4.2 Disk Operating System (DOS)
4.2.5 Booting the system with a DOS disk
A DOS boot disk is used to boot a computer to the DOS Prompt. On the very first section of a DOS disk is a section called the boot sector. The boot sector contains information about how the disk is organized. Sometimes, it contains a small optional master boot record that can access a larger, more powerful bootstrap loader program, which is located in the root directory.

In most cases, the master boot record is found at sector-1, head-0, and track-0 of the first logical hard drive (or in this case, the boot disk). The Master Boot Record (MBR) is the required boot record on any disks that are created as boot or system disk. If a boot disk possesses the master boot record, it can boot up the hardware system to the operating system. A disk that can boot a system to the OS is called a bootable disk, or simply, a system disk. It must contain the three system files mentioned in the previous section. Having a boot disk will greatly facilitate the process of preparing a hard drive and installing the OS.

Booting the System
Insert a bootable disk in the floppy disk drive, and turn on the computer. The BIOS will execute the bootstrap program (small BIOS program that initiates and controls much of the boot up routine) to move the master boot record into RAM and then begin the process of loading the operating system. Typically, if the system performs a standard DOS boot up it should print the Date and Time prompts on the monitor screen, followed by the DOS command line prompt (A:\), which indicates that DOS is operational and that the A: floppy drive (boot disk) is the currently active drive.

The figure summarizes the chain of events that culminates in the display of the DOS command line when a system boots with a DOS boot disk.