This is the ready check for a serial printer in a driver for the 6800:

How would you write this for the NMOS 6502?

I can see juggling bytes on the stack or using a temporary variable, but no direct translation.

Something like this - it puts the result in the carry flag rather than the negative flag:

A CMOS 6502 could do this all in one instruction but you asked for MMOS explicitly.

The interface was originally used for a parallel printer. Space was extremely limited, so they just read the port and the ready line was bit 7. I would rather not change the interface as that would make the parallel driver even bigger.

The best I could come up with is:

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The Ready line is bit 7 for the parallel port. It is bit 1 for the serial port.

George's code looks good to me. Yes, you'd have to make a minor change in every place from which this routine is called (to test Carry rather than the Sign). But that seems tolerable if you're replacing all the 6800 code with 6502 code anyway...

-- Jeff

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In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
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It looks like the 6800 version is nine bytes. If you have space for ten bytes and you have a "junk" zp location, you can just add an ROR zp to George's code:
When I was translating code from 6800 to 6502 I would be often annoyed by the need to tsta after a pula, but this is a good counterexample.

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Got a kilobyte lying fallow in your 65xx's memory map? Sprinkle some VTL02C on it and see how it grows on you!

Mike B. (about me) (learning how to github)

Is there any reason you can't trash X or Y? If not, here's another 10-byte attempt:

The ROR's can be replaced with LSR's if only the carry flag is pertinent.

I don't recommend it, unless you only JSR $EAxx ... PC will be one byte off ...

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Got a kilobyte lying fallow in your 65xx's memory map? Sprinkle some VTL02C on it and see how it grows on you!

Mike B. (about me) (learning how to github)

... and it works around that pesky ROR bug!

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Got a kilobyte lying fallow in your 65xx's memory map? Sprinkle some VTL02C on it and see how it grows on you!

Mike B. (about me) (learning how to github)

One of the goals of the project is to make it as easy as possible to port 6800 code over. Changing this interface would be yet another thing to forget to check.

The serial driver is under no space pressure.

The parallel driver is. The 6800 version uses 55 of the 56 bytes allotted. I've allocated 64 bytes for the 6502 version and barely managed to code golf it to fit.

I posted that subroutine from the serial driver to see whether I overlooked any coding tricks.

I do not have a spare zp location, but there are bytes to spare within the driver, that temporary variable I used, for instance.

I should have known that you would come up with something.

You could even ROR a read-only location, like something in ROM. The write will fail but the negative flag should be set correctly.

Interesting that you can do it with ROM. That will confuse someone trying to reserve engineer the code unless they happen to know that fact.