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random access memory
central processing unit
operational computer
Steps - By - Step Process Of Booting In Operating Systems
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The boot process is the set of steps required to take the computer from a no-power state to a fully operational computer that is ready for users task.

The below carefully explain the step-by-step in the boot process:

Power On:

Connect the computer to a power supply and presses the “power-on” button to enable the power supply activate the processor. In most cases, the power supply will have its own "kill switch" to prevent power from even reaching the power supply, thus preventing power from reaching the rest of the computer.

Central Processing Unit (CPU) Initialized:

Next, the motherboard of a computer causes the computer's clock to tick. Several ticks of the clock cause the process to begin self-initialization process which includes a quick hardware check of critical processor components. Once the processor assesses itself as "fit", the boot process continues.

Load the Local System ROM:

Alongside, the processor will references the system's Read-Only Memory (ROM) for firmware i.e. written specifically for a specific hardware implementation. The firmware is then loaded into a predetermined memory location. The boot process will fail if memory is absent. Once the firmware loads and verified, the boot process continues.

Run POST (Power-On Self Test):

Next, firmware test the systems Random Access Memory (RAM) fully; the firmware begins to inventory primary computer resources. The computer bus (Pathways of data), input-output (IO) device e.g. video card, mouse and keyboard, the memory components and specialized components use to detect SATA controllers (Not SATA disks), SCSI controllers (not SCSI hard disks), network controllers and USB controllers. All of the mentioned items must fit or be available or POST will fail.

Take Hardware Inventory:

In this step, the firmware continues by completing two specific functions: (1) inventory any devices attached to the specialized components identified in the previous step and (2) run a POST step for these newly inventoried devices. And the specialized controller usually performed the last step.

Determine OS Location:

Subsequently, the firmware now attempts to load an operating system (OS), the firmware of the system will assess whether any user interrupts occurs during the boot process. Most computers allow the user to stop the boot process at this point and enter a firmware configuration mode. From this mode, the user can override defaults for just one boot session or permanently alter the firmware configuration. Also, Read local firmware settings to assess the storage device that house the default boot records. Then it should prompt the user to select a boot device. This selection provides the user an opportunity to override default configurations and is necessary for systems to boot from floppy disks, CD-ROMs, or DVD-ROMs.

Locate boot record:

The Master Boot Record (MBR) has the software instructions required to start the operating system. Each hard disk and partition reserves the first 512 bytes of the hard disk (or logical block address #0) for boot records. The record associated with the disk (not the partitions on the disk) is known as the Master Boot Record. On Windows NT/2000/XP/Vista, this software called NTLDR (as in NT Loader). On Sun Solaris, this function is Sun's Boot PROM ("Programmable Read-Only Memory"). In Linux, two boot loaders exist: LILO (as in LInux LOader) and GRUB (as in GRand Unified Bootloader).

The software in the Master Boot Record (MBR) begins by loading configuration data. Once fully loaded, it boots the default without waiting, then pause for sometime before continuing. This time period could be as short as 1 second or as long as infinity. The resulting time period allows the user an opportunity to select an operating system and boot into a restricted version of an OS. For example, Windows XP sometimes detects the last boot attempts as "failed" and offers a "debug" boot as an alternative OS.

Load the OS:

Finally, the boot loader's load the operating system into memory and turn over control of the CPU to the operating system. The now running OS begins by gathering an inventory of attached devices. This inventory is much more thorough than the one performed formerly by the firmware. This includes all memory, processors, peripheral devices such as printers and scanners, joysticks, touch screens, advanced keyboards, mice, and video cards.

Load the first process:

Concluding stage of the boot process is similar for most OS installations. At this point, the operating system is now loaded and the system is ready to start the first process. A default first process configured when compiled the OS and boot loaders. In several cases, the boot loader will allow the user to override the default first process. The responsibilities of the first process vary widely, but the result is a fully booted operating system.

In Windows, the first process, detects hardware and returns the information to the firmware. Next, the firmware calls ntoskrnl.exe with the information returned.


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