Page table data for each page within a memory address space includes a write permission flag and a dirty-bit-modifier flag. The write permission flag is initialised to a value indicating that write access is not permitted. When a write access occurs, then the dirty-bit-modifier flag indicates whether or not the action of the write permission flag may be overridden. If the action of the write permission flag may be overridden, then the write access is permitted and the write permission flag is changed to indicate that write access is thereafter permitted. A page for which the write permission flag indicates that writes are permitted is a dirty page.