MEMORY MANAGEMENT

MEMORY MANAGEMENT

          Memory management is anxious with managing the primary memory. Memory contains of array of bytes or words each with their own address. The instructions are collected from the memory by the CPU based on the value program counter.

Functions of memory management:

* Keeping trace of status of each memory location.

* Determining the allocation policy. 

* Memory allocation technique. 

* De-allocation technique.

Address Binding:

          Programs are kept on the secondary storage disks as binary executable files. When the programs are to be implemented they are brought in to the main memory and placed within a process. The grouped of processes on the disk waiting to enter the main memory forms the input queue. One of the processes which are to be executed is collected from the queue and placed in the main memory. During the implementation it takes instruction and data from main memory. After the process terminates it yields back the memory space. During implementation the process will go through different steps and in each step the address is represented in different ways. In source program the address is symbolic. The compiler transfers the symbolic address to re-locatable address. The loader will change this re-locatable address to absolute address.

Binding of instructions and data can be finish at any step along the way:

Compile time:-If we know whether the process occupys in memory then absolute code can be generated. If the static address changes then it is required to re-compile the code from the beginning.

Load time:-If the compiler doesn’t know whether the process occupys in memory then it generates the re-locatable code. In this the binding is waited until the load time.

Execution time:-If the process is moved during its execution from one memory chunk to another then the binding is delayed until run time. Special hardware is used for this. Most of the general 

purpose operating system uses this method.

Logical versus physical address:

The address produced by the CPU is called logical address or virtual address. The address seen by the memory unit i.e., the one loaded in to the memory register is called the physical address. Compile time and load time address binding methods generate some logical and physical address. The implementation time addressing binding create different logical and physical address. Set of logical address space created by the programs is the logical address space. Set of physical address responding to these logical addresses is the physical address space. The mapping of virtual address to physical address during run time is done by the hardware device called memory management unit (MMU). The base register is also called re-location register. Value of the re-location register is added to every address generated by the user process at the time it is sent to memory.

The above figure shows that dynamic re-location which specifies mapping from virtual addresses space to physical address space and is performed by the hardware at run time. Re-location is performed by the hardware and is invisible to the user dynamic relocation makes it possible to move a partially implement process from one area of memory to another without affecting.

Dynamic Loading: 

For a process to be implemented it should be loaded in to the physical memory. The size of the process is restricted to the size of the physical memory. Dynamic loading is used to get better memory utilization. In dynamic loading the routine or steps will not be loaded until it is called. Whenever a routine is called, the calling routine first checks whether the called routine is already loaded or not. If it is not loaded it give raise to the loader to load the desired program in to the memory and updates the programs address table to indicate the change and control is moved to newly called routine.

Advantage: 

Gives better memory utilization. Unused routine is never loaded. Do not need specialoperating system support. This method is useful when large amount of codes are needed to handle in simultaneously occurring cases.

Dynamic linking and Shared libraries:

Some operating system supports only the static linking. In dynamic linking only the main program is packed in to the memory. If the main program asks for a procedure, the procedure is loaded and the link is established at the time of references. This linking is delayed until the execution time. With dynamic linking a “stub” is used in the image of each library referenced routine. A “stub” is a piece of code which is used to imply how to locate the appropriate memory resident library routine or how to load library if the routine is not already present. When “stub” is implement it checks whether the routine is present is memory or not. If not it packs the routine in to the memory. This feature can be used to update libraries i.e., library is restored by a new version and all the programs can make use of this library. More than one version of the library can be packed in memory at a time and each program uses its version of the library. Only the programs that are compiled with the new version are affected by the changes incorporated in it. Other programs connected before new version is installed will continue using older libraries this type of system is called “shared library”.