"BIOS and booting process"

"BIOS and booting process"

Abstract

BIOS is a set of small programs that are designed to operate major pc subsytems

Major PC subs/ms are video, disk, keyboard and so on. Each of these s/m services are invoked by a set of standard calls that are made as needed through OS

When the OS request a standard BIOS service, the particular BIOS program performs the appropriate fns that has been tailored to the particular hardware

Each mother board needs its own BIOS. BIOS resides on the mother board in the form of a read only memory.In addition to the system services, BIOS executes a POST(Power on self Test) program each time the PC is initialized 

  Booting Process 

PC initialization starts when you turn on the s/m. When all output voltages from the power supply are valid, the supply generates a Power Good(PG) logic signal.

When the mother board timer receives the PG signal, at this point CPU starts processing

The very first operation performed by a CPU is to fetch an instruction from address FFFF:000h, since this address is at the end of available ROM space, the instruction is almost always a jump command(JMP) followed by the actual BIOS ROM starting address

If there is no issues with the system, the system now needs to load the Operating system

First step here is to have the BIOS search for boot device in the disk. If there is no disk in the drive, the BIOS will search the next drive in the boot order.may be a hard disk.

BIOS loads  sector 1 from the hard drive' s  master partiton boot sector in to memory. And start the execution of the OS

Master boot record 

The master boot record is information that is normally stored in the first sector of the hard drive.

MBR contains a small data structure that identifies where an operating system is located in the drive so that the OS can be loaded in to the system's memory

The MBR contains two elements . Executable code and a partition table which identifes each partition residing on the hard drive

The MBR executable code begins the boot process by looking up the partition table to determine what partition holds the operating system

It then loads the boot sector of the partition containing the OS in to RAM, and transfers execution of the program  to the partition boot sector

The partition boot sector then finishes loading the os files in to RAM 

Partitioning 

Partitioning  the drive is the process of dividing the physical drive in to pieces(called partitions or logical volumes)

Partitioning is important because partition size and type will have important impact on both performance and disk space efficiency.

Formatting

Even after partitioning an OS cannot store files on a drive

A series of datastructures must be written to the drive

A volume boot sector, two copies of the File Allocation table and a root directory are written to each logical partition

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