As we have seen above,
Xinu processes all execute in the same address space,
and do not incur the overhead of a trap when making a system
call.
Such processes are called lightweight processes ( lwps),
since they have little associated state,
making context switches, process creation, and interprocess
communication relatively inexpensive.
These processes are to contrasted with Unix-like heavyweight processes
( hwps),
which run in separate address spaces and trap to the kernel to
make a system call.
Lightweight and heavyweight processes are complementary concepts in that one
can run multiple lightweight processes inside a heavyweight process.
In fact,
your assignments will be doing exactly this,
creating Xinu lwps within a Unix hwp.