Cache Architectures for Real-Time University of Embedded
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CACHES FOR
ES
UNIVE
k
me
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
RSITY OF MARYLAND
Cache Architecturesfor Real-TimeEmbedded Systems
Prof. Bruce Jacob
Electrical & Computer EngineeringUniversity of Maryland, College Par
OUTLINE:
• Cache Primer
• Memory Management Primer
• Caches & Embedded Systems
• Cache Architectures for Real-Ti
CACHES FOR
ES
M
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
What is a Cache?
Von Neumann Bottleneck
P
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M
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
What is a Cache?
P M
CACHE
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CK
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Cache Organizations
Fundamental Unit: CACHE BLO
Purpose: HOLD DATA
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REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Cache Organizations
...
A Simple Cache
PAGE 5
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REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Cache Organizations
ADDRESS
...
Cache Addressing Mechanism
DATA
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REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Cache Organizations
DATA OUT
ADDRESS
...
Cache Addressing Mechanism
DATA
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s
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Cache Organizations
DATA OUT
ADDRESS
...
Cache Addressing Mechanism
DATA
MODULO
block number
#blk
CACHE INDEX
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REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Cache Organizations
...
A Simple Cache
PAGE 9
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.
....
s)
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Cache Organizations
.....
..
A More Complex Cache(similar to having several cache
PAGE 10
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.
....
E)E SIZE)
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Cache Organizations
.....
..
A Single CACHE BLOCK (or LINParameter: BLOCK SIZE (or LIN
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.
....
nce class)
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Cache Organizations
.....
..
A Single CACHE SET (equivaleParameter: ASSOCIATIVITY
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.
....
AY)
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Cache Organizations
.....
..
A Single CACHE COLUMN (or W
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REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Cache Organizations
ADDRESS
Associative Lookup
DATA DATA
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REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Cache Organizations
DATA OUT
ADDRESS
Associative Lookup
DATA DATA
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REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Cache Organizations
DATA OUT
ADDRESS
The Purpose of TAGS
TAG DATA TAG DATA
(!CMP) (+CMP)
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ive
emory)
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Cache OrganizationsGiven 8 cache blocks ...
DirectMapped
2-WaySet Associative
4-WaySet Associat
8-Way Set Associative(Fully Associative, or Content-Addressable M
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DirectMapped
..
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Cache OrganizationsAssociativity vs. the Memory Space
n/4-WayAssoc.
n/2-WayAssoc.
FullyAssoc.
.
n = blocks in cache
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ESMORY
CEHES
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Memory Management
TRADITIONALLY:
The MANAGEMENT ofone’s USE of PHYSICAL ME
THIS TALK’S CONTEXT:
a DIFFERENT NAMESPAused for ADDRESSING CAC
i.e. VIRTUAL ADDRESSING
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PAGETABLE
(SOFTWARE)
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Memory ManagementWhat is a TLB?(Translation Lookaside Buffer)
APPLICATION
PAGETABLE
NAME SPACE
PHYSICALNAME SPACE
(HARDWARE)
MANY
APPLICATION
TLB
NAME SPACE
PHYSICALNAME SPACE
(HARDWARE
TODAY
CACHE)
MOONS AGO
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ed
ffset
CHE
ffset
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Cache AddressingPhysically Indexed, Physically Tagg
TLB
Virtual Page Number
ASID
Page O
Page Frame Number
Tag: Page Frame Number
Cache Data
Virtual Address
CA
Cache Index
Page Frame Number Page O
Physical Address
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d
HE
fset
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Cache AddressingPhysically Indexed, Virtually Tagge
CAC
Virtual Page Number Page Of
Virtual Page Number
Cache Index
Tag: Virtual Page Number
Cache Data
Virtual Address
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d
Offset
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Cache AddressingVirtually Indexed, Physically Tagge
CACHETLB
Virtual Page Number
ASID
Page
Page Frame Number
Cache Index
Tag: Page Frame Number
Cache Data
Virtual Address
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CHE
fset
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Cache AddressingVirtually Indexed, Virtually Tagged
CA
Virtual Page Number Page Of
Virtual Page Number
Cache Index
Tag: Virtual Page Number
Cache Data
Virtual Address
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set
ta)E
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
A Little More DetailVirtual Page Number Page Off
PFNVPN
TLB (CAM)Cache Block (DaP-Tag
DIRECT-MAPPED CACH
Match?
