Boot Up
A 'boot up' of a computer is the process of loading an operating system. The computer's virtual memory and random access memory (RAM) plays an important role in the boot up of a system. The process of boot up of an operating system involves the complete loading of the initialization files present in the primary partition. A failure in loading the.ini files of a system would result in a reboot.
For the booting of an operating system performed through a boot disk; most often the internal hard drive is used. The boot disk comprises of the boot.ini, NTLDR and Ntdetect files that take control of the loading of the operating system.
Booting Procedure
NOTE: Mac, UNIX, OS/2, and other operating systems follow different booting procedures. When a computer is switched on, the operating system is sequentially booted into the physical memory), the sequence is as follows:
- The moment the circuits of a computer are charged, the basic input-output system (BIOS) present on the read-only memory (ROM) chip is seizes control. The BIOS is in-built on the ROM chip and does not require a self-boot.
- To begin with, the BIOS conducts a POST (power-on-self-test) to ensure the components of a computer are in a working state. If the test is successful, the boot program of the BIOS searches for the boot disk that performs the definitive booting of the operating system on the hard disk.
- The search is carried out on the floppy drive. If there is no diskette in the floppy drive, the BIOS forced to look up its location on the hard drive.
- Once the BIOS identifies the location of the boot files, BIOS scans the first sector (a 512-byte area) of the location and transfers information from it into specific locations in the RAM. This data is known as the boot record or Master Boot Record. The boot record is then temporarily loaded into the hexadecimal address 7C00 of the RAM.
- The BIOS now bifurcates to a program present in the boot record. Thus, it gives boot records control over the computer. Boot records load the initial system file for instance, into the RAM from the hard disk or diskette for instance IO.SYS for DOS systems.
- The initial file then loads the remainder operating system into RAM. The boot record at this stage is no longer needed and therefore one can be overlay it with other data. (For instance, IO.SYS that has a program called SYSINIT)
- The initial file boots up a critical system file, which knows how to work with the BIOS. For example, SYSINIT loads MSDOS.SYS. System configuration file is the initial operating system file that is loaded. (for DOS, it's called CONFIG.SYS). The data present in the configuration file notifies the loading program about the specific OS files that needs to be loaded (for instance, specific device driver)
- Another special file that notifies the specific applications or commands the user wishes to include or perform, as part of the boot process is called WIN.INI. In DOS the file is renamed AUTOEXEC.BAT.
- The operating system is given total control over the computer when all operating system files have been loaded. It thereafter performs the requested initial commands and waits for the user input.
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