One of the delightful things in programming a 6502-based computer is the relative importance of the OS. In those times, you had complete control over the machine and you called the OS (or the ROM routines) to do whatever you wanted it to.

I remember that when I had to do floating-point math or calculate a screen address (something convoluted on an Apple II) I would simply call a subroutine in ROM and pick up the results.