A dirty memory subsystem includes storage operable to store redundant copies
of
dirty indicators. Each dirty indicator is associated with a respective block of
main memory and is settable to a predetermined state to indicate that the block
of main memory associated therewith has been dirtied. By providing redundant storage
for the dirty indicators, any difference between the stored copies of the dirty
indicators can be considered as indicative of memory corruption, for example as
a result of a cosmic ray impact. As the different copies can be stored in different
locations, it is unlikely that a cosmic ray impact would affect all copies equally.
If a difference between the stored copies is detected, then the dirty indicator
can be take as being unreliable and remedial action can be taken. For example,
it can be assumed that a block of main memory has been dirtied if any of the copies
of the dirty indicator has the predetermined state. Such a memory finds application
in a method of managing reinstatement of an equivalent memory state in the main
memory of a plurality of processing sets of a fault tolerant computer following
a lock step error.