This is how MS-DOS boots on a normal PC-compatible machine after it is initialized:

  1. The BIOS performs an INT19 interrupt to search the drives for a Master Boot Record.
  2. The Primary Bootstrap Loader moves the Master Boot Record into memory.
  3. The system executes the Secondary Bootstrap Loader from the Master Boot Record.
  4. The Secondary Bootstrap Loader moves IO.SYS and MSDOS.SYS into memory.
  5. IO.SYSlooks for the CONFIG.SYS file in the root directory.
  6. If the CONFIG.SYS is found, IO.SYS uses it to customize the environment in three read sequences (Devices, Install and Shell).
  7. IO.SYS loads the COMMAND.COM.
  8. The COMMAND.COM looks for the AUTOEXEC.BAT in the root directory.
  9. If the AUTOEXEC.BAT is found, COMMAND.COM carries out the script and executes the commands.
  10. 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.