History fifo with bypass wherein an order through queue is maintained irrespective of retrieval of data

Abstract

An apparatus and method for maintaining a circular FIFO (first-in, first-out) queue in an I/O (input-output) subsystem of a computer system such as a server, workstation, or storage machine. The queue is coupled to a bypass circuit, used to provide access to data items out of the order in which they were stored in the queue, thus bypassing the latency inherent in retrieving the items from the queue. Control logic maintains write and read pointers indicating locations in the queue for writing and reading data items. The write pointer is incremented upon every data event to the queue, thereby maintaining a history of data that has been written to the queue, which is useful for diagnostic purposes. A history flag is maintained to indicate whether the write pointer has wrapped around the addresses in the queue, indicating whether all data items in the queue are valid for diagnostic purposes.