floating point precision

[dsmith]

How do you increase floating point precision beyond the default of 6?
example:

double var = exp(-1.987654321123456789);
writeln(var);

--> 0.137016

Assuming this result is only an output format issue and that operations are
still using double's 64 places, if var above is passed to a function, are all
64 places passed?  Must it be passed by reference to make it so?