Abstract: A method is provided for diagnostic checking of a variable memory 14 in a safety critical system in order to detect variable memory failures; wherein the safety critical system comprises a central processing unit (CPU) with an operating system, an internal volatile memory 12 and an external volatile memory 14 including the variable memory 14; and the CPU can access a plurality of address spaces including one or more address spaces of the external volatile memory 14 that are utilised by the operating system and/or by a safety critical application of the safety critical system during normal use of the safety critical system.