A companion to IEEE 754 80-bit Extended double (long double) to 64-bit double unpack, this takes an IEEE 64-bit real (Perl NV) and packs into a 10-byte binary encoded IEEE 754 80-bit real.

Testing limited to Intel P4 and Perl 5.8.6/7 under WinXP.

Reinstated, as I believe it now correctly handles all cases.

(Be careful what you read on the internet. Not all reference material is created equal.)

sub IEEE64toIEEE80 {
    my( $mantissa, $exponent, $sign ) =
        unpack 'a52 a11 a1', unpack 'b64', pack 'd', $_[ 0 ];

    return unpack 'd', pack 'b80', '0' x 79 . $sign unless $mantissa =
+~ m[1] or $exponent =~ m[1];

    if( $exponent !~ m[0] ) { 
        $exponent     = '111111111111111';
    }
    else {
        $exponent = unpack( 'v', pack 'b11', $exponent ) - 1023;
        $exponent -= length( $1 ) if $exponent == -1023 and $mantissa 
+=~ m[(0+$)];
        $exponent = unpack 'b15', pack 'v', $exponent  + 16383;
    }
    return pack 'b80', '0'x 11 . $mantissa . '1' . $exponent . $sign;
}

[download]

What Perl thinks it's writing and what D reads back for a selection of values at the boundaries: 

c:\test>\test\to80bit.pl >junk.dat          c:\test>readreal80 junk.da
+t
FFFFFFFFFFFFFFFF -1.#QNAN                   -nan
FFF8000000000001 -1.#QNAN                   -nan
FFF8000000000000 -1.#IND                    -nan
FFF0000000000001 -1.#QNAN                   -nan
FFF0000000000000 -1.#INF                    -inf
FFEFFFFFFFFFFFFF -1.7976931348623157e+308   -1.797693134862315707900e+
+308
8010000000000000 -2.2250738585072014e-308   -2.225073858507201383200e-
+308
800FFFFFFFFFFFFF -2.2250738585072009e-308   -2.225073858507201136000e-
+308
8000000000000001 -4.9406564584124654e-324   -4.940656458412466538300e-
+324
8000000000000000  0                          0.0000000000000000000000
0000000000000000  0                          0.0000000000000000000000
0000000000000001  4.9406564584124654e-324    4.940656458412466538300e-
+324
000FFFFFFFFFFFFF  2.2250738585072009e-308    2.225073858507201136000e-
+308
0010000000000000  2.2250738585072014e-308    2.225073858507201383200e-
+308
7FEFFFFFFFFFFFFF  1.7976931348623157e+308    1.797693134862315707900e+
+308
7FF0000000000000  1.#INF                     inf
7FF0000000000001  1.#QNAN                    nan
7FF7FFFFFFFFFFFF  1.#QNAN                    nan
7FF8000000000000  1.#QNAN                    nan
7FFFFFFFFFFFFFFF  1.#QNAN                    nan

[download]

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