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For the rules used by the text interpreter for recognising double-precision integers, see Number Conversion.
A double precision number is represented by a cell pair, with the most
significant cell at the TOS. It is trivial to convert an unsigned single
to a double: simply push a 0 onto the TOS. Since numbers are
represented by Gforth using 2's complement arithmetic, converting a
signed single to a (signed) double requires sign-extension across the
most significant cell. This can be achieved using s>d. The moral
of the story is that you cannot convert a number without knowing whether
it represents an unsigned or a signed number.
These words are all defined for signed operands, but some of them also
work for unsigned numbers: d+, d-.
s>d n – d core “s-to-d”
d>s d – n double “d-to-s”
d+ d1 d2 – d double “d-plus”
d- d1 d2 – d double “d-minus”
dnegate d1 – d2 double “d-negate”
dabs d – ud double “d-abs”
dmin d1 d2 – d double “d-min”
dmax d1 d2 – d double “d-max”