The way a computer boots from a hard disk depends on the way that hard disk is partitioned and the way that operating system is being booted. When you turn on your computer, the central processing unit (CPU) takes control. It immediately executes the instructions built into the computer’s ROM BIOS, a program which contains the startup procedures. The last part of the BIOS instructions contains the boot routine. This routine is programmed to read the master boot record (MBR) from the first sector of the first physical hard disk. The MBR contains a master boot program and a partition table which describes all of the hard disk’s partitions. The BIOS boot routine executes the master boot program, which then continues the boot process. The master boot program looks over the partition table to see which primary partition is active. If there is only one primary partition, then the partition’s OS is loaded and booted into operation.