Computers today. Гурьянова В.И. - 26 стр.

       2. A multi-user system permits a number of different users to use the
computer at the same time, permits a number of terminals to be connected to one
computer. Each user has exclusive use of one terminal from which he or she
gives the computer instructions. When a user inputs an instruction from a termi-
nal, an answer comes back from the computer immediately.
       3. An interactive system permits a “conversation” between the user and
the CPU. This interaction may be displayed on a CRT (cathode ray tube, which
is similar to a television screen), or it may be printed out on paper by a teletype-
like printer.
       4. Time sharing means that when two or more jobs are done on the com-
puter at the same time, a small amount of CPU time is given to each job in suc-
cession. The computer does only a little of each job at a time. The operating sys-
tem has to keep track of how far along each job is and start the work from that
point when the job’s turn comes up. The rate at which a human being can feed
input into a computer is very slow compared to the rate at which the computer
can process it. In the time it takes the human to get his input typed in, the com-
puter can do other work and then get back to him. This means that any one user
does not have to worry about being made to wait very long, and it eliminates the
waste of CPU time that would occur without time sharing. Time sharing system
is sometimes abbreviated to TSS.
       5. Multi-programming concept is based on user’s jobs and computer
tasks. From the viewpoint of a computer (strictly speaking, a CPU), a task is a
work unit. From a user’s viewpoint, a work unit is a job (program) or a com-
mand. One job consists of one task or several tasks, and one task may invoke
other tasks in turn. A word “process” is used in the same sense as a task. A
multi-programming (or multi-job) system permits a single user to run several
programs (jobs) at the same time. A user can create another job before he com-
pletes the previous job.


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