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<h3 class="section">5.16 Internals</h3>
<a name="index-Limb"></a>
<p>A <em>limb</em> means the part of a multi-precision number that fits in a single
word. Usually a limb contains
32 or 64 bits. The C data type for a limb is <code>mp_limb_t</code>.
</p>
<p>The <code>mpfr_t</code> type is internally defined as a one-element
array of a structure, and <code>mpfr_ptr</code> is the C data type representing
a pointer to this structure.
The <code>mpfr_t</code> type consists of four fields:
</p>
<ul>
<li> The <code>_mpfr_prec</code> field is used to store the precision of
the variable (in bits); this is not less than <code>MPFR_PREC_MIN</code>.
</li><li> The <code>_mpfr_sign</code> field is used to store the sign of the variable.
</li><li> The <code>_mpfr_exp</code> field stores the exponent.
An exponent of 0 means a radix point just above the most significant
limb. Non-zero values <em>n</em> are a multiplier <em>2^n</em> relative to that
point.
A NaN, an infinity and a zero are indicated by special values of the exponent
field.
</li><li> Finally, the <code>_mpfr_d</code> field is a pointer to the limbs, least
significant limbs stored first.
The number of limbs in use is controlled by <code>_mpfr_prec</code>, namely
ceil(<code>_mpfr_prec</code>/<code>mp_bits_per_limb</code>).
Non-singular (i.e., different from NaN, Infinity or zero)
values always have the most significant bit of the most
significant limb set to 1. When the precision does not correspond to a
whole number of limbs, the excess bits at the low end of the data are zeros.
</li></ul>
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