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“Memory Management

Function

Contents  Basic memory management  Swapping  Virtual memory  Paging

Page replacement algorithmsModeling page replacement algorithmsDesign issues for paging systemsImplementation issues

^ Segmentation

Memory Management ^ Ideally programmers want memory that is^ ^ large^ ^ fast^ ^ non volatile ^ Memory hierarchy^ ^ small amount of fast, expensive memory – cache^ ^ some medium-speed, medium price main memory^ ^ gigabytes of slow, cheap disk storage ^ Memory manager handles the memory hierarchy

Multiprogramming with Fixed

Partitions ^ Fixed memory partitions^ ^ separate input queues for each partition^ ^ single input queue

Names and Binding

^ Symbolic names

^ Logical names

^ Physical names

○^ Symbolic Names: known in a context or path^ ^ file names, program names, printer/device names, user names ○^ Logical Names: used to label a specific entity^ ^ job number, major/minor device numbers, process id (pid), uid. ○^ Physical Names: address of entity^ ^ inode address on disk or memory^ ^ entry point or variable address^ ^ PCB address

Address Binding

^ Address binding^ ^ fixing a physical address to the logical address of a process’address space ^ Compile time binding^ ^ if program location is fixed and known ahead of time ^ Load time binding^ ^ if program location in memory is unknown until run-timeAND location is fixed ^ Execution time binding^ ^ if processes can be moved in memory during execution^ ^ Requires hardware support

Dynamic relocation with a base

register

^ Memory Management Unit (MMU) - dynamicallyconverts logical addresses into physical address ^ MMU contains base address register for running process

process

i Operatingsystem

Max addr^0

Max Mem

0 Physical memoryaddress

Relocation register for process

i^1000 +^ MMU Program generated address

Protection using base & limit

registers

^ Memory protection^ ^ Base register gives starting address for process^ ^ Limit register limits the offset accessible from therelocation register

base +

Physicaladdress^

memory register limitregister yes< no addressing^ error logicaladdress

Features of Memory Management^ ^ Relocation

1. Static2. Dynamic ^ Protection ^ Sharing ^ Logical organization ^ Physical organization ^ Memory Compaction

Various Methods used by OS  Swapping  Virtual Memory  Paging  Segmentation

Swapping

^ Allocating space for growing data segment ^ Allocating space for growing stack & data segment

Memory Management with Bit Maps^ 

Part of memory with 5 processes, 3 holes^ ^ tick marks show allocation units^ ^ shaded regions are free  Corresponding bit map  Same information as a list

Managing memory with linked listsSearching the list for space for a new process^ ^ First Fit^ ^ Next Fit

○^ Start from current location in the list  Best Fit ○^ Find the smallest hole that will work ○^ Tends to create lots of really small holes  Worst Fit ○^ Find the largest hole ○^ Remainder will be big  Quick Fit ○^ Keep separate lists for common sizes

Virtual Memory

^ Virtual Memory^ ○

Separation

of^ user

logical

memory

from

physical

memory. ○ Only^ PART

of the program needs to be in memory for execution. ○ Logical address space can be much larger than physicaladdress space. ○ Need to allow pages to be swapped in and out. ^ Virtual Memory can be implemented via^ ^ Paging^ ^ Segmentation