CS 152 Computer Architecture and
Engineering
Lecture 9 - Virtual Memory
Krste Asanovic
Electrical Engineering and Computer Sciences
University of California at Berkeley
http://www.eecs.berkeley.edu/~krste
http://inst.eecs.berkeley.edu/~cs152
February 21, 2012
CS152, Spring 2012
Last time in Lecture 9
• Protection and translation required for
multiprogramming
– Base and bounds was early simple scheme
• Page-based translation and protection avoids need
for memory compaction, easy allocation by OS
– But need to indirect in large page table on every access
• Address spaces accessed sparsely
– Can use multi-level page table to hold translation/protection
information, but implies multiple memory accesses per reference
• Address space access with locality
– Can use “translation lookaside buffer” (TLB) to cache address
translations (sometimes known as address translation cache)
– Still have to walk page tables on TLB miss, can be hardware or
software talk
• Virtual memory uses DRAM as a “cache” of disk
memory, allows very cheap main memory
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Memory Management
• Can separate into orthogonal functions:
– Translation (mapping of virtual address to physical address)
– Protection (permission to access word in memory)
– Virtual memory (transparent extension of memory space
using slower disk or flash storage)
• But most modern systems provide support for all the
above functions with a single page-based system
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Modern Virtual Memory Systems
Illusion of a large, private, uniform store
Protection & Privacy
OS
several users, each with their private
address space and one or more
shared address spaces
page table  name space
Demand Paging
Provides the ability to run programs
larger than the primary memory
useri
Primary
Memory
Swapping
Store
Hides differences in machine
configurations
The price is address translation on
each memory reference
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VA
mapping
TLB
PA
4
Hierarchical Page Table
31
22 21
p1
0
12 11
p2
offset
10-bit 10-bit
L1 index L2 index
offset
Root of the Current
Page Table
Physical Memory
Virtual Address
p2
p1
(Processor
Register)
Level 1
Page Table
page in primary memory
page in secondary memory
Level 2
Page Tables
PTE of a nonexistent page
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Data Pages
5
Page-Based Virtual-Memory Machine
(Hardware Page-Table Walk)
Page Fault?
Page Fault?
Protection violation?
Virtual
Physical
Address
Address
PC
Inst.
TLB
Inst.
Cache
Protection violation?
Virtual
Physical
Address
Address
D
Decode
E
+
Data
TLB
M
Data
Cache
W
Miss?
Miss?
Page-Table Base
Register
Physical
Address
Hardware Page
Table Walker
Memory Controller
Physical
Address
Physical Address
Main Memory (DRAM)
• Assumes page tables held in untranslated physical memory
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Address Translation:
putting it all together
Virtual Address
Restart instruction
hardware
hardware or software
software
TLB
Lookup
miss
hit
Protection
Check
Page Table
Walk
 memory
the page is
Page Fault
(OS loads page)
 memory
Update TLB
denied
Protection
Fault
permitted
Physical
Address
(to cache)
SEGFAULT
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Page Fault Handler
• When the referenced page is not in DRAM:
– The missing page is located (or created)
– It is brought in from disk, and page table is updated
Another job may be run on the CPU while the first job waits for
the requested page to be read from disk
– If no free pages are left, a page is swapped out
Pseudo-LRU replacement policy
• Since it takes a long time to transfer a page
(msecs), page faults are handled completely in
software by the OS
– Untranslated addressing mode is essential to allow
kernel to access page tables
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Handling VM-related exceptions
PC
Inst
TLB
Inst.
Cache
TLB miss? Page Fault?
Protection violation?
D
Decode
E
+
M
Data
TLB
Data
Cache
W
TLB miss? Page Fault?
Protection violation?
• Handling a TLB miss needs a hardware or software
mechanism to refill TLB
• Handling a page fault (e.g., page is on disk) needs a
restartable exception so software handler can resume
after retrieving page
– Precise exceptions are easy to restart
– Can be imprecise but restartable, but this complicates OS software
• Handling protection violation may abort process
– But often handled the same as a page fault
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Address Translation in CPU Pipeline
PC
Inst
TLB
Inst.
