How Operating System is loaded | Booting Sequence | Computer Boot Process | Easy Learning IT

How Operating System is loaded | Booting Sequence | Computer Boot Process | Easy Learning IT Classroom

Objectives;
• Booting Sequence
• How Operating System is loaded

The boot sequence is the order in which a computer searches for nonvolatile data storage devices containing program code to load the operating system (OS). Typically, a Macintosh structure uses ROM and Windows uses BIOS to start the boot sequence.
The address of the first instruction is fixed
It is stored in read-only-memory (ROM)

Booting Sequence Procedure:
• Program counter PC is loaded with the address of first instruction.
• Instruction register IR fetches the address and PC is incremented e.g.
• On i386 machines, ROM stores a Basic Input/Output System (BIOS)
• BIOS contains information on how to access storage devices
• Performs Power-On Self Test (POST)
• Checks memory and devices for their presence and correct operations.
• During this time you will hear memory counting, which consists of noises from floppy and hard disks followed by a final beep.
After POST;
• The master boot record (MBR) is loaded from the boot device (configured in BIOS)
• The MBR is stored at the first logical sector of the boot device (e.g. a hard drive) that
• Fits into a single 512-byte disk sector (boot sector)
• Describes the physical layout of the disk (e.g., number of tracks)
• Master boot record (MBR) functions:
• Identifies physical layout of the drive
• Identifies where and how Operating system is located so that it can be boot (loaded) into computers main storage or RAM.
After getting the Info. On the boot device;
• BIOS loads a more sophisticated loader from other sectors on a disk
• The more sophisticated loader loads the operating system
Operating System Loaders;
• In case of LINUX, MBR contains (LILO) Linux loader.
• Under Linux, this sophisticated loader is called LILO (Linux Loader).

Booting Sequence Procedure in Brief:
• A CPU jumps to a fixed address in ROM,
• Loads the BIOS,
• Performs POST,
• Loads MBR from the boot device,
• Loads an OS loader,
• Loads the kernel image,
• Sets the kernel mode, and
• Jumps to the OS entry point.

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