- default MS-DOS device drivers
- DOS initialization program

1. first sector of the boot disk is loaded into memory and executed
2. first 3 sectors of IO.SYS into memory and transfers control to it
3. Loads the rest of itself into memory.
4. Initializes each default device driver in turn (console, disk, serial port)
5. Loads the DOS kernel(MSDOS.SYS) and calls its initialization routine.
6. Processes the MSDOS.SYS, CONFIG.SYS
7. Loads COMMAND.COM