PCG: A software package for the iterative solution of linear systems on scalar, vector and parallel computers. The PCG package is a software system for solving systems of linear equations by means of preconditioned conjugate gradient (PCG)-type iterative methods on a variety of computer architectures. The software is designed to give high performance with a nearly identical user interface across different scalar, vector and parallel platforms, as well as across different programming models, such as shared-memory, data-parallel and message-passing programming interfaces. The basic features of the package are discussed, as well as techniques used to attain portability and high performance. Representative results for several computers are given