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+Long double format
+==================
+
+  Each long double is made up of two IEEE doubles.  The value of the
+long double is the sum of the values of the two parts (except for
+-0.0).  The most significant part is required to be the value of the
+long double rounded to the nearest double, as specified by IEEE.  For
+Inf values, the least significant part is required to be one of +0.0
+or -0.0.  No other requirements are made; so, for example, 1.0 may be
+represented as (1.0, +0.0) or (1.0, -0.0), and the low part of a NaN
+is don't-care.
+
+Classification
+--------------
+
+A long double can represent any value of the form
+  s * 2^e * sum(k=0...105: f_k * 2^(-k))
+where 's' is +1 or -1, 'e' is between 1022 and -968 inclusive, f_0 is
+1, and f_k for k>0 is 0 or 1.  These are the 'normal' long doubles.
+
+A long double can also represent any value of the form
+  s * 2^-968 * sum(k=0...105: f_k * 2^(-k))
+where 's' is +1 or -1, f_0 is 0, and f_k for k>0 is 0 or 1.  These are
+the 'subnormal' long doubles.
+
+There are four long doubles that represent zero, two that represent
++0.0 and two that represent -0.0.  The sign of the high part is the
+sign of the long double, and the sign of the low part is ignored.
+
+Likewise, there are four long doubles that represent infinities, two
+for +Inf and two for -Inf.
+
+Each NaN, quiet or signalling, that can be represented as a 'double'
+can be represented as a 'long double'.  In fact, there are 2^64
+equivalent representations for each one.
+
+There are certain other valid long doubles where both parts are
+nonzero but the low part represents a value which has a bit set below
+2^(e-105).  These, together with the subnormal long doubles, make up
+the denormal long doubles.
+
+Many possible long double bit patterns are not valid long doubles.
+These do not represent any value.
+
+Limits
+------
+
+The maximum representable long double is 2^1024-2^918.  The smallest
+*normal* positive long double is 2^-968.  The smallest denormalised
+positive long double is 2^-1074 (this is the same as for 'double').
+
+Conversions
+-----------
+
+A double can be converted to a long double by adding a zero low part.
+
+A long double can be converted to a double by removing the low part.
+
+Comparisons
+-----------
+
+Two long doubles can be compared by comparing the high parts, and if
+those compare equal, comparing the low parts.
+
+Arithmetic
+----------
+
+The unary negate operation operates by negating the low and high parts.
+
+An absolute or absolute-negate operation must be done by comparing
+against zero and negating if necessary.
+
+Addition and subtraction are performed using library routines.  They
+are not at present performed perfectly accurately, the result produced
+will be within 1ulp of the range generated by adding or subtracting
+1ulp from the input values, where a 'ulp' is 2^(e-106) given the
+exponent 'e'.  In the presence of cancellation, this may be
+arbitrarily inaccurate.  Subtraction is done by negation and addition.
+
+Multiplication is also performed using a library routine.  Its result
+will be within 2ulp of the correct result.
+
+Division is also performed using a library routine.  Its result will
+be within 3ulp of the correct result.
+
+
+Copyright (C) 2004 Free Software Foundation, Inc.
+
+Copying and distribution of this file, with or without modification,
+are permitted in any medium without royalty provided the copyright
+notice and this notice are preserved.