Method and apparatus for storing prior versions of modified values to facilitate reliable execution

1. Field of the Invention One embodiment of the present invention provides a system that facilitates reliable execution in a computer system by keeping track of write operations to a main memory of the computer system in order to undo the write operations if necessary. This system operates by receiving a write operation directed to the main memory at a memory controller, wherein the write operation includes data to be written to the main memory and a write address specifying a location in the main memory into which the data is to be written. Next, the system examines a log bit associated with the write address, wherein the log bit indicates whether an existing value from the write address in main memory has been copied to a checkpoint store. If the log bit is not set, the system creates a new entry for the write address in the checkpoint store; retrieves an existing value from the write address in the main memory; and stores the existing value to the new entry in the checkpoint store. The system then stores the data to be written to write address in the main memory. The system also periodically performs a checkpointing operation, which clears all entries from the checkpoint store.