Pr
ProtectionViolation?
AND
CACHE DATA
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ESMORY
UCTURESY
helpent
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Memory Management
TRADITIONALLY:
The MANAGEMENT ofone’s USE of PHYSICAL ME
NEW DEFINITION:
The MANAGEMENT of ALL STRassociated with MEMOR
WHAT WE WILL SEE:
VIRTUAL ADDRESSING canSIMPLIFY memory managem
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RAM
requires
d RAMs
SRAM0
SRAM1
DRAM
IBUF
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Cache vs. Scratch-pad
MAIN DIFFERENCE: scratch-pad EXPLICIT MANAGEMENT
UNIFORMADDRESS
SPACE
Traditional Caches
NON-UNIFORMADDRESS
SPACE
Scratch-Pa
I-CACHE
D-CACHE
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Suck
ISM
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Why Traditional Caches
NON-DETERMIN
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or MISS?
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #1
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133ABABC01 3 34
Time
HIT
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #1
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133ABABC01 3 34
0
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #1
1
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0 133ABABC01 3 34
0
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #1
3
1
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 01 33ABABC01 3 34
0
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #1
3
1
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 013 3ABABC01 3 34
0
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #1
ASet 3:
Set 2:
Set 1:
Set 0:
Refs: 0133 ABABC01 3 34
3
10
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Bruce Jacob
University ofMaryland
C Summer ’99
Example #1
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133A BABC01 3 34
AB
10
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #1
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133AB ABC01 3 34
AB
10
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Bruce Jacob
University ofMaryland
C Summer ’99
Example #1
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133ABA BC01 3 34
AB
10
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #1
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133ABAB C01 3 34
AB
1C
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #1
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133ABABC 01 3 34
AB
10
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #1
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133ABABC0 1 3 34
AB
1C
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #1
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133ABABC01 3 34
A3
1C
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IT or MISS?
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #2
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133ABABC
45645601 3 3
Time
H
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #2
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133
0
ABABC
45645601 3 3
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #2
1
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0 133
0
ABABC
45645601 3 3
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #2
3
1
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 01 33
0
ABABC
45645601 3 3
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #2
3
1
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 013 3
0
ABABC
45645601 3 3
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #2
3
1
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133
4
ABABC
45645601 3 3
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #2
3
5
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133
4
ABABC
45645601 3 3
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #2
3
5
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133
4
ABABC
45645601 3 3
6
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #2
3
5
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133
4
ABABC
45645601 3 3
6
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #2
3
5
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133
4
ABABC
45645601 3 3
6
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #2
3
5
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133
4
ABABC
45645601 3 3
6
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #2
3
5
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133
0
ABABC
45645601 3 3
6
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #2
3
1
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133
0
ABABC
45645601 3 3
6
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #2
3
1
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133
0
ABABC
45645601 3 3
6
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IT or MISS?
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #3
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133ABABC
45645601 3 3
Time
H
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REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #3
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133ABABC
45645601 3 3
0
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REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #3
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0 133ABABC
45645601 3 3
10
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REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #3
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 01 33ABABC
45645601 3 3
3
10
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #3
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 013 3ABABC
45645601 3 3
3
10
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #3
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133ABABC
45645601 3 3
3
10
A
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #3
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133ABABC
45645601 3 3
3
10
BA
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #3
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133ABABC
45645601 3 3
3
10
BA
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REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #3
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133ABA BC
45645601 3 3
3
10
BA
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #3
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133ABAB C
45645601 3 3
3
10
B
C
A
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #3
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133ABABC
45645601 3 3
3
10
B
C
A
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #3
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133ABABC
45645601 3 3
3
10
B
C
A
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #3
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133ABABC
45645601 3 3
3
10
B
C
A
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #3
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133ABABC
45645601 3 3
3
10
B
C
A
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SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #3
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133ABABC
45645601 3 3
3
10
54
6
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HIT?or MISS?
