The sys command actually jumps to anything in the 6510 address space, be it RAM, ROM or an IO device.. :-) Not that it's useful to jump to an IO device.

The C-64's CPU had two special memory locations, 0 and 1. Toggling these mem locations changes your memory map, for example makes your IO, ROM etc disappear so you can use the whole 64kB of memory for what ever you like. No prebuilt kernal routines, though.. You've got to build all the lowlevel stuff yourself after that.