A multiprocessor computer system has a multiplicity of sub-systems and a main memory coupled to a system controller. Some of the sub-systems are data processors, each having a respective cache memory that stores multiple blocks of data and a respective set of master cache tags (Etags), including one Etag for each data block stored by the cache memory. Each data processor includes an interface for sending memory transaction requests to the system controller and for receiving cache transaction requests from the system controller corresponding to memory transcation requests by other ones of the data processors. The system controller includes transaction activation logic for activating each said memory transaction request when it meets predefined activation criteria, and for blocking each said memory transaction request until the predefined activation criteria are met. An active transaction status table stores status data representing memory transaction requests that have been activated, including an address value for each activated transaction. The transaction activation logic includes comparator logic for comparing each memory transaction request with the active transaction status data for all activated memory transaction requests so as to detect whether activation of a particular memory transaction request would violate the predefined activation criteria. With certain exceptions concerning writeback transactions, an incoming transaction for accessing a data block that maps to the same cache line a pending, previously activated transaction, will be blocked until the pending transaction that maps to the same cache line is completed.