A computing device (10) includes a trusted execution environment (TEE) manager (40) that manages a switchover from non-trusted software (116) to trusted software (118). The TEE manager (40) includes memory (90) configured to store password-bearing, immediate-operand instructions (54). At the point of switching between the non-trusted software (116) and the trusted software (118) the memory (90) may be accessed as instruction fetches, and its contents fetched into a CPU core (24) as instructions. Immediate-operand portions (60) of the immediate-operand instructions (54) provide passwords, which are written back into guess registers (80) within the TEE manager (40). When a predetermined relationship between the instructions (54) and guesses in guess registers (80) is identified, actual execution of the immediate-operand instructions (54) is verified, the TEE mode of operation is signaled, and security-sensitive hardware (44) is enabled for use by a privileged routine (42) portion of the trusted software (118).