An apparatus includes a first processing resource to execute a program code, and a second processing resource separate from the first processing resource. The program code includes an embedded execution unit. The execution unit, during execution of the program code, calculates a first security value for a part of the program code. The second processing resource runs a validation program. The validation program receives the first security value, checks the first security value against a second security value calculated from a corresponding part of a reference copy of the program code to obtain a check result, returns the check result to the execution unit. The execution unit performs a security-related action in response to a check result indicating a mismatch between the first security value and the second security value.