Hi Guys,

I'm trying to get my head around adding numbers that a greater than 8 bit.

I've managed to get 16 bit addition working to an extent.

Here I'm adding 6000 to the previous contents of result, which is working ok if the result is below 65536.



I'll only ever be adding 16 bit number at maximum. How would I go about expanding this to flow over in to a 24 bit result? I have provision for "result + 2" allocated in RAM.

I've tried a few different things but am struggling to get a 24 bit result.

I thought (logically) something like this at the end of the code, but that doesn't seem to work at all.



Math on the 6502 is definitely not my strong point and is one of those areas that always trips me up.

Any advice is greatly appreciated.


[Edited to reflect that "CLC" was being called prior to the routine - Pointing this out to reflect the context of Garth's reply]

_________________
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?

Thanks Garth! I'll give that a try.

I should have clarified that I am in fact doing "CLC" in the line above the snippet of code up to (I'll edit and add it now).

So is the method of the ADC in my second code snippet failing because the carry bit is already (potentially) set from the addition prior to it?

_________________
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?

Thanks again, your solution works well and I understand where you have gone with it there.

I'll have to get out the pencils and paper to fully understand your reasoning as to why my method failed. Which is not a bad thing, I'm sure it will click when I analyse it much deeper.

Garths listing is excellent if you are keeping a running tally, but if you just want to add two 16-bit numbers and know the immediate total, then the code should be:

Otherwise you risk having a random value in the RESULT+2 position without zeroing it first.

The real code to prevent addition errors should be:

Thanks Rob!

Yeah, in my particular case I'm adding back to the 24 bit number each time.