1. A computing device, comprising: an input bridge, which is coupled to receive program instructions; one or more system outputs; an output bridge, having a first state in which output is allowed to pass through the output bridge from the processing core to the one or more system outputs, and a second state in which the output bridge is stalled in a manner inhibiting output from the processing core to the one or more system outputs; a processing core, which is coupled to receive the program instructions from the input bridge and execute the program instructions, wherein the program instructions include both program instructions which are capable of outputting signals through the output bridge and program instructions that do not send data to the one or more system outputs, and wherein the processing core can execute program instructions that do not send data to the one or more system outputs both when the output bridge is in the first state and when the output bridge is in the second state; and authentication logic, which is coupled to receive and authenticate the program instructions while the processing core executes the program instructions, and to entirely inhibit output of signals through the output bridge until it has been determined that the received program instructions that are capable of outputting signals through the output bridge have been authenticated.