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COMP 310:
Operating Systems
Lecture 14:
Memory Management
October 6, 2004
Christine Alvarado
Today’s Topics
Begin Memory Management
Virtual memory:
Fixed Partitions
Variable Partitions
Paging
Segmentation
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Memory Management - Lecture Slides | COMP 310, Study notes of Operating Systems
Material Type: Notes; Class: Operating Systems; Subject: Computer Science; University: University of San Diego; Term: Fall 2004;
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COMP 310:
Operating Systems
Lecture 14:
Memory Management
October 6, 2004
Christine Alvarado
Today’s Topics
Begin Memory Management
Virtual memory:
Fixed Partitions Variable Partitions Paging Segmentation
What does the OS have to manage? Programs: Processes, Threads, Synchronization I/O: Hardware, Interrupts, Direct Memory Access Memory/Disk: Virtual Memory, Paging, Naming Memory Management
Goals of Memory Management
To provide convenient abstraction for programming To maximize performance given scarce memory To protect processes and their memory
Mechanisms
Physical and virtual addressing Techniques: Partitioning, paging, segmentation Page table management, TLBs, VM tricks
Policies
Page replacement algorithms
Physical Memory: To Run a Program… Memory My Program Physical Memory: To Run a Program Memory My Program
Physical Memory: Multi- processing Memory My Program Your Program Multi-programming and Memory
Want multiple processes in memory at once
Why?
Multi-programming changes memory access
strategy
Don’t let processes access memory directly
OS allows processes to THINK they have direct
access… but they really don’t
How can we implement VM?
Fixed Partitions
Paging
Segmentation
Fixed Partitions
Physical Memory broken up into (surprise) Fixed
Partitions, all same size
My program’s memory is all in one place, but I don’t exactly know where it starts P P P P2’s Base Offset Base Register
Advantages
Easy to implement, fast context switch
P
P
P
P2’s Base Offset Base Register
Disadvantages
Internal fragmentation: I may not use all of
my space
P
P
P
P2’s Base Offset Base Register
Disadvantages
External Fragmentation: empty holes in memory
P
P
P
P2’s Base Offset Base Register < + P2’s limit Limit Register Recap
Internal Fragmentation
External Fragmentation
It’s a trade-off between the two
Paging
Paging uses fixed size units for physical and
virtual memory.
A single job might get more than one unit
P
Page 1 Page 3 Page 2 Page N Virtual Memory Physical Memory Paging
Mapping from VM to PM invisible to program
Must provide protection:
Program can’t access memory outside its VM
space.
Page Table Illustration Physical memory Page frame Offset Physical Address Page number Offset Virtual Address Page frame Page Table Paging Example
Pages are 4K
VPN is 20 bits (2^20 VPNs), offset is 12 bits
Virtual Address is 0x
Page number is?
Offset is?
Look up page number in page table to get
page frame (let’s say it’s 0x16000)
What is the PA?
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