MEMORY MANAGEMENT

MEMORY MANAGEMENT:

1.Introduction:

•  Program must be brought into memory and placed within a process for it to be run.
• Input queue – collection of processes on the disk that are waiting to be brought into memory to run the program.
• User programs go through several steps before being run. 

2.Address binding:

•  Compile time: If memory location known a priori, absolute code can be generated; must recompile code if starting location changes.
• Load time: Must generate relocatable code if memory location is not known at compile time.
• Execution time: Binding delayed until run time if the process can be moved during its execution from one memory segment to another. Need hardware support for address maps (e.g., base and limit registers).
• Address binding of instructions and data to memory addresses can happen at three different stages.

3.Dynamic loading:

• Routine is not loaded until it is called. Better memory-space utilization; unused routine is never loaded.
• Useful when large amounts of code are needed to handle infrequently occurring cases.
• No special support from the operating system is required implemented through program design.

4.Dynamic linking:

• Linking postponed until execution time.
• Small piece of code, stub, used to locate the appropriate memory-resident library routine.
• Stub replaces itself with the address of the routine, and executes the routine.
• Operating system needed to check if routine is in processes’ memory address.
• Dynamic linking is particularly useful for libraries.

5.Overlays:

• Keep in memory only those instructions and data that are needed at any given time.
• Needed when process is larger than amount of memory allocated to it.
• Implemented by user, no special support needed from operating system, programming design of overlay structure is complex.

6.Swapping:

• A process can be swapped temporarily out of memory to a backing store, and then brought back into memory for continued execution.
• Backing store – fast disk large enough to accommodate copies of all memory images for all users; must provide direct access to these memory images.
•  Roll out, roll in – swapping variant used for priority-based scheduling algorithms;
• lower-priority process is swapped out so higher-priority process can be loaded and executed.
• Major part of swap time is transfer time; total transfer time is directly proportional to the amount of memory swapped.
• Modified versions of swapping are found on many systems, i.e., UNIX, Linux, and Windows.