Cache
TLB miss? Page Fault?
Protection violation?
D
Decode
E
+
M
Data
TLB
Data
Cache
W
TLB miss? Page Fault?
Protection violation?
• Need to cope with additional latency of TLB:
– slow down the clock?
– pipeline the TLB and cache access?
– virtual address caches
– parallel TLB/cache access
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Virtual-Address Caches
CPU
VA
PA
TLB
Physical
Cache
Primary
Memory
Alternative: place the cache before the TLB
VA
CPU
Virtual
Cache
TLB
PA
Primary
Memory (StrongARM)
• one-step process in case of a hit (+)
• cache needs to be flushed on a context switch unless address
space identifiers (ASIDs) included in tags (-)
• aliasing problems due to the sharing of pages (-)
• maintaining cache coherence (-) (see later in course)
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Virtually Addressed Cache
(Virtual Index/Virtual Tag)
Virtual
Address
Virtual
Address
PC
Inst.
Cache
Miss?
D
Decode
E
+
M
Data
Cache
W
Miss?
Inst.
TLB
Page-Table Base
Register
Hardware Page
Table Walker
Physical
Address
Instruction data
Memory Controller
Data
TLB
Physical
Address
Physical Address
Main Memory (DRAM)
Translate on miss
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Aliasing in Virtual-Address Caches
VA1
Page Table
Data Pages
PA
VA2
Two virtual pages share
one physical page
Tag
Data
VA1
1st Copy of Data at PA
VA2
2nd Copy of Data at PA
Virtual cache can have two
copies of same physical data.
Writes to one copy not visible
to reads of other!
General Solution: Prevent aliases coexisting in cache
Software (i.e., OS) solution for direct-mapped cache
VAs of shared pages must agree in cache index bits; this
ensures all VAs accessing same PA will conflict in directmapped cache (early SPARCs)
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Concurrent Access to TLB & Cache
(Virtual Index/Physical Tag)
VA
VPN
L
TLB
PA
PPN
b
k
Page Offset
Tag
=
hit?
Virtual
Index
Direct-map Cache
2L blocks
2b-byte block
Physical Tag
Data
Index L is available without consulting the TLB
cache and TLB accesses can begin simultaneously!
Tag comparison is made after both accesses are completed
Cases: L + b = k, L + b < k, L + b > k
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Virtual-Index Physical-Tag Caches:
Associative Organization
VA
VPN
TLB
PA
PPN
a
L = k-b
k
Virtual
Index
2a
b
Direct-map
2L blocks
Direct-map
2L blocks
Phy.
Tag
Page Offset
=
Tag
hit?
=
2a
Data
After the PPN is known, 2a physical tags are compared
How does this scheme scale to larger caches?
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Concurrent Access to TLB & Large L1
The problem with L1 > Page size
Virtual Index
VA
VPN
a
Page Offset
b
TLB
PA
PPN
Page Offset
L1 PA cache
Direct-map
VA1 PPNa
Data
VA2 PPNa
Data
b
=
Tag
hit?
Can VA1 and VA2 both map to PA ?
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CS152 Administrivia
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A solution via
CPU
RF
Second Level Cache
L1
Instruction
Cache
Memory
Unified L2
Cache
L1 Data
Cache
Memory
Memory
Memory
Usually a common L2 cache backs up both
Instruction and Data L1 caches
L2 is “inclusive” of both Instruction and Data caches
• Inclusive means L2 has copy of any line in either L1
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Anti-Aliasing Using L2: MIPS R10000
Virtual Index
VA
VPN
TLB
PA
PPN
a
Page Offset
b
into L2 tag
Page Offset
VA1 PPNa
Data
VA2 PPNa
Data
b
PPN
Tag
• Suppose VA1 and VA2 both map to PA and VA1
is already in L1, L2 (VA1  VA2)
• After VA2 is resolved to PA, a collision will be
detected in L2.
• VA1 will be purged from L1 and L2, and VA2 will
be loaded  no aliasing !
February 21, 2012
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L1 PA cache
Direct-map
PA
=
a1
hit?
