A computer system comprising: a system memory (14A, 14B); a plurality of active devices including a first active device, a second active device and a third active device, wherein each of said plurality of active devices is configured to access data stored in said system memory and includes a processing unit, a write-back cache configured to store data accessed by said active device and an interface that interfaces said active device to an address network for conveying address transactions between said active device and said system memory and to a data network for conveying data transactions between said active device and said system memory, wherein said address network serves as an ordering point for transactions involving a given cache block and implements an ordered broadcasts property, wherein: said first active device, in response to receipt of a read-to-own address transaction on said address network from said third active device requesting a writable copy of a cache block when said first active device has owner status and a read access right for said cache block, is operable to change its ownership status for said cache block from owner to non-owner and to send a corresponding data packet to said third active device on said data network and to change its access right for said cache block from said read access right to invalid; said second active device, in response to receipt of said read-to-own address transaction on said address network from said third active device to request a writable copy of a cache block when said second active device has non-owner status and a read access right for said cache block, is operable to change its access right for said cache block from said read access right to invalid; and said third active device, in response to-receipt of its own read-to-own address transaction on said address network, is operable to change its ownership status for said cache block from non-owner to owner and; in response receipt of said corresponding data packet from said first active device through said data network, is operable to change its access right to a write access right. a system memory (14A, 14B); a plurality of active devices including a first active device, a second active device and a third active device, wherein each of said plurality of active devices is configured to access data stored in said system memory and includes a processing unit, a write-back cache configured to store data accessed by said active device and an interface that interfaces said active device to an address network for conveying address transactions between said active device and said system memory and to a data network for conveying data transactions between said active device and said system memory, wherein said address network serves as an ordering point for transactions involving a given cache block and implements an ordered broadcasts property, wherein: said first active device, in response to receipt of a read-to-own address transaction on said address network from said third active device requesting a writable copy of a cache block when said first active device has owner status and a read access right for said cache block, is operable to change its ownership status for said cache block from owner to non-owner and to send a corresponding data packet to said third active device on said data network and to change its access right for said cache block from said read access right to invalid; said second active device, in response to receipt of said read-to-own address transaction on said address network from said third active device to request a writable copy of a cache block when said second active device has non-owner status and a read access right for said cache block, is operable to change its access right for said cache block from said read access right to invalid; and said third active device, in response to-receipt of its own read-to-own address transaction on said address network, is operable to change its ownership status for said cache block from non-owner to owner and; in response receipt of said corresponding data packet from said first active device through said data network, is operable to change its access right to a write access right. said first active device, in response to receipt of a read-to-own address transaction on said address network from said third active device requesting a writable copy of a cache block when said first active device has owner status and a read access right for said cache block, is operable to change its ownership status for said cache block from owner to non-owner and to send a corresponding data packet to said third active device on said data network and to change its access right for said cache block from said read access right to invalid; said second active device, in response to receipt of said read-to-own address transaction on said address network from said third active device to request a writable copy of a cache block when said second active device has non-owner status and a read access right for said cache block, is operable to change its access right for said cache block from said read access right to invalid; and to change its access right for said cache block from said read access right to invalid; and said third active device, in response to-receipt of its own read-to-own address transaction on said address network, is operable to change its ownership status for said cache block from non-owner to owner and; in response receipt of said corresponding data packet from said first active device through said data network, is operable to change its access right to a write access right.