Each time a computer boots up, it goes through an initial series of processes. This sequence of events is aptly named a "boot sequence." During the boot sequence, the computer activates the necessary hardware components and loads the appropriate software so that a user can interact with the machine.
The boot sequence starts by accessing the the computer's BIOS on Windows PCs or the system ROM on a Macintosh. The BIOS and ROM contain basic instructions that tell the computer how to boot up. These instructions are then passed to the computer's CPU, which begins loading information into the system RAM. Once a valid boot disk or startup disk is found, the computer begins loading the operating system into the system memory. After the operating system finishes loading, the computer is ready to be used.
The boot sequence can take anywhere from a few seconds to several minutes, depending on the computer's configuration. If the system is booting from a CD or DVD, the boot time may be significantly longer than if the computer is booted from a hard drive. Also, if your computer was turned off unexpectedly, the boot time might increase since the system may perform some additional checks to make sure everything is OK.