Hardware Reference
In-Depth Information
yte(MiB). Since then theBIOShasgrownsomewhat insize, butitstill must runas16-bit
x86codeandfitintoameager128-kibibyte(KiB)windowofmemory.Inaddition,add-on
cards that need to be functional at boot time (such as video and RAID cards) must contain
a ROM of limited size with 16-bit x86 code. This is why the POST, BIOS Setup, RAID
Setup, and other preboot code you normally interact with usually runs in text mode and
must be navigated using only the keyboard. All of this looks and feels crude in comparis-
on to the high-res graphical mouse-driven interfaces found in modern OSs.
Another issue is that because the BIOS is so limited in size and functionality (it exists
mainly to test/initialize the hardware, and then find and load an OS), a PC without an OS
installed (or with one that has been corrupted) has little or no functionality besides enter-
ing the BIOS Setup. In other words, you cannot run extensive diagnostics, go online with
a web browser, send or receive email, or read or write files.
But perhaps the most important limitation with the PC BIOS is that it was only designed
to boot from MBR-formatted disks. Because the MBR can only define up to four primary
partitions on a disk of just under 2.2TB in maximum size, BIOS-based systems cannot
boot from a drive that is 2.2TB or greater in capacity. That started to be a problem a few
years ago for drive arrays as it became possible to combine multiple drives in an array
potentially larger than that. But the 2.2-terabyte (TB) limitation started to become a real
problem in 2010 when the first 3TB drives were released to the market.
The solution to the limitations caused by the MBR format is a more flexible partitioning
scheme called the GUID (globally unique identifier) Partition Table (GPT). The GPT is
a newer boot sector and format design that supports up to 128 partitions on a disk of up
to 9.4 zettabytes (ZB, where 1 zettabyte = 1 billion terabytes or 1 sextillion bytes). While
the GPT breaks the 2.2TB barrier, GPT-formatted drives can only be used as data drives
becausethePCBIOSsupportsonlyMBRandcannotbootfromaGPTdrive.Thesolution
totheGPTbootproblemisUEFI,whichreplacestheBIOSbootloaderwithaUEFIload-
er that supports both MBR and GPT-formatted drives, thus breaking the 2.2TB limitation.
Note that UEFI does not completely replace the BIOS, at least not yet. At a minimum, it
runs on top of the BIOS as a replacement for the boot loader and runtime services. UEFI
doesnotspecificallydefinePOSTorBIOSSetupfunctionality,butitdoesallowenhanced
versions of those preboot functions to be created along with additional preboot applica-
tions. Because of this, a motherboard with UEFI firmware may look exactly like a stand-
ard BIOS Setup from the user's perspective, with the only difference being a UEFI Boot
option somewhere in the BIOS Setup. Enabling this option allows the UEFI boot loader
to replace the BIOS boot loader, thus enabling the ability to boot from a drive 2.2TB or
greater in capacity.
