A can only have a value between 0 and 255. If I multiply too large a value by 3, I overflow 255, so 256/3 = 85 jumps are all the method I presented can cope with - this is usually not much of a problem.

As for playing around with the PC, in what context do you want to alter it - give us an example and I'm sure one of us will give you some guidance here. Esko's code is one way of indirectly manipulating the PC.