Requests that map to the same mcGroup are never reordered. Reordering between the mcGroup instances is controlled with the ORDERING parameter. When set to "NORM," reordering is enabled and the arbiter implements a round-robin priority plan, selecting in priority order among the mcGroups with a command that is safe to issue to the SDRAM.
The timing of when it is safe to issue a command to the SDRAM can vary on the target bank or bank group and its page status. This often contributes to reordering.
When the ORDERING parameter is set to "STRICT," all requests have their CAS commands issued in the order in which the requests were accepted at the native interface. STRICT ordering overrides all other controller mechanisms, such as the tendency to coalesce read requests, and can therefore degrade data bandwidth usage in some workloads.