cs552-s10-memory_management

cs552-s10-memory_management - Memory Management Click to...

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Click to edit Master subtitle style 1/20/11 Memory Management
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1/20/11 Background Program must be brought (from disk) into memory and placed within a process for it to be run Main memory and registers are only storage CPU can access directly Register access in one CPU clock (or less)
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1/20/11 Many Ways to Do Memory Management! We will cover a variety of them. Many of them are not mutually exclusive, and can be hybridized in various ways. What you need to learn is to get some idea of the possibilities so that when you encounter a specific one, you will be able to grasp it quickly.
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1/20/11 The Problem There is a finite amount of RAM on any machine. There is a bus with finite speed, non- zero latency. There is a CPU with a given clock speed. There is a disk with a given speed. How do we manage memory so that
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1/20/11 Base and Limit Registers Put every process at a different location in memory. A pair of base and limit registers define the logical
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1/20/11
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1/20/11 Logical vs. Physical Address Space The concept of a logical address space that is bound to a separate physical address space is central to proper memory management. Logical address – generated by the CPU; also referred to as virtual address.
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1/20/11 Memory-Management Unit (MMU) Hardware device that maps virtual to physical address. In a simple MMU scheme, the value in the relocation register is added to every address generated by a user process at the time it is sent to memory. The user program deals with
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1/20/11 Simple MMU.
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1/20/11 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
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1/20/11 Schematic View of Swapping
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1/20/11 Contiguous Allocation Main memory usually into two partitions: Resident operating system, usually held in low memory with interrupt vector. User processes then held in high memory. Relocation registers used to protect user processes from
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1/20/11 Hardware Support for Relocation and Limit Registers
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1/20/11 Multiple-partition allocation Hole – block of available memory; holes of various size are scattered throughout memory When a process arrives, it is OS process 5 process 8 process 2 OS process 5 process 2 OS process 5 process 2 OS process 5 process 9 process 2 process 9 process 10
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1/20/11 Dynamic Storage-Allocation Problem First-fit : Allocate the first hole that is big enough Best-fit : Allocate the smallest hole that is big enough; must search
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cs552-s10-memory_management - Memory Management Click to...

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