a method for memory checkpointing which does not involve copying of data and that is virtually instantaneous involves designating the active location at a checkpoint as being the backup for the checkpoint . until there is a new write operation after the checkpoint , this location is regarded as both the active and the backup location . at the first write operation after the checkpoint , the write is carried out to the other , previously non - designated location . control of writing and indicating of the status of the locations is carried out by updating two binary bits for each pair of locations , namely , a first modification indicator bit and a second role indicator bit . the modification bits are stored on a resettable ram for almost instantaneous checkpointing where the location granularity is that of a write operation . the method obviates the need for copying the contents of an active location at a checkpoint .