3.8.1 BIOS

Learn about legacy BIOS, CMOS, UEFI, and EEPROM.

BIOS

The legacy firmware that's been mostly replaced by UEFI. UEFI improves software interop and fixes the address limitations of BIOS, and it also provides better security against rootkit attacks (malware that targets the bootloader). UEFI also has faster startup times, support for 2.2 TB storage drives, support for 64-bit firmware device drivers, and backwards compatibility for both BIOS and UEFI hardware. Since most PCs now use UEFI, BIOS has become a term that refers to both legacy BIOS and UEFI.

CMOS

CMOS chips have been mostly replaced by EEPROM chips, which are non-volatile memory used in computers to store small amounts of data. EEPROM replaced EPROM, allowing for bytes to be erased and reprogrammed, so that uses could now update their BIOS without taking apart the PC.

CMOS also stores system date/time and BIOS settings, so that when the system powers down that those settings aren't lost.

ROM

In order to work, PCs have a built in Read-Only Memory chip that houses important functions. Devices attached to the motherboard call these functions the same way they call for data in the RAM. The BIOS is actually the collection of software that's housed on the ROM.

Devices

Devices that are connected to the system can be categorized into three categories:

• Non-configurable
• Configurable
• Custom

Non-configurable

Can be handled by the BIOS alone, since they don't have any parameters that change during runtime (aka doesn't communicate with the CPU) Includes peripheral devices like:

• keyboard
• system
• speaker

Configurable

Devices whose settings can be changed or upgraded from time-to-time. BIOS can't handle these alone, since the quantities, presence, or settings can change from system to system. Things like hard drives that can have different storage capacities would require different BIOS chips since BIOS can't dynamically change any info it needs, so it needs to communicate with the CPU through a second RAM chip called the EEPROM chip.

Custom

Any card that is inserted into the motherboard's PCI expansion slots. Includes the following devices:

• sound cards
• network adapters
• video cards Only problem with custom devices is that there's no feasible way to build a BIOS ROM chip or a CMOS chip that's compatible with all types of devices that you could possibly plug into a PC.

To resolve this, there's something called Option ROM that's installed on the devices themselves. OPROM is run during the OS boot, and it loads up firmware drivers that the motherboard uses to communicate with the driver.

You can also use device drivers. which tell the CPU how to handle and talk to the connected device (e.g. sound card or network board)

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