Memory Management

Main memory is a large array of words or bytes, its own address. Main memory is a repository of quickly accessible data shared
by the CPU and I/O devices. The central processor reads instructions from main
memory during the instruction-fetch cycle and both reads and writes data from
main memory during the data-fetch cycle (on a Von Neumann architecture).
The main memory is generally the only large storage device that the CPU is able
to address and access directly. For example, for the CPU to process data from
disk, those data must first be transferred to main memory by CPU-generated
I/O calls. In the same way, instructions must be in memory for the CPU to
execute them.
For a program to be executed, it must be mapped to absolute addresses and
loaded into memory. As the program executes, it accesses program instructions
and data from memory by generating these absolute addresses. Eventually,
the program terminates, its memory space is declared available, and the next
program can be loaded and executed.
To improve both the utilization of the CPU and the speed of the computer's
response to its users, general-purpose computers must keep several programs
in memory, creating a need for memory management. Many different memorymanagement
schemes are used. These schemes reflect various approaches, and
the effectiveness of any given algorithm depends on the situation. In selecting a
memory-management scheme for a specific system, we must take into account
many factors—especially on the hardware design of the system. Each algorithm
requires its own hardware support.
The operating system is responsible for the following activities in connection
with memory management:

•  Keeping track of which parts of memory are currently being used and by whom
•  Deciding which processes (or parts thereof) and data to move into and out of memory
•  Allocating and deallocating memory space as needed