The interval arithmetic in GlobSol, as of June 1998, was based, at the lowest level, on the FORTRAN 77 library INTLIB, ACM Transactions on Mathematical Software Algorithm 737; see http://interval.louisiana.edu/preprints/intlib_toms_algorithm.ps (Postscript, approx. 165KB) or http://interval.louisiana.edu/preprints/intlib_toms_algorithm.dvi (TeX DVI, approx. 50KB) for a preprint of the corresponding report. Also, [8, §2.1,] gives a description of INTLIB.
Also as of June, 1998, an interval data type was supplied via operator overloading in Fortran 90, based on the module INTERVAL_ARITHMETIC ACM Transactions on Mathematical Software Algorithm 763. For a preprint of the report, see http://interval.louisiana.edu/preprints/interval_arithmetic.ps (Postscript, approx. 125KB) , or http://interval.louisiana.edu/preprints/interval_arithmetic.dvi (TeX DVI, approx. 35KB). The module in GlobSol has been modified slightly from the published version, to extend some capabilities, and to make it faster on certain compilers at the possible expense of some portability.
Both INTLIB and module INTERVAL_ARITHMETIC can be accessed directly in user programs if GlobSol is installed. INTLIB is in the library intlib.alt/intlib.a (on Unix systems with the default build process). Module INTERVAL_ARITHMETIC is in the library overload/overload.a, and, for compiling programs that use that module, the directory overload should be included in the search path for module information.
There are plans to eventually use interval computations in GlobSol from an interval data type intrinsically supported by the compiler.