Supporting out-of-order operation using tags.

Some initiators, particularly out-of-order CPU cores, issue multiple outstanding reads and can do useful work as soon as any of them are serviced.

Some targets, particularly DRAM, can provide better performance by servicing requests out-of-order.

If we multiplex a pair of in-order busses onto a common bus, yet tag all of the traffic from each bus on the common bus according to its in-order initiator, we have a tagged, out-of-order bus.

The semantics are that for any given tag the requests and replies must be kept in order.

There is no difference between demultiplexing to separate in-order targets and a single target that understands tags. Hence the targets may be virtual.