to four IDE/EIDE/SATA drives (including CD-ROM drives), SCSI drives, USB and network cards.
• Plug-and-Play support The BIOS must detect and configure PnP devices during POST.