For the rules used by the text interpreter for recognising double-precision integers, see Literals.
A double precision number is represented by a cell pair, with the most
significant cell at the top-of-stack (TOS). It is trivial to convert
an unsigned single to a double: simply push a 0 onto the
TOS. Numbers are represented by Gforth using 2’s complement
arithmetic, so converting a signed single to a (signed) double
requires sign-extension across the most significant cell. This can be
achieved using s>d. You cannot convert a number from
single-cell to double-cell without knowing whether it represents an
unsigned or a signed number. By contrast, in 2’s complement
arithmetic the conversion from double to single just drops the
most significant cell, and d>s just documents the intent.
D+ and d- are defined for signed operands, but also work
for unsigned numbers.
s>d ( n – d ) core “s-to-d”
d>s ( d – n ) double “d-to-s”
d+ ( ud1 ud2 – ud ) 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”