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memory management - part 3 (9)

memory management - part 3 (9) - COP 4600 Summer 2011...

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COP 4600: Intro To OS (Memory Management – Part 3) Page 1 © Dr. Mark Llewellyn COP 4600 – Summer 2011 Introduction To Operating Systems Memory Management – Part 3 Department of Electrical Engineering and Computer Science Computer Science Division University of Central Florida Instructor : Dr. Mark Llewellyn [email protected] HEC 236, 407-823-2790 http://www.cs.ucf.edu/courses/cop4600/sum2011
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COP 4600: Intro To OS (Memory Management – Part 3) Page 2 © Dr. Mark Llewellyn Memory Management Memory Management Methods Contiguous Allocation Non-Contiguous Allocation Single Partition Multiple Partition Fixed Allocation Dynamic Allocation Segmentation Paging "Basic" Paging Demand Paging (Virtual Memory)
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COP 4600: Intro To OS (Memory Management – Part 3) Page 3 © Dr. Mark Llewellyn Virtual Memory The memory management schemes that we have seen so far are necessary because of one basic requirement: The instructions being executed by the processor must be in the physical memory of the machine. The first approach to meeting this requirement is to place the entire logical address space (i.e., the entire process) into the main memory. Comparing simple paging and simple segmentation, on one hand, with fixed and dynamic partitioning, on the other, we have the foundations for a more sophisticated memory management system. There are two characteristics of paging and segmentation which are key: 1. All memory references within a process are logical addresses that are dynamically translated into physical addresses at run time. This means that a process can be swapped in and out of main memory such that it occupies different regions of main memory at different times during its lifetime. 2. A process may be broken up into a number of chunks (either pages or segments) and these chunks need not be contiguously located in main memory during execution. The combination of dynamic run-time address translation and the use of a page or segment table permits this.
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COP 4600: Intro To OS (Memory Management – Part 3) Page 4 © Dr. Mark Llewellyn Virtual Memory (cont.) Putting these two key elements together allows us to realize that it is not necessary that all of the pages or segments of a process be in main memory during execution. If the page or segment that holds the next instruction to be fetched and the page or segment that holds the next data location to be access are both in main memory, then at least for a time, the execution may proceed. The ability to execute a program that is only partially in main memory provides many benefits, some of which are: A program is no longer constrained in size by the amount of physical memory. Programmers could write programs for a very large virtual address space . The degree of multiprogramming can be increased since each user program could take less physical memory.
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