This is a lightly modified version of the 6502 "Hello World" code from Ben Eater's video series.
This version uses ca65 instead of vasm, as ca65 is available in the cc65 Debian package.
I also included the NMI and IRQ vectors to fully flesh out the 6502 vectors
table. This feels more correct, at least because now the vector table starts
at $fffa (its actual starting location) rather than $fffc (the location of
the reset vector entry).
The 6522 I/O port controller is referenced using symbolic names (e.g. io_b)
rather than address literals (e.g. $6000).