Virtual Memory
-Neha Jain
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Topics Covered
Introduction
Virtual Memory
Address Translation
Thrashing and Protection
References
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Introduction
Memory Hierarchy
CPU
Functional units
Registers
Internal cache
Memory Content: Cache  Main Mem 
Secondary Mem
Memory Parameters:
Access Time: increase with distance from CPU
Cost/Bit: decrease with distance from CPU
External Cache
Capacity: increase with distance from CPU
Main Memory
Secondary Memory
(Disk)
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Cache : use SRAM (faster and costly)
Main Memory : use DRAM (slower and
cheap)
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Principle of Locality
Temporal Locality : Item tend to be
referenced again soon
Spatial Locality : Nearby items tend to be
referenced again soon
Cache Memory
-Takes advantage of principle of locality
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Virtual Memory
A technique that uses main memory as a
“cache” for secondary storage
Basic Terminology




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Page : Fixed number of bytes recognized by
operating system
Page Fault : Occurs when accessed page is not in
main memory
Physical Address : Address in main memory
Virtual Address: Addresses that application
programmers deal with and are translated by MMU to
physical address
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MMU:
Memory
management
unit
Virtual
Address
Physical
address
Cache
Data
Data
Processor
Physical
address
Main
memory
Virtual Address
31 30 29 28 27 ……………....13 12 11………….…….…2 1 0
Virtual Page Number
Page Offset
Translation
29 28 27 ……………………...13 12 11………….…….…2 1 0
Physical page number
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Page Offset
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Physical Address
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Segmented Memory
Memory Management
Unit (MMU)
S1
x
Segment
Table
S3
S1
free
S4
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x
free
S2
S3
Physical Memory
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Mapping Segmented Address
Virtual Address
Physical Memory
Segment # Displacement
Segment Table
V prot limit base
Protection
Compare segment
Limit to displacement
+
datum
Physical
address
Segment# : specifies which segment to use( base address and length)
Displacement
: offset from base address
for actual memory access
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Page table for address
translation
Virtual page
number
Page table
Physical page or
Valid disk address
Physical memory
1
1
1
1
0
1
1
0
1
1
0
1
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Disk storage
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Page Tables
The page table ,PC and registers specifies the state or process of
a program
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Swap
Space : Space on hard diskELEC6200-001
for all the pages of a process
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Making Address Translation Fast
By Translation-lookaside Buffer
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TLBs and Caches
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Who handles page faults?
Operating System (in system software)
- Look up page table
- Choose a physical page to replace
- Bring referenced page from disk into
chosen physical page
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Thrashing is a disaster and it occurs
when program does not follow locality
principle and continuously swaps pages
from main memory to disk
Protection
- One of the way is by having different
physical addresses for a same virtual
address
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References
http://www.eng.auburn.edu/users/nelsovp/
courses/elec5200_6200/
http://www.eng.auburn.edu/users/agrawvd/
E5200/course.html
http://webster.cs.ucr.edu/AoA/Windows/HT
ML/MemoryArchitecturea3.html
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