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0 pointsSomeone4y ago0 comments

Not only C, any language that thinks there’s other things than global state.

If all your functions are void foo(void) and you don’t use local variables (or your language doesn’t support recursion, in which case all locals can be given a fixed address), targeting 6502 is fine (it also helps if you avoid floating point, use 8-bit variables where possible, etc)

Not supporting recursion also means you can statically compute maximum stack depth. That way, you can avoid linking code that would overflow the stack.

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royjacobs4y ago

The cool thing about LLVM-MOS specifically it that by using the zero page as virtual registers you sort-of get the same output with 'regular' code as opposed to this 'global variables' style of programming.

I recall a tutorial for 'cc65 optimizations'[0] which basically destroys a well-structured C program in order to do all of these optimizations (like making everything global) and it was absolutely terrible, code-wise. Well, the end result was probably fine, but it's just a shame these 'optimizations' were needed.

[0] I think it was this one: https://github.com/ilmenit/CC65-Advanced-Optimizations

SomeoneOP4y ago

Nice article, but it doesn’t mention the really gnarly stuff such as using the fact that a subroutine happens to return with some flags set, or with some fixed value in the X register to shave of some initialization instruction in the code calling it.

A main advantage of the 6502 is that it only has 64 kilobytes of memory ;-). That means sufficiently advanced and motivated programmers can keep the entire program in their head, and also nudges them to avoid bloat such as the use of 16-bit integers.

cmrdporcupine4y ago

Zero page is great, but has limitations, for sure. Lots of moving stuff back and forth into the accumulator in order to do anything with it. And not relocatable like in the 6809 or 65816 "direct page".

Some nice simple extensions to the 02 architecture would be:

1) relocatable direct page and stack like in the 816. 2) some way of aliasing A to a direct page address to avoid doing it by hand.

tom_4y ago

I think you could use zero page as the data stack. Treat X is your frame pointer, and the zero page "address" is then the offset into the frame. Most instructions have a zp,X addressing mode. (This scheme works well with (zp,X) too.) LDY zp,X and STY zp,X are available, and the useful read instructions have abs,Y forms. So you can do lookups into global tables with an 8-bit local variable index without having to save X.

You'd need a little region for making use of (zp),Y, probably callee-saved, putting previous values on the return stack with PHA.

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