A virtual copy of data stored in a first memory is created in a second
memory. Creating the virtual copy includes, in one embodiment, creating
first and second tables in memory each one of which comprises a plurality
of multibit entries. Each entry of the first table corresponds to a
respective memory region of the first memory. Each entry of the second
table corresponds to a respective memory region of the second memory. The
first bit of the first and second tables indicates whether the
corresponding memory region of the first and second memories,
respectively, contains valid data. The second bit of the first and second
tables indicates whether data in the corresponding memory region of the
first and second memories, respectively, has been modified since the
creation of the first and second tables, respectively.