6502.org

Greetings,

I spent a couple of enjoyable hours reading through some of this boards threads last night. This is my first post.

I have recently returned to 65xx assembly after a very long hiatus (my last time was in the early 80s).

For the heck of it (and to keep my mind sharp), my first foray back is to cobble together an Atari 2600 4k game binary.

As I experiment and learn the 2600, I find myself wishing I had a set of fast constant-time 6507 flag to mask conversion snippets.
Before I wrack my brain reinventing the wheel, I thought I'd check if anyone on this board knows of an existing set.

What I'm looking for is a set of "bithacks" for converting processor flag states (or boolean combination of processor flag states) to canonical 8-bit masks in A, in the least number of cycles but in constant-time.

By canonical mask I mean -1/0 (all bits on or all bits off), similar to Forth flags.

By constant-time I mean either branchless or using "ballasted" branches, so that all conditional paths through the code end up taking exactly the same number of cycles.

I'm perfectly fine with using undocumented 6507 opcodes, as they seem to be commonly used in 2600 homebrews.

Here are a couple of examples (I have no idea if these are optimal or not).

~C -> -1/0:
C -> -1/0:
... and so on and so forth, including useful boolean combinations of N, V, C, Z.

Cheers,
Mark

Welcome, Mark, and congratulations on ending your very long hiatus!

Here are two possible approaches. I'm pretty sure I understand what we're trying to achieve. But I'm less sure I've gotten all the details correct, as it's late and I'm getting sleepy! Maybe this will at least help you get started.

-- Jeff

Very fast but also very bulky: A bit slower but a great deal smaller:

Thanks very much for the kind welcome and the code.

It's not exactly what I'm looking for, in the sense that I'm after the fastest routine per test rather than a generic approach.

For example if one is attempting to convert ~C to a mask, I'm guessing lda #0 sbc #0 at 4mc may be the fastest constant-time option, and perhaps lda #0 sbc #0 eor #$ff at 6mc is best for converting C to a mask, I don't really know. In summary, I'm after the best mask generator for each commonly useful SR flags test.

Cheers,
Mark

Thank you.

If it does turn out to be infeasible, then I'll at least try to reproduce some of the Bxx tests as mask producers to similar ends, e.g.

A = -(A>=0) [edited for correction (twice!)] In Forth-83 test/query words returned -1/0, Forth-79 and Fig-Forth used 1/0.
Although I'm not a fan of the later ANS-Forth, I adopted some of the controversial changes of '83, particularly the -1/0 test results and the use of floored division.

As to why -1/0 is better, you can use a result directly as a mask.
Also, unlike C for example you don't need different operators for logical and bitwise operations, since they're the equivalent when using -1/0 as canonical flags.

Below is an example where I'm using the result of a comparison as a mask for the subsequent AND.

Sorry—I didn't mean 1 versus -1 for a "true" flag, but that IF, WHILE, UNTIL, ABORT", ?DUP, ?EXIT, ?<anything> only look to see if it's 0 or not. Things that produce a flag, like 0=, >, etc. do produce a -1 as a true flag; but I've seldom found that it matters. In the rare case that it does matter, you can use 0<> to change any non-0 number into a -1.

Yes, in the grand scheme it's a very minor difference, sometimes a small convenience, other times a small inconvenience. Which you favor is more a question of personal coding style and taste than anything else.
And of course, with Forth you can always have it your way.

The reason I'm keen on mask generators is that, in the display kernel of a 2600 game, I'd like to avoid branches wherever I can easily do so, since all control flow paths will need to be balanced to consume the exact same number of cycles, and ballasting branches is a pain.

It's an HP-16C Computer Scientist, bought it at Macy's in San Francisco in the early 80s.

Very very cool
I was just reading last night or this morning about your use of HP-IL on your workbench computers page.

Welcome, Mark! An interesting quest, to be sure. And rather jumping in at the deep end of deterministic runtime, to pick the 2600. But I look forward to seeing some ingenious ideas.

You might find the resources linked here useful, if any are new to you:
https://alienbill.com/2600/

And there's an in-browser IDE for the 2600 which might help when you want to test things:
https://8bitworkshop.com/v3.9.0/?file=e ... atform=vcs

Thanks for the welcome and the resources BigEd.