  A shared read/write register emulation provides the illusion of shared-memory
on top of message-passing models. The main hurdle with such emulations is
dealing with server faults in the system. Several crash-tolerant register
emulations in static systems require algorithms to replicate the value of the
shared register onto a majority of servers. Majority correctness is necessary
for such emulations. Byzantine faults are considered to be the worst kind of
faults that can happen in any distributed system. Emulating a
Byzantine-tolerant register requires replicating the register value on to more
than two-thirds of the servers. Emulating a register in a dynamic system where
servers and clients can enter and leave the system and be faulty is harder than
in static systems. There are several crash-tolerant register emulations for
dynamic systems. This paper presents the first emulation of a multi-reader
multi-writer atomic register in a system that can withstand nodes continually
entering and leaving, imposes no upper bound on the system size and can
tolerate Byzantine servers. The algorithm works as long as the number of
servers entering and leaving during a fixed time interval is at most a constant
fraction of the system size at the beginning of the interval, and as long as
the number of Byzantine servers in the system is at most f. Although our
algorithm requires that there be a constant known upper bound on the number of
Byzantine servers, this restriction is unavoidable, as we show that it is
impossible to emulate an atomic register if the system size and maximum number
of servers that can be Byzantine in the system is unknown to the nodes.
