Well, that is not entirely true. The C64, like the Apple 2 line had a very limited OS in ROM. It did have BASIC and some rudimentary commands to enable some peripherals.
It did not have a disk operating system in ROM. For the disk OS your needed to boot up a peripheral (a floppy drive, typically) that had a bootstrap program to then load the disk OS addendums.
In the Apple 2 line, you typically would type "PR#6" (for slot 6) to start up the disk controller in that slot, where it usually was.
In the C64 I believe you had to type "load *,8,1" (don?t blast me if I got that slightly wrong, I didn?t have a C64).
The point is, to get the computer to use some program that was on an external peripheral, you needed to do another boot. I did like the occasional ?flip the switch? and the built in OS with BASIC was there, and I was able to do some things. But if I wanted to then save some program I just typed in I would then have to save it to cassette tape (because those tape routines were in ROM) boot to start up a disk drive, bring the saved program off of tape, then save it to floppy.
OR make sure that I boot up the floppy at turn on so that the DOS would be there, if needed.
I had the (Commodore) Amiga 1000 (a truly great computer, and a great multi-tasking OS), it did not have any usable OS in ROM at all. All it had was a ?bootstrap? program whose only purpose was to start up the floppy to then read in the OS.
That limitation was ?remedied? in the later Amiga?s, like the 500 and the 2000 and 3000 that followed.
I made modifications to my Amiga 1000 to add in third-party ROMs into the case of the Amiga that was the OS, but even with that I still had to boot to a connected hard drive to load in the Disk Operating System. I even added additional memory INSIDE my Amiga 1000 case to then have 1.75 MB of memory (much more than the standard 256K or if you had the expansion extra 256K to then have 512K).
But with the added third-part ROM, the extra memory, and the hard drive, the boot was just a few seconds and I just loved that machine.