Sorry That is not a complete answer but i hope it can help you .
"One of the most important aspects of operating systems is the ability
to multiprogram. A single program cannot, in general, keep either the CPU
or the I/O devices busy at all times. Single users frequently have multiple
programs running. Multiprogramming increases CPU utilization by organizing
jobs (code and data) so that the CPU always has one to execute.
The idea is as follows: The operating system keeps several jobs in memory
simultaneously (Figure 1.9). Since, in general, main memory is too small to
accommodate all jobs, the jobs are kept initially on the disk in the job pool.
This pool consists of all processes residing on disk awaiting allocation of main
The set of jobs in memory can be a subset of the jobs kept in the job
pool. The operating system picks and begins to execute one of the jobs in
memory.Eventually, the job may have to wait for some task, such as an I/O
operation, to complete. In a non-multiprogrammed system, the CPU would sit
idle. In a multiprogrammed system, the operating system simply switches to,
and executes, another job. When that job needs to wait, the CPU switches to
another job, and so on. Eventually, the first job finishes waiting and gets the
CPU back. As long as at least one job needs to execute, the CPU is never idle.
This idea is common in other life situations. A lawyer does not work for
only one client at a time, for example. While one case is waiting to go to trial
or have papers typed, the lawyer can work on another case. If he has enough
clients, the lawyer will never be idle for lack of work. (Idle lawyers tend to
become politicians, so there is a certain social value in keeping lawyers busy.) Multiprogrammed systems provide an environment in which the various
system resources (for example, CPU, memory, and peripheral devices) are
utilized effectively, but they do not provide for user interaction with the
computer system. Time sharing (or multitasking) is a logical extension of
multiprogramming. In time-sharing systems, the CPU executes multiple jobs
by switching among them, but the switches occur so frequently that the users
can interact with each program while it is running."
"Time sharing and multiprogramming require that several jobs be kept
simultaneously in memory. If several jobs are ready to be brought into memory,
and if there is not enough room for all of them, then the system must choose
among them. Making this decision involves job scheduling."
'Having several programs in memory at
the same time requires some form of memory management'
Operating System Concepts Essentials -Abraham Silberschatz -second edition  section [1.4]