A system is often affected by changes occurring outside the
system. Such changes are said to occur in the system
environment. In modeling a system, it is necessary to decide on the
boundary between the system and its environment.
E.g. When studying cache memory using simulation, one has to decide
where is the boundary of the system. It can be simply the CPU and
cache, or it can include main memory, disk, O.S., compiler, or even
user programs.
