Methods and apparatus for placing a processing unit into one or more of a plurality of operational modes are disclosed wherein: the apparatus includes a local memory, a bus operable to carry information to and from the local memory, one or more arithmetic processing units operable to process data and operatively coupled to the local memory, and a security circuit operable to place the apparatus into the operational modes; and the plurality of operational modes includes a first mode whereby the apparatus and an external device may initiate a transfer of information into or out of the memory over the bus, a second mode whereby neither the apparatus nor the external device may initiate a transfer of information into or out of the memory over the bus, and a third mode whereby the apparatus may initiate a transfer of information into or out of the memory over the bus, but the external device may not initiate a transfer of information into or out of the memory over the bus.