1. An apparatus for ordering operations and data received by a first bus having a first ordering policy in accordance with a second ordering policy which is different from the first ordering policy, and for transferring the ordered data on a second bus having the second ordering policy, the apparatus comprising: a plurality of execution units for storing operations and executing the transfer of data between the first and second buses, each one of the execution units being assigned to a group which represents a class of operations wherein at least one first unit for a first class of operation is assigned to a single first class operation at a time and at least one second unit for a second class of operation is assigned to a single second class operation at a time; intra prioritizing means, for each group, for prioritizing the stored operations in accordance with the second ordering policy exclusive of the operations stored in the other groups; and inter prioritizing means for determining which one of the prioritized operations can proceed to execution in accordance with the second ordering policy.