System for arbitrating communication requests using multi-pass control

Abstract

A system control unit (SCU), adapted to operating a plurality of central processor units (CPUs) in a parallel fashion in combination with at least one input/output (I/O) unit and for allowing the CPUs and I/O units to controllably access address segments of a system memory, arbitrates communication requests received at the SCU ports from the CPUs and I/O units in such a manner that available system resources are optimally used, while at the same time guaranteeing that all requests are granted within a reasonable period of time. Incoming communication requests are stored, and from there these incoming communication requests are selected, on the basis of a pre-defined prioritizing scheme, commands corresponding to requests that are to be arbitrated. For the command corresponding to each request selected for being arbitrated, there is generated a first vector defining all system resources that are required for executing the command. A second vector is generated representative of all system resources that are in fact available at the time of arbitration. The first and second vectors are compared, and the selected command is honored if all corresponding required resources are available; otherwise, the request corresponding to the command is placed on reserved status, and the availability of corresponding required resources for the reserved request is rechecked, and the reserved request is subsequently honored by executing the associated command when corresponding reserved resources become available.