With respect to the basic concepts and abstractions provided by operating systems:

A. a key concept in all operating systems is the process; to provide a place to keep them, most operating systems have the concept of a directory as a way to group processes together.

B. to keep multiple programs from interfering with each other, each one has its own address space; and, most addresses are 128 bits long.

C. system calls are needed to create files, remove files, read files, and write files; and, every file within a directory hierarchy can be specified by giving its path name from the top of the directory hierarchy (the root directory), called a relative path.

D. files in UNIX are protected by assigning each one a 9-bit binary protection code, consisting of three 3-bit fields; the 3 bits are known as the rwx bits (read, write, or execute)