It was included in the BIOS of the original IBM PC and most other early non-clone IBM PC models and was started whenever the computer did not find a bootable floppy disk at power on.
It was included in the BIOS ROM of the original IBM PC. Cassette BASIC provided the default user interface if there was no floppy disk drive installed, or if the boot code did not find a bootable floppy disk at power on.