In a set of registers, each individually addressable by register
operations using a corresponding register identification, at least one
register of the set of registers is an extended register having multiple
storage locations. Values stored in the multiple storage locations are
accessed, for example, according to the order in which they have been
stored. Less than all of the multiple storage locations are accessible by
a register operation at a given time. Older versions of software that do
not recognize extended registers identify the extended register as having
only one storage location. An extended register can be, for example, a
stack register, a queue register, or a mixed register and values stored
in the multiple storage locations are read and stored according to the
characteristics of the register.