BCSLIB-EXT Iterative Solver is an industrial strength preconditioned iterative solver for large sparse matrices derived from PDE or PDE systems. The scalar version of the solver is designed to operate on general sparse matrices without regard to matrix structure. The block version of the iterative solver is designed specifically to take advantage of block structure in matrices, such as arises in solutions of partial differential equation system, where several variables are being solved at each node. Neither the matrix structure, not matrix values need be symmetric; in fact, this solver is designed specifically as a non-symmetric matrix solver. This solver has been successfully used in several National Labs and industrial firms.