IRQ loader
C64 loader that pulls data off the disk in the background while music and effects keep running.
An ordinary fastloader takes over the whole machine while it loads: the screen freezes and the music stops. An IRQ loader instead lets music and graphics run off raster interrupts while the main program receives data from the 1541, which runs its own transfer code on its own CPU. The trick is that the transfer has to survive being interrupted halfway through a byte without losing sync.
IRQ loaders are what made the trackmo possible — the demo that moves from part to part without a pause. Popular ones include Krill's loader, Linus Åkesson's Spindle and Bitfire, and C64 sceners debate the choice with the same seriousness other people reserve for programming languages.