System providing multiple outstanding information requests

In a system which includes a common bus to which a plurality of units are connected for the transfer of information, such as a data processing system, information may be transferred during asynchronously generated information transfer cycles. Logic is provided for enabling a first unit to transfer first information to a second unit during a first request transfer cycle requesting that the second unit transfer second information to the first unit during a later first response transfer cycle. Logic is also provided to enable the first unit to transfer third information to a third unit during a second request transfer cycle requesting that the third unit transfer fourth information to the first unit during a later second response transfer cycle. Logic is provided that enable the first unit to transfer the third information before receiving the second information and logic is further provided that enables the first unit to receive the second information before or after receiving the fourth information. Logic is provided for enabling any other unit to communicate over the common bus during the time between the first request transfer cycle and the latest transfer cycle associated with the transfer of the second and fourth information.