[stella] Very nasty 6502 tricks, don't use them! :-)

Subject: [stella] Very nasty 6502 tricks, don't use them! :-)
From: Manuel Polik <manuel.polik@xxxxxxxxxxx>
Date: Fri, 05 Jan 2001 15:54:21 +0100
And yet another post.

I want to describe two very nasty 6502 programming tricks, if you
promise to never ever use them and if so, that you don't claim that I
told you.

Ok, here's one I found by myself, it's utilized a lot in the Alpha demo.

Let's say you're in Subroutine1 which would end like this:

JSR Subroutine2

If you are totally aware what you are doing, you might consider to save
a Byte and some Cycles, by just doing:

JMP Subroutine2

instead. (Remember: Don't ever do this - and I didn't tell you!)

Here's the next, which I found in Starmaster:

Imagine you're doing a fast loop, where you'd need three variables, not
just X and Y.
Now you could use a temporary variable or push and pop the third value
to the stack.

But if you need to be faster, consider doing this to temporarily store
the variable away:


and get it back with


of course. This is way faster than PLA / PHA, 


- Don't do this within a subroutine!
- Don't do any JSRs or other stack handling between TXS and TSX!
- Don't forget to restore the Stack to $FF after the loop!
  (If you construct your loop in a way, that X ends with $FF, this 
   would cost just an additional TXS after the loop)

(And again: Remember: Don't ever do this - and I didn't tell you!)


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