FORTRAN-XSC. A portable Fortran 90 module library for accurate and reliable scientific computing. Fortran 90, the new international Fortran standard released in 1991, offers a multitude of new features and enhancements compared with FORTRAN 77. However, numerical problems persist since the new standard contains no accuracy requirements for the arithmetic operators and mathematical functions, making reliable computation extremely difficult. Even with IEEE arithmetic, results may vary widely depending on code optimization and vectorization. [R. Hammer, IMACS Am. Comput. Appl. Math. 7, 467-482 (1990); D. Ratz, ibid. 499-514 (1990)].