PVSio is a PVS package that extends the ground evaluator with a predefined library of imperative programming language features such as side effects, unbounded loops, input/output operations, floating point arithmetic, exception handling, pretty printing, and parsing. The PVSio library is implemented via semantic attachments. PVSio is extensible as it provides a high level interface for writing user-defined semantic attachments. It also provides a simpler Emacs interface and a stand alone Emacs-free interface to the ground evaluator. Finally, PVSio includes proof rules that safely integrates the ground evaluator into the PVS theorem prover. These proof rules use the efficient Common Lisp code generated by the ground evaluator to simplify ground expressions in sequent formulas.