On the Commodore C128 it was possible to auto-boot a floppy disk that's in the drive when the system is powered up. This does NOT work on the C64.
This only works if Track 1 Sector 0 is allocated and contains a specific signature.
Boot sector format
|00||Signature||Auto-boot Signature "CBM"|
|03||Additional sectors||Track/Sector address or 0x0000 for none|
|05||Bank||Bank for additional sectors, default 0x00|
|06||Load count||Number of sectors to load, default 0x00|
|07||Boot message||0x00 terminated string, can be just 0x00 for "BOOTING..."|
|08+b||Program name||0x00 terminated string, can be just 0x00 for none|
|09+b+p||Boot Loader||Boot Loader code, padded with 0x00|
This is from lemon64.com which has an example of a boot sector in DASM.
In this instance it autoboot's CPM:
This is from C128BOOT.txt which seems to be different to the above.
The code for the above loader is: