Method for implementing a checkpoint between pairs of memory locations using two indicators to indicate the status of each associated pair of memory locations

Claim 1

A method carried out by a processor for implementing a checkpoint between pairs of memory locations, the method comprising the steps of: (a) storing in a memory circuit a modification indicator associated with each pair of memory locations, each modification indicator indicating if a write operation has taken place to one of the pairs of memory locations since a previous checkpoint and wherein said memory circuit is resettable such that all modification indicators are simultaneously changed to a default value; (b) storing a role indicator associated with each said pair of memory locations, each role indicator indicating status of the associated pair of memory locations when read in combination with the associated modification indicator, each said role indicator being stored in a second memory circuit separate from the resettable memory circuit storing the modification indicators; (c) implementing a current checkpoint for all memory locations by resetting the resettable memory circuit so that all modification indicators have said default value, the role indicators being unchanged at a checkpoint, the combination of role and modification indicators for each pair of locations thereby indicating: if a location has been modified since the previous checkpoint, that said location now has active and backup status and that the other location of its pair of memory locations is now non-designated, and if neither location has been modified since the previous checkpoint, that the status of both locations is unchanged, and (d) for each memory location pair, at a first write operation to a location of said pair after the current checkpoint: updating the associated modification indicator to indicate that a write operation has occurred to one of the memory locations; updating the associated role indicator so that in combination with the modification indicator said role indicator indicates that said non-designated location is changed to active status and that the other location is changed to backup status; and writing data to the active location and subsequently writing data to said active location until the next checkpoint.

Claim 2

A method carried out by a processor for implementing a checkpoint between pairs of memory locations, the method comprising the steps of: (a) storing in a memory circuit a modification indicator associated with each pair of memory locations, each modification indicator indicating if a write operation has taken place to one of the pairs of memory locations since a previous checkpoint and wherein said memory circuit is resettable such that all modification indicators are simultaneously changed to a default value; (b) storing a role indicator associated with each said pair of memory locations, each role indicator indicating status of the associated pair of memory locations when read in combination with the associated modification indicator, each said role indicator being stored in a second memory circuit separate from the resettable memory circuit storing the modification indicators; (c) implementing a current checkpoint for all memory locations by resetting the resettable memory circuit so that all modification indicators have said default value, the role indicators being unchanged at a checkpoint, the combination of role and modification indicators for each pair of locations thereby indicating: if a location has been modified since the previous checkpoint, that said location now has active and backup status and that the other location of its pair of memory locations is now non-designated, and if neither location has been modified since the previous checkpoint, that the status of both locations is unchanged, and (d) for each memory location pair, at a first write operation to a location of said pair after the current checkpoint: updating the associated modification indicator to indicate that a write operation has occurred to one of the memory locations; updating the associated role indicator so that in combination with the modification indicator said role indicator indicates that said non-designated location is changed to active status and that the other location is changed to backup status; and writing data to the active location and subsequently writing data to said active location until the next checkpoint.