Data
Direct-Mapped L2
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Anti-Aliasing using L2 for a Virtually
Addressed L1
VA
VPN
Page Offset
Virtual
Index & Tag
b
TLB
PA
PPN
Tag
Page Offset
February 21, 2012
VA2
Data
“Virtual
Tag”
Physical
Index & Tag
CS152, Spring 2012
Data
L1 VA Cache
b
Physically-addressed L2 can also be
used to avoid aliases in virtuallyaddressed L1
VA1
PA
VA1
Data
L2 PA Cache
L2 “contains” L1
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Atlas Revisited
• One PAR for each physical page
PAR’s
• PAR’s contain the VPN’s of the pages
resident in primary memory
PPN
• Advantage: The size is proportional to
the size of the primary memory
VPN
• What is the disadvantage ?
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Hashed Page Table:
Approximating Associative Addressing
VPN
d
Virtual Address
Page Table
PID
hash
Offset
+
PA of PTE
Base of Table
VPN PID PPN
• Hashed Page Table is typically 2 to 3 times
larger than the number of PPN’s to reduce
collision probability
• It can also contain DPN’s for some nonresident pages (not common)
• If a translation cannot be resolved in this table
then the software consults a data structure
that has an entry for every existing page (e.g.,
full page table)
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VPN PID DPN
VPN PID
Primary
Memory
22
Power PC: Hashed Page Table
VPN
hash
d
Offset
80-bit VA
+
PA of Slot
Page Table
VPN
VPN
PPN
Base of Table
•
•
•
•
Each hash table slot has 8 PTE's <VPN,PPN> that are
searched sequentially
If the first hash slot fails, an alternate hash function is
used to look in another slot
All these steps are done in hardware!
Hashed Table is typically 2 to 3 times larger than the
number of physical pages
The full backup Page Table is a software data structure
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Primary
Memory
23
VM features track historical uses:
• Bare machine, only physical addresses
– One program owned entire machine
• Batch-style multiprogramming
– Several programs sharing CPU while waiting for I/O
– Base & bound: translation and protection between programs (not virtual
memory)
– Problem with external fragmentation (holes in memory), needed occasional
memory defragmentation as new jobs arrived
• Time sharing
– More interactive programs, waiting for user. Also, more jobs/second.
– Motivated move to fixed-size page translation and protection, no external
fragmentation (but now internal fragmentation, wasted bytes in page)
– Motivated adoption of virtual memory to allow more jobs to share limited
physical memory resources while holding working set in memory
• Virtual Machine Monitors
– Run multiple operating systems on one machine
– Idea from 1970s IBM mainframes, now common on laptops
» e.g., run Windows on top of Mac OS X
– Hardware support for two levels of translation/protection
» Guest OS virtual -> Guest OS physical -> Host machine physical
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Virtual Memory Use Today - 1
• Servers/desktops/laptops/smartphones have full
demand-paged virtual memory
–
–
–
–
Portability between machines with different memory sizes
Protection between multiple users or multiple tasks
Share small physical memory among active tasks
Simplifies implementation of some OS features
• Vector supercomputers have translation and protection
but rarely complete demand-paging
• (Older Crays: base&bound, Japanese & Cray X1/X2: pages)
– Don’t waste expensive CPU time thrashing to disk (make jobs fit in
memory)
– Mostly run in batch mode (run set of jobs that fits in memory)
– Difficult to implement restartable vector instructions
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Virtual Memory Use Today - 2
• Most embedded processors and DSPs provide physical
addressing only
– Can’t afford area/speed/power budget for virtual memory support
– Often there is no secondary storage to swap to!
– Programs custom written for particular memory configuration in
product
– Difficult to implement restartable instructions for exposed architectures
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Acknowledgements
• These slides contain material developed and
copyright by:
–
–
–
–
–
–
Arvind (MIT)
Krste Asanovic (MIT/UCB)
Joel Emer (Intel/MIT)
James Hoe (CMU)
John Kubiatowicz (UCB)
David Patterson (UCB)
• MIT material derived from course 6.823
• UCB material derived from course CS252
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