MEMORY MANAGEMENT

Memory
Monoprogramming without Swapping or Paging
Monoprogramming with fixed partitions
Swapping
Virtual Memory

MEMORY

Memory is the electronic holding place for instructions and data that the computer's microprocessor can reach quickly. When the computer is in normal operation, its memory usually contains the main parts of the operating system and some or all of the application programs and related data that are being used. Memory is often used as a shorter synonym for random access memory (RAM). This kind of memory is located on one or more microchips that are physically close to the microprocessor in the computer. Most desktop and notebook computers sold today include at least 16 megabytes of RAM, and are upgradeable to include more. The more RAM you have, the less frequently the computer has to access instructions and data from the more slowly accessed hard disk form of storage.
Memory is sometimes distinguished from storage, or the physical medium that holds the much larger amounts of data that won't fit into RAM and may not be immediately needed there. Storage devices include hard disks, floppy disks, CD-ROM, and tape backup systems. The terms auxiliary storage, auxiliary memory, and secondary memory have also been used for this kind of data repository.
Additional kinds of integrated and quickly accessible memory are read-only memory (ROM), programmable ROM (PROMO), erasable programmable ROM (EPROM). These are used to keep special programs and data, such as the basic input/output system, that need to be in the computer all the time.


The memory is a resource that needs to be managed carefully. Most computers have a memory hierarchy, with a small amount of very fast, expensive, volatile cache memory, some number of megabytes of medium-speed, medium-price, volatile main memory (RAM), and hundreds of thousands of megabytes of slow, cheap, non-volatile disk storage. It is the job of the operating system to coordinate how these memories are used.
The part of the operating system that manages the memory hierarchy is the memory manager. It keeps track of parts of memory that are in use and those that are not in use, to allocate memory to processes when they need it and de-allocate it when they are done, and to manage swapping between main memory and disk when main memory is too small to hold all the processes.
Systems for managing memory can be divided into two categories: the system of moving processes back and forth between main memory and disk during execution (known as swapping and paging) and the process that does not do so (that is, no swapping and ping).

Monoprogramming without Swapping or Paging

The most simple memory management scheme is to run one program at a time, sharing the memory between that program and the operating system. As shown in the diagram bellow, there are three variations of this type. The operating system may be at the bottom of the memory in TAM (random access memory), as shown in the diagram 1, or it may be in ROM (read-only memory) at the top of the memory, as shown in diagram 2, or the device drivers may be at the top of the memory in a ROM and the rest of the system in RAM down bellow

With the system organised in this way, only one process at a time can be running. As soon as the user types a command, the operating system copies the request program from a disk to memory and executes it. When the process finishes, the operating system displays a prompt character and waits for a new command. When it receives the command, it loads a new program into memory overwriting the first one.

Monoprogramming with fixed partitions

It is often desirable to allow multiple processes to run at the same time, even on simple operating systems in which multiprogramming is sometimes used. On time-sharing systems, having multiple processes in memory at once means that when one process is blocked waiting for the I/O to finish, another one can use the CPU. This way, multiprogramming increases the CPU utilisation. It is however preferable to be able to run two or more programs at once even on personal computers.

Swapping

The process of organising memory into fixed partitions on batch system is simple compared to time sharing systems or graphically oriented personal computers. On batch systems, each job is loaded into a partition when it gets to the head of the queue. It stays in memory until it has finished. As long as enough jobs can be kept in memory to keep the CPU busy all the time, there is no reason to use anything more complicated. On time-sharing machines, sometimes there is not enough main memory to hold all the currently active processes, therefore excess processes must be kept on disk and brought in to run dynamically.
Swapping as an approach to memory management consists of bringing each process in its entirety, running it for a while, then putting it back on the disk. For details and illustrations, see OS design and implementation, A.S. Tanenbaum & A.S. Woodhull, Prentice Hall 1997, pg.310.


Virtual Memory

Another strategy for managing memory is the virtual memory, which allows programs to run even when they are only partially in main memory. The basic idea behind this strategy is that the combine size of the program, data, and stack may exceed the amount of physical memory available for it. The operating system keeps those parts of the program currently in use in main memory, and the rest on the disk.

Virtual memory can also work in a multiprogramming system, with bits and pieces pf many programs in memory at once. While a program is waiting for a part of itself to be brought in, it is waiting for I/O and cannot run, so the CPU can be given to another process, the same way as for any other multiprogramming system. Most virtual memory systems use a technique called paging.