@Houl The 6502 did not have a reasonably orthogonal instruction set which means it isn't RISC. Some people would add the lack of a flat memory model, but that just makes it a poor C target.

I love the 6502, but I personally wish zero page could be moved around -- something WDC variants call direct page -- and the stack could similarly be relocated. The first change would increase performance *a lot* and the second would make real multitasking possible.

@profoundlynerdy @Houl

A mobile direct page is the gateway feature leading to register windows and the Sparc architecture!

Probably the most interesting feature (at the time) for the Sparc architecture was the use of register windows. The basic idea is that the processor has 32 general purpose registers divided between globals, "in", "local" and "out". When you make a call, the processor renames things a bit so that the out registers become the in registers at the next call level.

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@profoundlynerdy @Houl

Register windows are a bit like a (small) page zero because you can get to them with a short address. Being able to efficiently roll to the next window was pretty cool and is like being able to have a new page zero when you call a function.

In fact, however, register windows turned out very bad if you account for context switches because tons of unused registers had to be flushed to the stack.

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@profoundlynerdy @Houl

Thus my "gateway feature" comment. Windows seemed cool at the time.

Register windows can also never be removed from the arch due to compatibility. Thus the drug metaphor.

(that may be more than you wanted to know 🙂

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@profoundlynerdy @Houl

@ted_dunning @Houl That was exactly the kind of thing I wanted to know. I'm an #assemblylanguage geek. Thanks! I've *cough* spent more time looking at #motorola #M6809 documents today than I should have. Damned ADHD. :-P

Do you think the arch will live on in some way? Even #Fujitsu bailed on it.