[Gambas-user] Hex string to float

[nando]

Two Questions:
1) Is the HEX you show below the HEX representation of 4B of a IEEE 4B Float?
2) How do you get these values? (read in from file? something else?)
-Fernando


---------- Original Message -----------
From: "Ron_1st" <ronstk at ...239...>
To: gambas-user at lists.sourceforge.net
Sent: Fri, 12 Dec 2008 19:42:09 +0100
Subject: Re: [Gambas-user] Hex string to float

> On Friday 12 December 2008, Ron wrote:
> > Ron schreef:
> > >
> > > Hi,
> > >
> > > Anyone have a routine to convert a hex string to a float?
> > > Like these:
> > > B6FBEB3A = -0.000007507766895287204
> > > 3C5D2104 = 0.01349664106965065
> > >
> > > Find lots of vb code, but those use copymem functions.
> > >
> > > Thanks in advance!
> > >
> > To answer my own question:
> > 
> > I ended up with this so far:
> > 
> > PUBLIC SUB Main()
> > 
> >   PRINT HexToFloat("3C5D2104")
> >   PRINT HexToFloat("B6FBEB3A")
> > 
> > END
> > 
> > PUBLIC FUNCTION HexToFloat(sHex AS String) AS Float
> > 
> >   DIM sTemp AS String
> >   DIM iSign, iExponent AS Integer
> >   DIM fTemp, fMant, fResult AS Float
> > 
> >   PRINT "Value = " & sHex
> >   ' calculate sign
> >   sTemp = Mid(sHex, 1, 2)
> >   fTemp = Val("&H" & sTemp) AND &H80
> >   iSign = IIf(fTemp = 128, -1, 1)
> >   PRINT "Sign = " & iSign
> > 
> >   ' calculate exponent
> >   sTemp = Mid(sHex, 1, 3)
> >   fTemp = Val("&H" & sTemp) AND &H7F8
> >   iExponent = fTemp / 2 ^ 3 - 127
> >   PRINT "Exponent = " & iExponent
> > 
> >   ' calculate mantissa
> >   sTemp = Mid(sHex, 3, 6)
> >   fTemp = Val("&H" & sTemp) AND &H7FFFFF
> >   fMant = (fTemp / 2 ^ 23) + 1
> >   PRINT "Mantissa = " & fMant
> > 
> >   fResult = iSign * fMant * 2 ^ iExponent
> > 
> >   RETURN fResult
> > 
> > END
> > 
> > Value = 3C5D2104
> > Sign = 1
> > Exponent = -7
> > Mantissa = 1.727570056915
> > 
> > Result: 0.01349664107
> > 
> > Value = B6FBEB3A
> > Sign = -1
> > Exponent = -18
> > Mantissa = 1.968116044998
> > 
> > Result: -7.507766895287E-6
> > 
> > Regards,
> > Ron_2nd
> > 
> >
> 
> Hi Ron :)
> It's a very long time ago I had to manage same 'problem'
> 
> First you need to know what notation is in the hex number.
> I only know a little of two of them and second you cant do
> strict hex to float. Its binary written in short by hex digits.
> 
> 1) The left most bit is the sign of the number and every bit is 1/2
>   01001001 is positive 1*0.5 + 0*.25 + 0*.125 + 1*0.062 + 0*0.031 +0*0.016 +1*0.008
> in hex 0x49
> 
> 2) the leftmost is the sign, then 8 bits for the exponent and followed
> 23 bits for the mantisa. (it looks to me you have/use this notation)
> 
> The first step is to convert the hex number to binary.
> Seperate the sign bit from the bits and loop the remaining bits 
> is the base method in both notations.
> The bad part is the fractional part but if you have a fixed count of bits 
> you can first multiply by a fix number.
> i.e. if you have 8 bits (1 sign 7 number) you multiply by 128, 
> for 16 bits (1 sign,15 number) you multiply by 65536.
> After dont the calculations you know you need to divide by that number.
> 
> I did it in Z80 machine language and the method was good enough, using 
> a quarter of a sinus table (180 steps of 0.5 grad per byte) and usage 
> of pitagoras to draw circles on the screen.
> Her the multiply was done by shift left, divide by shift right.
> The table was the sinus table value for 90 degree multiplied by 256
> 
> If floppy disks have a lifetime of centuries the I maybe can find it back
> on a real 5.25 floppy disk :)
> 
> links:
> google:floating number in binary
> IEEE 754-2008 - Wikipedia, the free encyclopedia
>   http://en.wikipedia.org/wiki/IEEE_754
> IEEE Standard 754 Floating-Point
>   http://steve.hollasch.net/cgindex/coding/ieeefloat.html
> Binary floating point and .NET
>   http://www.yoda.arachsys.com/csharp/floatingpoint.html
>   Read the part:
>     What exactly does a floating point number look like in memory?
> 
> IEEE Floating-Point Binary Representation
>   http://hankfiles.pcvsconsole.com/answer.php?file=454
> 
> So the keyword is 'IEEE754' for search.
> 
> google:floating number in binary  z80
> (nice results)
> Understanding Floating Point Formats
>   http://aplawrence.com/Basics/floatingpoint.html (perl script)
> 
> Best regards
> Ron_1st
> 
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------- End of Original Message -------