Memory Management
In these tutorials, we will go through another important function of the Operating System – the memory management.
Memory is central to the operation of a modern computer system. Memory is a large array of words or bytes, each location with its own address. Interaction is achieved through a sequence of reads/writes of specific memory address. The CPU fetches from the program from the hard disk and stores in memory. If a program is to be executed, it must be mapped to absolute addresses and loaded into memory.
In a multiprogramming environment, in order to improve both the CPU utilisation and the speed of the computer’s response, several processes must be kept in memory.
There are many different algorithms depending on the particular situation to manage the memory. Selection of a memory management scheme for a specific system depends upon many factors, but especially upon 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:
• Keep track of which parts of memory are currently being used and by whom.
• Decide which processes are to be loaded into memory when memory space becomes available.
• Allocate and deallocate memory space as needed.
In the multiprogramming environment operating system dynamically allocates memory to multiple processes. Thus memory plays a significant role in the important aspects of computer system like performance, S/W support, reliability and stability.
Memory can be broadly classified into two categories–the primary memory (like cache and RAM) and the secondary memory (like magnetic tape, disk etc.). The memory is a resource that needs effective and efficient management. The part of OS that perform this vital task of memory management is known as memory manager.
In multiprogramming system, as available memory is shared among number of processes, so the allocation speed and the efficient memory utilisation (in terms of minimal overheads and reuse/relocation of released memory block) are of prime concern. Protection is difficult to achieve with relocation requirement, as location of process and absolute address in memory is unpredictable. But at run-time, it can be done. Fortunately, we have mechanisms supporting protection like processor (hardware) support that is able to abort the instructions violating protection and trying to interrupt other processes.
These tutorials collectively depicts such memory management related responsibilities in detail by the OS. Further we will discuss, the basic approaches of allocation are of two types:
Contiguous Memory Allocation: Each programs data and instructions are allocated a single contiguous space in memory.
Non-Contiguous Memory Allocation: Each programs data and instructions are allocated memory space that is not continuous. This unit focuses on contiguous memory allocation scheme.