As always, the answer is on piclist.com.... <GRIN>

http://www.piclist.com/techref/piclist/codegen/constdivmul.htm

James Newton, PICList Admin #3
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----- Original Message -----
From: Jonathan Newport <jonathan.newport@VANDERBILT.EDU>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Tuesday, June 26, 2001 16:48
Subject: [PIC]: floating point headaches


Hello all:

    I haven't been messing around with pics for a while....but just
picked it back up again a couple of weeks ago and forgot how much fun it
was.
Now, onto the plea for help....

    I admit...I've ALWAYS been scared of floating point routines, but
the time has come where I MUST use them....the Microchip routines scare
me worse than any of them....but maybe I'm just being stupid and
ignorant of binary math...in any case...I've found these routines which
look good to me and I think I can use.  What I'm trying to do is
multiply the 10-bit A/D value that I obtained from my F876 by
0.0048828125 to get the resultant voltage...the only thing I don't know
how to do is get the A/D value (an integer) into floating point
notation, actually, I guess I don't know how to get the 0.0048828 value
into floating point notation either, but that's just a constant...how
would I convert an integer to a float?  the float routines that I'm
using use two 16-bit arguments with an 8-bit mantissa and an extra sign
byte (which I don't know what does yet) here's one of the routines (some
call statements may be missing...I just gave a snippit):


;*******************************************************************
;   Binary Floating Point Multiplication :
;   ACCb(16 bits)EXP(b) * ACCa(16 bits)EXPa -> ACCb(16 bits)EXPb

F_mpy:  call  S_SIGN
  call  setup
mloop:    bcf  STATUS,C      ; clear carry bit       ??????????
  rrf  ACCdHI,F         ;rotate d right
  rrf  ACCdLO,F
  btfsc  STATUS,C      ;need to add?
  call  D_add
  rrf  ACCbHI,F
  rrf  ACCbLO,F
  rrf  ACCcHI,F
  rrf  ACCcLO,F
  decfsz  temp, F          ;loop until all bits checked
  goto  mloop

  movf  EXPa,W
  addwf  EXPb, F

  IF  Mode16
  movf  ACCbHI, F
  btfss  STATUS,Z
  goto  finup            ; if ACCbHI != 0
  movf     ACCbLO, F
  btfss    STATUS,Z
  goto     Shft08           ; if ACCbLO != 0 && ACCbHI == 0

  movf  ACCcHI,W
  movwf    ACCbHI           ; if ACCb == 0, then move ACCc to ACCb
  movf     ACCcLO,W
  movwf    ACCbLO
  movlw  .16
  addwf    EXPb, F
  goto      finup

Shft08:   movf     ACCbLO,W
  movwf    ACCbHI
  movf     ACCcHI,W
  movwf    ACCbLO
  movlw    .8
  addwf    EXPb,F

  ENDIF                     ; matching endif for IF Mode16

finup:    btfss    sign,MSB
  goto     F_norm

  decf     ACCcLO,F         ; negate ACCc
  comf     ACCcLO,F
  btfsc    STATUS,Z
  decf     ACCcHI,F
  comf     ACCcHI,F
  btfsc    STATUS,Z

neg_B:    decf     ACCbLO,F         ; negate ACCb
  comf     ACCbLO,F
  btfsc    STATUS,Z
  decf     ACCbHI,F
  comf     ACCbHI,F

  goto     F_norm



thanks for your time!

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