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Example #3
Set 3:
Set 2:
Set 1:
Set 0:
Refs: 0133ABABC
45645601 3 3
3
10
–/B
5/–4/C
6/A
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nce)
t
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Traditional Caches
Require TAGS
Soon into program execution,contents of cache are indeterminate(thus the term “hit rate” for performa
Set associativity delays problems,but only to a point
Associativity > 2 does not implemenTRUE Least-Recently-Used
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in)
d RAMs
SRAM0
SRAM1
DRAM
IBUF
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Scratch-pad RAMs (aga
Traditional Caches Scratch-Pa
I-CACHE
D-CACHE
Require EXPLICIT MANAGEMENT
UNIFORMADDRESS
SPACE
NON-UNIFORMADDRESS
SPACE
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ent:
rved
CTS
sES
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Scratch-Pad RAMs
No TAGS (save die area)
As long as everything fits, GREAT!
Otherwise, addressing is impedim
CONTIGUITY must be prese
DISTANCE BETWEEN OBJEmust be preserved
DSPs go one step further:
Multiply-accumulate requireTWO DISJOINT DATA SPAC
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GONAL
ARD
ES
es)
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Scratch-Pad RAMs
Access to memory is NON-ORTHOSeparate spaces are DISJOINT
Bottom Line: COMPILATION IS H
Trend: UNIFORM ADDRESS SPAC
(i.e. more like traditional cach
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E
sse
CACHE
che Organization
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
WHAT WE WANT
Guaranteed slow access-time
CODE
Statistically fast access-time
Guaranteed fast access-time
DATA
AddressSpace
DATA
AddressSpace
COD
DATA
AddreSpac
CACHE
No Cache Traditional Cache Ideal Ca
CODE
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T
D LAYOUT
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
WHY IT’S DIFFICULT
DATA NAME => DATA PLACEMENMust Group Data & InstructionsSo as to Minimize Cache Conflicts
CODE
DATA
BAD LAYOUT GOO
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ESs ...
Mts)
easily
atter ...
...
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Data Placement
DATA SPACE
• Relatively easy to rearrange item
• ... Unless part of a LARGER ITE(cannot rearrange array elemen
CODE SPACE
• Can move FUNCTIONS around
• PORTIONS of code is another m
THERE IS A FAMILIAR SOLUTION
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OVERLAP:ds 1-way
O-WAY
assoc.
RLAP:ds 2-wayassoc.
EE-WAYRLAP:ds 3-wayassoc.
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Solution #1
A BIG, HIGHLY ASSOCIATIVE CAC+ ability to PIN DOWN CACHE LIN
CODE
DATA
NO Nee
TW
set
OVENeeset
THROVENeeset
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ent items
TION
ystems)
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Solution #1
• Choose items to cache,Bring each into the cache,Pin each down
• Can CACHE/NOT-CACHE adjac
• Must know CACHE ORGANIZAat COMPILE TIME(not huge issue for embedded s
• SIMPLEST, but perhapsMOST EXPENSIVE solution
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0
MAX
NOTCACHED
CACHED
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Solution #2 (var. on #1)
Software-Managed Caches
Top bits determinememory-access behavior(CACHED/NON-CACHED)
Other possibilities:
• Physical/virtual
• Faulting/non-faulting
• Which cache ormemory structure
Enables on-the-fly decision-makingRe: memory behavior
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/ YES; // NO
MAYBE
ENCE
0000
cache_it
;
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Application Behavior
int *array = malloc (N * sizeof int); /int *stream = malloc (N * sizeof int)int *mix = malloc (N * sizeof int); //
for (i=0; i<N; i++)x = array[i]; // CACHED REFER
stream |= MIN_NEG_INT; // 0x8000for (i=0; i<N; i++)
x = stream[i]; // NON-CACHED
for (i=0; i<N; i++) // DEPENDS ONx = (cache_it (i)) ? mix[i]
: (mix | MIN_NEG_INT)[i]
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ESISIONS
IZATION
CACHE
MENT ...
