Upon powering on the machine a POST (Power on Self Test) is executed, this does a quick test on things such as the memory and power supply of a machine. POST transfers control to CMOS (Complementary Metal Oxide Semiconductor) which contains the BIOS (Basic Input Output System). The BIOS loads various low level settings such as boot-order and network settings. The BIOS is also a very useful tool for testing memory or hard drives, as well as finding out basic information such as serial numbers and asset tags. Note if POST fails it is usually identified by various light or audible beep codes which often can be searched for by model on the manufacturers' website.
The BIOS searches attached disk drives and peripherals (such as USB flash drives) for a valid Master Boot Record (MBR). The MBR contains a series of tasks for loading an Operating System (OS). If a valid MBR is found, instructions for loading the OS are read. If no valid MBRs are detected by the BIOS, an error message is displayed.
Instructions from the MBR are read. These instructions check the floppy, CD-ROM, or hard drive (not necessarily in that order). The MBR points to the boot sector, which locates IO.SYS and loads the file into RAM.
IO.SYS is a file that loads low level drivers for hardware devices like the keyboard, serial and parallel ports, floppy drive, and hard drive. Next, IO.SYS will run Sysinit which in turn loads Msdos.sys into RAM.
MSDOS.SYS in turn helps manage the input and output for the hard disk. It processes the commands in Config.sys. Config.sys loads all the other device drivers and manages memory for optimization.
Command.com is loaded into RAM. Command.com is a file that stores all your internal commands.
Command.com then processes Autoexec.bat which will set the initial configuration of DOS. The user can add what programs to start each time the computer boots.