In one aspect of the present invention, a circuit is provided which implements an instruction set architecture defining a first instruction group, a second instruction group to enter a high-reliability mode of operation, and a third instruction group to enter a non-high-reliability mode of operation. The circuit includes means for causing the circuit to enter the high-reliability mode of operation in response to receiving the second instruction group; means for causing the circuit to enter the non-high-reliability mode of operation in response to receiving the third instruction group; first execution means for executing the first instruction group in the high-reliability mode of operation if the circuit is in the high-reliability mode of operation; and second execution means for executing the first instruction group in the non-high-reliability mode of operation if the circuit is in the non-high-reliability mode of operation.