The USR filetype on the Commodore VC 1541 disk drive is a less common sequential file that according to some book sources can be executed as a program directly on the drive. The drive has its own 6502 processor and 2K of RAM. While there exist a high number of program examples and demos for the C64, I could not find a single listing of a USR program. Would it theoretically be possible to provide a number of disk-executed commands as USR files, that for example to reverse a file on disk, compress a file, etc.?
The 2K RAM with even less available is meagre, but could a program theoretically use disk space as external memory? Running locally at the drive a program could benefit from the native floppy speed, avoiding the serial protocol bottleneck.
I know of C64 programs using a VC 1541 as a co-processor for fractal or 3D computations. I'm also aware of the programs that play music using the stepper motor, but as far as I remember those programs copied the code from the C64 over the serial program into the floppy memory.
I would be interested, however, in the USR programs. Does there exist a list of meaningful USR programs or a tutorial on how to program them?