Data Storage - Boot Sector Startup Processes

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Initially, the startup process is independent of disk format and operating system. The unique characteristics of operating and file systems become important when the boot sectors executable boot code starts.

Windows Disk

Two sectors are critical to start a computer on Windows:

These sectors contain both executable code and the data required to run the code. Computers use this boot sectors to run instructions during startup.

The initial startup process is summarized in the following steps:

  • The system BIOS and the CPU initiate the power-on self test (POST).
  • The BIOS searches for a boot device (typically a disk).
  • The BIOS loads the first physical sector of the boot device into memory and transfers CPU execution to that memory address.

On different Devices

On Hard Disk

If the boot device is on a hard disk, the BIOS loads the MBR. The master boot code in the MBR loads the boot sector of the active partition, and transfers CPU execution to that memory address. On computers that are running Windows 2000, the executable boot code in the boot sector finds NT Loader, loads it into memory, and transfers execution to that file.

Windows 2000 cannot start up from a spanned, striped, or RAID-5 volume running dynamic disk. These disk structures cannot be registered into the MBRs partition table, so a system partition using these structures is not startable. Windows 2000 must be fully loaded into memory before they can be used.

On Floppy Disk

If there is a floppy disk in drive A, the system BIOS loads the first sector (the boot sector) of the disk into memory. If the disk is startable formatted by MS-DOS with core operating system files applied the boot sector loads into memory and uses the executable boot code to transfer CPU execution to Io.sys, a core MS-DOS operating system file. If the floppy disk is not bootable, the executable boot code displays an error message such as:

Non-System disk or disk error
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This error will not appear on normally functioning systems that are configured to look for the startup files on drive C first. On many computers, an option in the CMOS setup program allows the user to set the sequence of installed disks that the system searches for the startup files.

If you get similar errors when trying to start the computer from the hard disk, the boot sector might be corrupted.

Boot and system partition

The system partition is defined as the partition containing the files needed for the initial system startup. For Windows NT, the files are, NTLDR, Boot.ini, and sometimes Ntbootdd.sys.

A boot partition is defined as the partition containing the system files. For Windows NT, this is the partition containing the %SystemRoot%\System32 folder.

The system partition and boot partition can be on the same partition or on different partitions. Because there can be multiple operating systems installed on a single computer, a computer can have multiple boot partitions, but a computer has only a single system partition.

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