(Processor Resource/Systems Manager) Hardware circuits and microcode built into IBM System/390 mainframes that support logical partitions (LPARs). LPARs allow the running of multiple system images on a single processor complex. Each such image has a full complement of CPUs (dedicated or shared), central storage, expanded storage and channels. ESCON and FICON channels may be shared across images using the MIF feature. Pronounced "priz-em," PR/SM was derived from VM, the IBM operating system that provides multiple system images in software. See LPAR, MIF and VM.