Convert a 'signed' Word to a Long with minimum manipulation ?

[mr.sneezy]

Yes. The method above works well. I did not try the other way to see if it's tighter code though, I was happy to just push onwards.

I discovered a issues with the rest of my calculations though, and I'd appreciate a comment from the 'Long literates' among us.

I'll try to explain without a huge dialogue, if it's too vague let me know. Here goes...

OK, this code, all the variables are Longs, except AC5, AC6 and MD which are Words.

X1 = ((lUtemp - AC6) * AC5) >>15  'find X1 - Note I needed the extra () around AC5 to get it to work
         X2 = (lMC << 11) / (X1 + MD)      'Find X2
        B5 =  X1 + X2                               'Find B5 from X1 and X2
        lCTemp = (B5 + 8) / 16    '>> 4      'Hey presto, lCTemp appears...

Firstly I needed two sets of parenthesis around the X1 calculation, or it gave a bad result with only one around the 'lUTemp -AC6'.
The original C code was like this though ( I changed the 2^15 to the shift right by 15).
X1 = (UT - AC6) * AC5 / 2^15

Second, in the last calculation, I got a bad result whenever the value B5 became negative, until I got rid of the >> 4 and made that a '/ 16'. Then the value of lCTemp worked OK below 0 degC (I used freezer spray). I thought a >> 4 shift was equivalent to a / 16, but here in this syntax it seems not.

Can someone fill me in on why the shifts didn't quite work like a numeric divide here ?
Perhaps something to do with signed math again ?

In the end with the second brackets and a numeric divide the results were good, I'd just like to know why my 'fixes' worked so I don't bugger up the next stage...

Thanks,
Martin

[Kamikaze47]

PBP's shift algorithms dont work on negative numbers. You either have to divide instead, or do something like this:

lCTemp=(B5+8)>>4
IF lCTemp.27 THEN lCTemp=lCTemp&$F0000000

(Untested)

*edit* note that the division will always round down (towards negative infinity) when shifting no matter if the number if positive or negative. e.g. 5/2 = 2, but -5/2 = -3 (only for shifting - not division)

[mr.sneezy]

Thanks.
It sounds like I should just avoid using the Shift type divides then with Longs...
Martin

[Kamikaze47]

Just to correct myself, the code in my last post should have been:

lCTemp=(B5+8)>>4
IF lCTemp.27 THEN lCTemp=lCTemp|$F0000000

I accidentally used an AND instead of an OR