OS II-Lec 6 memory management

MEMORY MANAGEMENT
- In a uniprogramming system, main memory is divided into two parts: one part for the operating system (resident monitor, kernel) and one part for the program currently being executed.
- In a multiprogramming system, the "user" part of memory must be further subdivided to accommodate multiple processes. The task of subdivision is carried out dynamically by the operating system and is known as memory management.
Effective memory management is vital in a multiprogramming system. If only a few processes are in memory, then for much of the time all of the processes will be waiting for I/O and the processor will be idle. Thus memory needs to be allocated to ensure a reasonable supply of ready processes to consume available processor time.
7.1 MEMORY MANAGEMENT REQUIREMENTS
While surveying the various mechanisms and policies associated with memory management, it is helpful to keep in mind the requirements that memory management is intended to satisfy. [LIST93] suggests five requirements:
o Relocation?o Protection?o Sharing?o Logical organization o Physical organization
Relocation
In a multiprogramming system, the available main memory is generally shared among a number of processes. Typically, it is not possible for the programmer to know in advance which other programs will be resident in main memory at the time of execution of his or her program. In addition, we would like to be able to swap active processes in and out of main memory to maximize processor utilization by providing


a large pool of ready processes to execute. Once a program has been swapped out to disk, it would be quite limiting to declare that when it is next swapped back in, it must be placed in the same main memory region as before. Instead, we may need to relocate the process to a different area of memory.
Thus, we cannot know ahead of time where a program will be placed, and we must allow that the program may be moved about in main memory due to swapping. These facts raise some technical concerns related to addressing, as illustrated in Figure 7.1. The figure depicts a process image. For simplicity, let us assume that the process image occupies a contiguous region of main memory. Clearly, the operating system will need to know the location of process control information and of the execution stack, as well as the entry point to begin execution of the program for this process. Because the operating system is