Each time you turn on your computer, it will check on the control file

In the PC bootup sequence, the first sector of the boot disk is loaded into memory and executed. If this is the DOS boot sector, it loads the first three sectors of IO.SYS into memory and transfers control to it. IO.SYS then: loads the rest of itself into memory-initializes each default device driver - loads the DOS kernel and calls its initialization routine. The kernel is stored in MSDOS.SYS- processes the MSDOS.SYS- processes the CONFIG.SYS file- loads COMMAND.COM- displays the bootsplash in Windows.