On Sun, Feb 05, 2006 at 01:39:25PM +0100, Jonas Maebe wrote:
>
> On 05 Feb 2006, at 13:32, Tom Verhoeff wrote:
>
> >Question 2: Any suggestions as to how to make it more platform
> >independent,
> >or how to support multiple platforms in an easy way?
>
> The only differences are little/big endian. So if it works on PPC and
> I386, you have support for all platforms. Just use {$ifdef
> FPC_LITTLE_ENDIAN}
[This is about IEEE floating-point numbers]
Two further questions.
Which platforms use which endian? (Of course, I could just experiment,
or maybe even read the FPC source, but asking is simpler. :-)
When I looked at my code again, it seems to me that the change that I made
to support both PPC and I386 is not endian-related, but I may be wrong,
or it may be more complicated.
So, here are some pieces of the code, it uses a variant array to
access the bits of the IEEE floating-point numbers in Single and Double:
-----
interface
const
SignIndexSingle = 31; { position of sign bit in single format }
ExpSizeSingle = 8; { size of exponent field in single format }
MaxExpSingle = 255;
ExpBiasSingle = 127;
EminSingle = 1 - ExpBiasSingle;
EmaxSingle = MaxExpSingle - 1 - ExpBiasSingle;
FracSizeSingle = 23; { size of fraction field in single format }
type
BitIndexSingle = 0 .. SignIndexSingle;
ConvertSingle = record
case Boolean of
False: ( d: Single )
; True : ( s: set of BitIndexSingle ) { i in s == bit i = 1 }
end;
procedure WriteBitsSingle ( const x: Single );
procedure WriteBinarySingle ( const x: Single );
function PowerOf2Single ( const x: Single; n: Integer ): Single;
const
SignIndexDouble = 63; { position of sign bit in double format }
ExpSizeDouble = 11; { size of exponent field in double format }
MaxExpDouble = 2047;
ExpBiasDouble = 1023;
EminDouble = 1 - ExpBiasDouble;
EmaxDouble = MaxExpSingle - 1 - ExpBiasDouble;
FracSizeDouble = 52; { size of fraction field in double format }
type
BitIndexDouble = 0 .. SignIndexDouble;
ConvertDouble = record
case Boolean of
False: ( d: Double )
; True : ( s: set of BitIndexDouble ) { i in s == bit i = 1 }
end;
procedure WriteBitsDouble ( const x: Double );
procedure WriteBinaryDouble ( const x: Double );
procedure FlipBitDouble ( var x: Double; i: BitIndexDouble );
function PowerOf2Double ( const x: Double; n: Integer ): Double;
-----
It turns out that my code for the Single type works on both PPC
and I386 without change. However for Double, I had to write e.g.
-----
procedure WriteBitsDouble ( const x: Double );
var c: ConvertDouble; i, i_: BitIndexDouble;
begin
with c do begin
d := x
; for i := SignIndexDouble downto 0 do begin
{$IFDEF CPUPOWERPC}
i_ := (i + 32) mod 64
{$ENDIF}
{$IFDEF CPUI386}
i_ := i
{$ENDIF}
; if i_ in s then write ( '1' )
else write ( '0' )
; if i in [ SignIndexDouble, SignIndexDouble - ExpSizeDouble ] then
write ( ' ' )
end { for i }
end { with c }
end; { WriteBitsDouble }
-----
That is, the lower and upper 32 bits (4 bytes) got swapped.
Maybe this is the case because both the 64-bit set and the Double
get allocated differently depending on endianess?
Can someone enlighten me?
Thanks,
Tom
--
E-MAIL: T.Verhoeff @ TUE.NL | Fac. of Math. & Computing Science
PHONE: +31 40 247 41 25 | Eindhoven University of Technology
FAX: +31 40 247 54 04 | PO Box 513, NL-5600 MB Eindhoven
http://www.win.tue.nl/~wstomv/ | The Netherlands
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