What is Boot? How does it Work?

To boot or boot up means to start your computer system, usually by turning on the power and/or pushing the “on” button. It’s called “booting” because the computer is going inside itself and turning itself on (doing a lot of preliminary checking and adjusting before it’s ready to run your programs). Hence the machine is considered to be “pulling itself up by its own bootstraps.”

 When the computer is first turned on or restarted, it reads the startup instructions found in the ROM BIOS chips. These instructions tell the computer to check the system over (a series of tests called the POST). Certain information (such as the amount of memory and the number and type of disk drives) about the PC is stored in a special chip called CMOS, and that information is also verified during boot. The last thing that happens during boot is the loading of the operating system, which is found on the hard disk drive or on a floppy disk in drive A. The computer cannot do anything without first loading an operating system into memory, because it’s the operating system that manages all of the computer’s basic functions.

Information in the operating system files continues the booting process. During a PC boot, the CONFIG.SYS file is located, and its instructions are executed. The CONFIG.SYS is a special file that fine-tunes the PC, customizing it so it can access optional peripherals (such as the mouse or the modem) and unused areas in memory. Next, the AUTO EXECBAT file is located, and its instructions are executed. The AUTOEXEC BAT file contains commands (such as those to start a particular program or change the prompt) that the user wants run at boot. Once the startup files have been found and executed, the computer is fully booted and ready to go.

How Does it Work

DOS is made up of three parts: IO.SYS, MSDOS.SYS, and COMMAND.COM. IO.SYS works with ROM BIOS to control the computer’s input/output functions; MSDOS.SYS(sometimes called the kernel) manages files, runs programs, and performs basic system functions; COMMAND. COM performs all the DOS commands. In addition,

COMMAND.COM functions as the overall “manager” for the computer. During the boot process, the three parts of the operating system are loaded into memory one at a time. After IO.SYS is loaded, it checks to see if all the system components are responding properly. MSDOS.SYS is loaded next so it can perform the commands in the CONFIG.SYS (a special configuration file). COMMAND.COM is loaded last and is kept in memory so that DOS commands (such as COPY) can be executed when needed. COMMAND.COM then executes the commands in AUTOEXEC.BAT, which completes the boot process.