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

[Acetronics2]

Hi,

I' d try a first thing ...

 
if Myword.15 then
 
MyWord = Myword * -1 ' absolute value
MyLong = MyWord * -1 ' the negative number
 
Else
 
MyLong = MyWord
 
Endif

certainly not the smartest ... but simple.

*** IDEA : TO BE CONFIRMED ***
Alain

[mr.sneezy]

I guess I should see which way uses least code space too, but I just tryed to complile the first suggestion and it did that OK. I'll only know if the code runs as expected later today, when I burn a PIC and try it.
I added this to my code.

    lAC2        var Long            'Long variables for calibration values actually negative in my sensor
    lAC3        var Long 
    lMB         var Long
    lMC         var Long

'Convert unsigned PBP Word var to signed PBP Long var.

        lAC2 = AC2                           'copy word to long   
        if AC2.15 then lAC2.HIGHWORD = $FFFF 'Check for negative, set top 16bits to all 1's if true 
        lAC3 = AC3                           'copy word to long
        if AC3.15 then lAC3.HIGHWORD = $FFFF 'Check for negative, set top 16bits to all 1's if true
        lMB = MB                           'copy word to long
        if MB.15 then lMB.HIGHWORD = $FFFF 'Check for negative, set top 16bits to all 1's if true
        lMC = MC                           'copy word to long
        if MC.15 then lMC.HIGHWORD = $FFFF 'Check for negative, set top 16bits to all 1's if true

[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