Abstract: An apparatus has tag checking circuitry responsive to a target address to: identify a guard tag stored in a memory system in association with a block of one or more memory locations, the block containing a target memory location identified by the target address, perform a tag check based on the guard tag and an address tag associated with the target address, and in response to detecting a mismatch in the tag check, perform an error response action. The apparatus also has tag mapping storage circuitry to store mapping information indicative of a mapping between guard tag values and corresponding address tag values. The tag checking circuitry remaps at least one of the guard tag and the address tag based on the mapping information stored by the tag mapping storage circuitry to generate a remapped tag for use in the tag check.