This is how MS-DOS boots
on a normal PC
after it is initialized
- The BIOS performs an INT19 interrupt to search the drives for a Master Boot Record.
- The Primary Bootstrap Loader moves the Master Boot Record into memory.
- The system executes the Secondary Bootstrap Loader from the Master Boot Record.
- The Secondary Bootstrap Loader moves IO.SYS and MSDOS.SYS into memory.
- IO.SYSlooks for the CONFIG.SYS file in the root directory.
- If the CONFIG.SYS is found, IO.SYS uses it to customize the environment in three read sequences (Devices, Install and Shell).
- IO.SYS loads the COMMAND.COM.
- The COMMAND.COM looks for the AUTOEXEC.BAT in the root directory.
- If the AUTOEXEC.BAT is found, COMMAND.COM carries out the script and executes the commands.
- If there is no AUTOEXEC.BAT, then the COMMAND.COM displays the current time and date, then stops with the command prompt.
The system is now ready to use.