On systems of this type it is necessary to trap into the
kernel to send or receive a message. The network interface uses
physical addresses to access main memory on the node
. This simplifies the design of
the network interface, but requires access in supervisor mode to
prevent user applications from destroying or reading portions of main
memory that must be protected. Protecting the network interface
is also necessary to prevent sending of data to arbitrary nodes in
the system. This architectural model is the most common. The Intel
Paragon, even though it has two and three CPUs on each node, can be
operated in this manner under the Puma operating system.