I'm essentially debouncing. Set LASTKB = MILLIS at a point in time, compare 16 bit timer value MILLIS to it continuously and check if 100 ticks have passed.
First I subtract the two 16 bit numbers. Then subtract the 100 and check carry.

Surely there must be a much better way to do this?

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You could add the 100 to MILLIS before you save the LASTKB value.

The all you need in the checking code is a compare.

The obvious improvement is to set LASTKB = MILLIS + 100 instead. Then


... as Martin said while I was typing. I will leave replacing "sec / sbc" with "cmp" as my contribution.

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Of course! Much better!

Thank you all!

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New new new new new video out! Serial Bootloader for my 65uino
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Personally, I would set the timer as a countdown timer you want to elapse, then use the IRQ to call the timer routine that decrements instead.

This way should be faster:

LDA MILLIS
BNE :1

DEC MILLIS+1 ; to use the full 16-bit value, then increment this by 1 when initializing values
BMI ALARM ; and use BEQ instead

:1 DEC MILLIS

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http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Technically I could time my main loop roughly and do



That would probably be the least overhead, running only every x loops - but not very consistent, timing wise.

If I used the ISR to decrement, like Rob suggested, it would be more consistent but the more things that need timing would quickly bloat the ISR.

_________________
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New new new new new video out! Serial Bootloader for my 65uino
Also, check out: I2C on a 6502 Single Board Computer
and Complete hardware overview of my 6502 SBC R1

The debouncing would still affect the count-up timer the same way as the extra ticks will still be added to the timer . The number of variables used would still be the same for a count-up timer vs countdown timer. The same number of variables would be needed for multi-tasking for each method. The countdown way eliminates the clock altogether except at the beginning when one has to calculate the difference between the starting MILLIS time and the target time.

Doing a 16-bit comparison takes 5 cycles longer on each pass using the count-up timer for a multitask environment. I see way more benefits to using a countdown timer vs count-up timer. The only real difference is one can more readily see the time elapsed with a count-up timer vs time-remaining for a countdown timer.

There shouldn't be any extra bloat as the calculations to set the timer are pretty much the same with each method and depending on the use-method, may be faster for a countdown timer.

For a count-up timer you have to subtract the desired elapsed time from the current clock's time. For a countdown timer, you just set the desired elapsed time with values.

Hi!


The above (and all the other code samples given) suffer from a data race: the ISR could fire just between the first LDA MILLIS and the second LDA MILLIS+1, and the roll-up will invalidate the count:

- Suppose MILLS = $0100
- LDA MILLS ; A = 0
- Now, the ISR fires and MILLS = $00FF
- LDA MILLS+1 : A = 0

So, instead of reading $0100 or $00FF, you are reading $0000, an invalid value.

To avoid this, you must read the value properly, the only way to do it in the presence of NMI is:


Now, if you have multiple tasks waiting for a given time to perform some action, it is better to have a priority queue with all the tasks, so you only check the earliest task available each loop.

Have Fun!

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?