The Boot Process
In computer terms, boot up means to start or get going. In Tutorial 6, we discussed POST (power-on self test). Once POST is complete (the basic hardware is determined to be OK), the computer loads software that tells it how to run. This software is the operating system (OS). The boot sequence for operating systems will vary. Several steps are involved in the MS-DOS system startup process:
- The ROM BIOS looks for an operating system. It checks for the presence of IO.SYS and MSDOS.SYS, first searching the A (floppy disk) drive and then the C drive.
- The operating system processes the CONFIG.SYS file (if present). The CONFIG.SYS contains information to configure the system environment including special memory management overlays and hardware drivers.
- COMMAND.COM is loaded.
- The operating system processes the AUTOEXEC.BAT file (if present). The AUTOEXEC.BAT loads programs and user-defined settings.
- If no programs (such as Windows) have been invoked, COMMAND.COM presents the active-drive prompt and waits for a command.
The process of starting from a no-power condition is called a cold boot. Occasionally, a system might require a reset-for instance, when the computer locks up or runs out of memory. Resetting can be accomplished without turning off the computer by holding down the CTRL, ALT, and DEL keys at the same time. This is called a warm boot.