Problem with word variables treated as Byte

[Edward LaBudde]

Hi all, having a problem with GCB.  Here is a subroutine for finding phase angles.  Everything works fine for the first 3 quadrants.  The calculated phase for Img=100, Real = 100 (1st quad) is 44 degrees. For Img = -100, Real =100 (4th quad) the phase is 58 degrees not 314 as it should be (GCB adds 14 not 270).  It appears that GCB is treating Phasex as a byte not a word variable.  Here is the code and .asm snippet showing the problem.  Nothing I have tried can fix this problem!

Can someone help me correct this problem or is there a bug in GCB?  Thanks, ED.

[code]
Sub Phase (Img, Real)           
    Dim Phasex as word          ‘Note word variable
    Dim Real as integer
    Dim Img as integer
    Dim ImgTemp as word
    Dim RealTemp as word
    Dim tan as word
    Dim H2 as word
    call Sign (img)             ‘Find sign
    If Img = 0 then IMG = 1*Signx       'keep from divide by zero
    Call Quad (Img,Real)            'Find quadrant
    ImgTemp = IMG*IMG           'Take Absolute value
    Call SQRT (ImgTemp)
    Img = ImgTemp
    RealTemp = Real*Real            ‘Take Absolute value
    Call SQRT (RealTemp)
    Real = RealTemp
    if img < Real then          'Keep Tan > 1
        Tan = Real/Img
    Else
        Tan = Img/Real
    End If
    H2 = 157 - 400*tan/(4*tan*tan + 1)  'Approximate Phase in degrees (+/- 2 degree accuracy)
    if Img < Real then
        Phasex = 90-H2*4/7
    else
        Phasex = H2*4/7
    end if
    If Quadx = 1 then Phasex = Phasex
    If Quadx = 2 Then Phasex = Phasex + 90
    If Quadx = 3 Then Phasex = Phasex + 180
    If Quadx = 4 Then Phasex =  [word]Phasex + [word]270 'problem here Phasex acts like a Byte not word
Return
Here is the .asm file at Quad 3 and 4
movlw   3
    subwf   QUADX,W
    btfss   STATUS, Z
    goto    ENDIF6
    movlw   180 -------------------This is correct
    addwf   PHASEX,F
    movlw   0
    btfsc   STATUS,C
    addlw   1
    addwf   PHASEX_H,F
ENDIF6
    movlw   4
    subwf   QUADX,W
    btfss   STATUS, Z
    goto    ENDIF7
    movlw   14 -------------------------Not correct! It is adding 14 to Phasex not 270  e.g. 14 = 44+270-256-44  acting like a byte overflow
    addwf   PHASEX,F
    movlw   1
    btfsc   STATUS,C
    addlw   1
    addwf   PHASEX_H,F
ENDIF7
    return
[/code]

[Frank]

Hi Ed,

The assembly looks all right for the 4th quadrant - its adding 14 to PHASEX and then adding 1 to PHASEX_H which is the name GCBasic gives to the high byte of the word so the assembly code is adding 1 x 256 + 14 = 270 to the PHASEX word as a whole.

Maybe the problem is later in your code - if PHASEX gets assigned to a byte variable later then you would lose the high byte and just see 58. You might also try moving the Phasex dim to the top of the program outside the sub or else make Phasex one of the subs parameters so its explicitly returned like this -

sub Phase(Img , Real, out Phasex as word)

and delete the dim for  Phasex in the next  line. I think GCBasic treats variables differently if they are dim'ed inside a sub - they become something like local variables. Outside the sub they often seem to be treated just like bytes regardless of what they were dim'ed as inside the sub. That may be by design though as I think it does allow certain system variables to be used as bytes or words depending on the sub using them.

Frank

[Edward LaBudde]

Frank thanks for your input.  When I went back today to check to see if the high Byte of Phasex was set to zero, I was shocked to find it was working correctly today!!!  I have no Idea what was wrong yesterday, as it is the identical setup.

I did run into a new problem.  I found that if the SQRT subroutine is called more than once, it returns a value of “0” on the second pass.  I am not able to discover why.  The only way to make it work correctly is to mage a separate subroutine for each call each with a different name e.g. Sqrt, Sqrt2.    This uses up a lot more memory for all the new variables. Worst yet, Each subroutine can only be called once. 

This is a real problem as I was intending to use this in an instrument requiring updates at a frequent rate.  If put in a loop it only gives the correct answer on the first pass.

Try this your self-using: You will see that Vd is 5 and Vn is 0

Any Ideas Hugh?

[Code]
call SQRT(25)
Vd = SQRTx 
call SQRT(100)
Vn = SQRTx
Sub SQRT (value)             
    Dim Value as word
    Dim Guess_old as word
    Dim Guess_new as word  
    Dim Delta as word
    Dim SQRTX as word
    Guess_old = 0
    Guess_new = 0
    Count = 0
    If value <= 50 then
        Guess_div = 1
    else
        Guess_div = 50
    end if
    Guess_old = value/Guess_div 
    Do until Delta = 0
        Count += 1
        Guess_new = (Guess_old + value/Guess_old)/2
        Delta = Guess_new - Guess_old
        Guess_old = Guess_new
    Loop 
    Sqrtx = Guess_new
Return
[/code]

[Edward LaBudde]

Hi All, I found the problem with the second pass!  It was due to leaving the Variable delta = 0 after the first pass.  This requires
Delta to be initialized at the start of the routine to a number other than 0 e.g. Delta = 1.  That fixes the problem.  How embarrassing!

Sub SQRT (value)            'Tested time = 99 us Adding in/out will stop copy back 
    Dim Value as word
    Dim Guess_old as word
    Dim Guess_new as word  
    Dim Delta as word
    Dim SQRTX as word
    Guess_old = 0
    Guess_new = 0
    Count = 0
    Delta = 1
    If value <= 50 then
        Guess_div = 1
    else
        Guess_div = 50
    end if
    Guess_old = value/Guess_div 
    Do until Delta = 0
        Count += 1
        Guess_new = (Guess_old + value/Guess_old)/2
        Delta = Guess_new - Guess_old
        Guess_old = Guess_new
    Loop 
    Sqrtx = Guess_new
Return