Hardware Boot Sequence

Vocabulary BIOS = Basic Input Output System UEFI = Unified Extensible Firmware Interface POST= Power On Self Test BR = Boot Record (aka MBR) BC = Boot Code (aka MBC) GUID = Globally Unique Identifier GPT = GUID Partition Table

BIOS  Instructions used to start the computer from a cold start. (power off to power on).  The BIOS instructions are written on non-volatile RAM.  EEP-ROM is the common media choice for the BIOS, installed on the motherboard.  The BIOS instructions are based on the chip-set installed on the motherboard

BIOS  The BIOS primary functions are  Issue the instructions for starting the hardware at boot time.  Load the OS from the boot device.

Hardware Boot Sequence  Turn on the power switch  All memory and cache is empty at startup.  A reset signal is generated by the chipset to the CPU until the power is ready.  The CPU powers up and reads address xFFFF0 from the ROM. This contains a jump instruction to the start of the BIOS instructions.

POST  The POST (Power On Self Test)  Provide power to the motherboard-speaker  This allows error-code signals to be broadcast by the mobo-speaker when errors are detected  Test if components on the motherboard are receiving power and functioning as intended.

Hardware Boot Sequence  Video and keyboard are activated early in the boot sequence. (video start instruction in ROM address is xC000)  Other devices are activated like the disk drives, optical drives, sound-card, NIC, USB bus, etc.  BIOS setup becomes available for the user.

BIOS: User Interface There is a curses based user interface that allows: a)Selection of a boot device b)Setting the system clock c)Configuring hardware (includes enabling or disabling some devices) d)Setting passwords for booting, & the UI, etc. e)Also shows access to some system config information: memory size, disk size, etc.

BIOS Completion  Memory count  Search for the OS Boot device  Optical drive  USB device  Network device  HDD

HDD Boot Sequence  If the HDD is the boot device then,...

HDD Boot Sequence  The BR is read from the first cylinder, first track, first sector.  The BR contains the BC (boot code) and a partition table.

HDD Boot Sequence  The boot code will do one of two things when executed: 1.Load a boot loader (GRUB, LILO, MS-BM) 1.Allows one to choose from different OSes or different versions of the same OS. 2.Load the OS if no boot loader is available.

Partition Table  The PT contains information about 4 primary partitions.  Only one of the 4 primary partitions is designated as the active partition  The active partition contains the OS for booting.  One of the 4 primary partitions can be designated as the extended partition.  The extended partition can be divided into additional logical partitions.

BIOS/UEFI In the beginning there was the BIOS Intel creates the Extensible Firmware Interface in 1998 UEFI now supersedes EFI – UEFI can run on-top-of the traditional BIOS or in place of the BIOS.

BIOS/UEFI BIOS is often used to describe UEFI Modern OSes allow backward compatibility between the traditional BIOS boot record and the UEFI GPT.

GPT Globally Unique Identifier – This is required to allow a file system to exceed 2TB. The 2TB limit is a result of limitations of the original BR partition table (Each partition has only16 bytes of storage in the partition table.)