The Amiga had a large part of its regular OS libraries in ROM, and the base OS was available directly to the bootloader on the floppy. You could ignore it and

The Amiga had a large part of its regular OS libraries in ROM, and the base OS was available directly to the bootloader on the floppy. You could ignore it and access the hardware directly, if you wanted, and many games did, but you could also use the OS if you wanted to. The default bootloader would basically just say "launch the default shell and tell it to load the startup shell script file".