How the BIOS boots

How the BIOS boots
The BIOS runs off the PROM, EPROM or, most commonly, flash memory when the computer is powered on. It initializes several motherboard components and peripherals, including:
• the clock generator,
• the processors and caches,
• the chipset (memory controller and I/O controller),
• the system memory,
• all PCI devices (by assigning bus numbers and resources),
• the primary graphics controller,
• mass storage controllers (such as SATA and IDE controllers),
• various I/O controllers (such keyboard/mouse and USB).
Finally, it loads the boot loader for the operating system, and transfers control to it. The entire process is known as Power-on self-test (POST). On the original IBM PC, the hardware only needed minimal configuration and POST was indeed used for testing; on modern systems, most of POST actually consists of hardware configuration.
Once system memory is initialized, the BIOS typically copies/decompresses itself into that memory and keeps executing from it.
Nearly all BIOS implementations can optionally execute a setup program interfacing the nonvolatile BIOS memory (CMOS). This memory holds user-customizable configuration data (time, date, hard drive details, etc.) accessed by BIOS code. The 80x86 source code for early PC and AT BIOS was included with the IBM Technical Reference Manual.
In most modern BIOS implementations, users select which device boots first: CD, hard disk, floppy disk, flash keydrive and the like. This is particularly useful for installing operating systems or booting to Live CDs, and for selecting the order of testing for the presence of bootable media.
Some BIOS's allow the user to select the operating system to load (e.g. load another OS from the second hard disk), though this is more often handled by a second-stage boot loader.
[edit] The BIOS Chip and BIOS Recovery


ROM with BIOS
Before 1990 or so BIOSes were held on ROM chips that could not be altered. As its complexity and need for updates grew, BIOS firmware was subsequently stored on EEPROM or flash memory devices. The first flash chips attached to the ISA bus. Starting in 1998, the BIOS flash moved to the LPC bus, a functional replacement for ISA, following a new standard implementation known as "firmware hub" (FWH). In 2006, the first systems supporting a Serial Peripheral Interface (SPI) appeared, and the BIOS flash moved again.
EEPROM chips are advantageous because they can easily be updated by the user; hardware manufacturers frequently issue BIOS updates to upgrade their products, improve compatibility and remove bugs. However, the risk is that an improperly executed or aborted BIOS update can render the computer or device unusable. To recover from BIOS corruption, some new motherboards have a backup BIOS (i.e. they are referred to as "Dual BIOS" boards, Gigabyte even offers a motherboard with quad BIOS). Also, most BIOSes have a "boot block" which is a portion of the ROM that runs first and is not updateable. This code will verify that the rest of the BIOS is intact (via checksum, hash, etc.) before transferring control to it. If the boot block detects that the main BIOS is corrupted, then it will typically initiate a recovery process, by booting to a removable device (floppy, CD or USB memory) so that the user can try flashing again.
Due to the limitation on the number of times that flash memory can be flashed, a flash-based BIOS is vulnerable to "flash-burn" viruses that repeatedly write to the flash, permanently corrupting the chip. Such attacks can be prevented by some form of write-protection, the ultimate protection being the replacement of the flash memory with a true ROM.
[edit] Firmware on adapter cards
A computer system can contain several BIOS firmware chips. The motherboard BIOS typically contains code to access fundamental hardware components such as the keyboard, floppy drives, ATA (IDE) hard disk controllers, USB human interface devices, and storage devices. In addition, plug-in adapter cards such as SCSI, RAID, Network interface cards, and video boards often include their own BIOS, complementing or replacing the system BIOS code for the given component.
In some devices that can be used by add-in adapters and actually directly integrated on the motherboard, the add-in ROM may also be stored as separate code on the main BIOS flash chip. It may then be possible to upgrade this "add-in" BIOS (sometimes called an option ROM) separately from the main BIOS code.
Add-in cards usually only require such an add-in BIOS if they:
• Need to be used prior to the time that the operating system loads (e.g. they may be used as part of the process which loads (bootstraps) the operating system), and:
• Are not sufficiently simple, or generic in operation to be handled by the main BIOS directly
Older operating systems such as DOS, as well as bootloaders, may continue to make use of the BIOS to handle input and output. However, most modern operating systems will interact with hardware devices directly by using their own device drivers to directly access the hardware. Occasionally these add-in BIOSs are still called by modern operating systems, in order to carry out specific tasks such as preliminary device initialization.
To find these memory mapped expansion ROMs during boot, PC BIOS implementations scan real memory from 0xC8000 to 0xF0000 on 2 kibibyte boundaries looking for a 0x55 0xaa signature, which is immediately followed by a byte indicating the number of 512 byte blocks the expansion ROM occupies in real memory. The BIOS then jumps to the offset immediately after the size byte, at which point the expansion ROM code takes over and uses BIOS services to provide a user configuration interface, register interrupt vectors for use by post-boot applications, or display diagnostic information.
For UNIX and Windows/DOS systems there is a utility with which BIOS firmware software can be dumped at http://www.linuks.mine.nu/ree/
There is a tool to flash the BIOS from Linux at http://packages.debian.org/flashrom
[edit] The BIOS boot specification
If the expansion ROM wishes to change the way the system boots (such as from a network device or a SCSI adapter for which the BIOS has no driver code), it can use the BIOS Boot Specification (BBS) API to register its ability to do so. Once the expansion ROMs have registered using the BBS APIs, the user can select among the available boot options from within the BIOS's user interface. This is why most BBS compliant PC BIOS implementations will not allow the user to enter the BIOS's user interface until the expansion ROMs have finished executing and registering themselves with the BBS API.
[edit] The Rise and Fall of the BIOS
Older operating systems such as DOS relied on the BIOS to carry out most input-output tasks within the PC. A variety of technical reasons eventually made it inefficient—especially for more recent operating systems written for the Intel 80386 such as Linux and Microsoft Windows—to invoke the BIOS directly. Such operating systems instead used their own better-performing native drivers and were also much easier to extend to support new hardware. As such, the BIOS was mostly relegated to bootstrapping to the point where the operating system's own drivers could take control of the hardware.
There was a similar transition for the Apple Macintosh, where the system software originally relied heavily on the ToolBox—a set of drivers and other useful routines stored in ROM—but later discarded this in favour of software drivers.
In recent years, however, by way of systems such as ACPI, the BIOS has taken on more complex functions such as power management, hot swapping and thermal management. This has led to renewed reliance on the BIOS by operating system producers, and an increase in complexity of BIOS code. This in turn led Intel to develop Extensible Firmware Interface (EFI), a specification which defines the firmware which replaces the functionality of the legacy BIOS. EFI is now driven by The Unified EFI Forum, an industry Special Interest Group (SIG). Linux has supported EFI via elilo boot loader since early 2000. Microsoft announced that support for EFI in Windows Vista is only available for the 64-bit versions starting with SP1, and for Windows Server 2008.
The Open Source community increased their effort to develop a replacement for proprietary BIOSes and their future incarnations with an open sourced counterpart through the LinuxBIOS and OpenBIOS/Open Firmware projects. So far, those projects have met with some success, with AMD providing product specifications for a number of fairly recent chipsets, and Google sponsoring the project. Motherboard manufacturer Tyan offers LinuxBIOS next to the standard BIOS with their Opteron line of motherboards. MSI and Gigabyte have followed suit with the MSI K9ND MS-9282 and MSI K9SD MS-9185 resp. the M57SLI-S4 modems.


What is the difference between CMOS & BIOS

CMOS is a type of memory chip that stores the BIOS information. The terms often are used interchangeably.

CMOS stands for Complementary Metal Oxide Semiconductor and is pronounced "sea moss." It describes the material out of which the chip is made. A CMOS chip will store information as long as it receives power that is usually supplied by a battery.

The BIOS is the Basic Input Output System. On startup, the BIOS tests the system and prepares the computer for operation by searching for system components and configuring memory to access the system hardware. It then loads the operating system and passes control to it.

The BIOS can be configured with information about system components in the Setup screen that can be accessed during bootup. Usually the F1, F2, DEL keys are used to access the BIOS Setup. Because the BIOS is stored on a CMOS chip the Setup is also referred to the CMOS Setup.