What are various Memory Management Requirements in OS?

 

Memory Management Requirements:

 

There are so many methods, policies for memory management. To observe these methods suggest five requirements.

 

1. Relocation
2. Protection
3. Sharing
4. Logical organization
5. Physical organization

 

   1.   Relocation:

 

Relocation is a mechanism to convert the logical address into physical address. An address generated by CPU is said to be logical address and an address generated by Memory Manager is said to be physical address.

 

Physical address = Contents of Relocation Register + Logical Address

 

Relocation is necessary at the time of swap in a process from one backing store to main memory. Almost all times, the process occupies the same location at the time of swap in. But, sometimes it is not possible; as such the relocation is required. Consider the following fig:

Fig: Relocation

    2. Protection:

 

The word protection means provide security from unauthorized usage of memory. An OS can protect the memory with the help of a base and limit registers. Base registers consist of the starting address of the next process. The Limit registers specifies the boundary of that job. The Limit register is also said to be fencing register. Consider the following fig:

Fig: Main Memory Protection

The base register holds the smallest legal physical memory address; the limit register contains the size of the process. Consider the below figure. It depicts the hardware protection mechanism with base and limit registers.

 

Fig: Memory protection with Base and Limit

If the logical address is greater than the contents of a base register, it is authorized access, otherwise; it is a trap to operating system. If the physical address (logical + base) is less than the base limit, it causes no problems; otherwise it is a trap to the operating system.

3.    Sharing:

 

Generally, protection mechanisms are required at the time of access to the same portion of main memory by several processes. Accessing the same portion of main memory by a number of processes is said to be “sharing”. Suppose, to say if number of processes are executing same program, it is advantageous to allow each process to access same copy of the program rather than it having its own copy. If each process maintains a separate copy of that program, a lot of main memory will be wasted.

 

For ex: 3 users want to take their resumes using word processor. Then the 3 users share the word processor in the server; in the place of individual copy of word processor. Consider the following fig. that depicts the sharing of word processor.

Fig: Memory sharing

4.    Logical Organization:

 

We know that main memory in a computer system is organized as a linear or one dimensional address space that consists of a sequence of bytes or words. Secondary memory, at its physical level, is similarly organized. Although this organization closely mirrors the actual machine hardware, it does not correspond to the way in which programs are typically constructed.

 

Most programs are organized into modules, some of which are modifiable (read only, execute only) and some of which contain data that may be modified. If the operating system and computer hardware can effectively deal with user programs and data in the form of modules of some sort, then a number of advantages can be realized.

 5.    Physical Organization:

 

A computer memory is organized into at least two levels: main memory and secondary memory. Main memory provides fast access at a relatively high cost. In addition, main memory is volatile i.e. it does not provide permanent storage. Secondary memory is slower and cheaper than main memory and it is usually not volatile. Thus, secondary memory of large capacity can be provided to allow for long-term storage of programs and data, while a smaller main memory holds programs and data currently in use.