What happens when you turn on your computer?

When you first turn your computer on, the only thing it's capable of doing is finding the BIOS ROM (Read Only Memory) chip on your mainboard. This BIOS chip has a program burned onto it that knows where to look for, and how to access, the different expansion slots, ports, drives, and the Operating System.
The computer begins the start up process (or boot sequence). It loads the programming code and instructions on the BIOS chip into memory and then carries out the instructions in order. It takes an internal and external inventory of equipment and performs several self-tests. It looks for the video card, loads its information and driver into RAM, then looks for any other cards in expansion slots and loads their configuration into memory. The BIOS program then does a simple memory check and count.
The routine then compares the information its gathered with the information stored in the CMOS chip's setup program. If there's any discrepancies, it halts the boot process and informs the operator.
Now the computer has to locate the OS. It first looks on the floppy drive. If there is a disk in the drive, it must have a copy of the OS system files on it. If it does, the operating system is loaded into RAM (Random Access Memory). If the disk doesn't contain any operating files, then the computer halts and gives an error message. You then have to remove the disk and press a key to continue.
If there is no disk in the floppy drive, then the computer checks the hard drive for the Operating System files and, once found, continues to load the OS into memory (RAM).

DOS Boot-up Sequence
IO.SYS: A hidden file in the root directory of the primary drive. This file provides the basic I/O capabilities for the system, allowing it the ability to communicate with the different peripherals. IO.SYS directs the overall process of loading the Operating System.
MSDOS.SYS: Also a hidden file in the root directory sometimes called the kernal for DOS. When an application needs to access a device or peripheral, this file translates the request into actions that IO.SYS can perform.
CONFIG.SYS: A user-configurable text file that usually contains device drivers and system setup values.
COMMAND.COM: This is the Command Interpreter. It can accept commands from the user, launch programs and pass this Information to MSDOS.SYS.
AUTOEXEC.BAT: Another user-configurable text file that is used to set system variables and load TSRs.

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