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Solution #2
Advantages over Solution #1:
Allows DYNAMIC CACHE DEC
LESS TIED to CACHE ORGAN
Many of the same weaknesses:
Requires BIG CACHE
Requires SET ASSOCIATIVE
Have to deal with DATA PLACE
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T
OVERLAP:ds 1-way
O-WAY
assoc.
RLAP:ds 2-wayassoc.
EE-WAYRLAP:ds 3-wayassoc.
TE DATA
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Issue: Data Placement
DATA NAME => DATA PLACEMEN
CODE
DATA
NO Nee
TW
set
OVENeeset
THROVENeeset
TO MINIMIZE OVERLAP, RELOCA
PAGE 84CACHES FOR
ESEMENT
n
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Issue: Data Placement
GOALS:
• Disassociate NAME and PLAC
• Fine-grained code/data relocatioat granularity of TLB pageor (better) cache line
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CACHES FOR
ES
D LAYOUT
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Enter Virtual Memory
Disassociates NAME from PLACE
Allows you to go from THIS:
CODE
DATA
BAD LAYOUT GOO
PAGE 86
CACHES FOR
ES
OD LAYOUT
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Enter Virtual Memory
Disassociates NAME from PLACE
... to THIS:
CODE
DATA
GOOD LAYOUT GO
TRANSLATIONADDRESS
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ES
ation
TCH-PAD
Page Offset
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Real-Time TLB Organiz
Works with either CACHE or SCRA
Physical Address
Page Number
Physical Page #SRAM Page #
Translation Lookaside Buffer:
PAGE withing SRAM
SEARCHONPHYS. PAGENUMBER
SRAM (Cache or Scratch-Pad RAM)
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CACHES FOR
ES
32 entries)
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Solution #3
Fully-Associative Real-Time TLB+ Direct-Mapped SRAM
• TLB must fully map SRAM(8KB SRAM, 256-byte page =>
• Can place ANY 256-byte pageANYWHERE in the SRAM
• Benefit: simple SRAM design
• Drawback: fully assoc. TLB
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ES
3
REAL-TIMEEMBEDDEDSYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Variations on Solution #
WANT A LARGER CACHE ?
• Larger TLB
• Larger Page Size
WANT A SMALLER TLB ?
• Smaller Cache
• Larger Page Size
WANT LESS ASSOCIATIVITY ?
• That’s a little more involved ...
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ES
s
DirectMapped
y.
1
1
1
1
1
1
1
1
...
n
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Set-Associative RT-TLBAssociativity vs. the Memory Space
2-WaAssoc
4-WayAssoc.
FullyAssoc.
...
n = entries in TLB
n
2
2
4
4
2
2...
...
n/2n/4
n/2-Way
n/2
n/2
Assoc.
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ES
s
blems
lifted
ORTH IT
ION
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Set-Associative RT-TLB
LIMITING CASE:
Direct-Mapped TLB
Direct-Mapped SRAM
Same set of data placement prowe had with NO TLB ...
EXCEPT: contiguity restriction
Bottom Line: PROBABLY NOT W
INTERMEDIATE SOLUTIONS:
Obvious Trade-Offs Exist
NEED MORE INVESTIGAT
PAGE 92
CACHES FOR
ES NFLICT)
PHYS. MEM
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Solution #4
What if SRAM Still Too Small?
(i.e. — previous solution reduces COproblems, not CAPACITY problems
Real-Time SRAM-Management
BROUGHT IN AT START
BROUGHT INON DEMAND
SRAM
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ES
ED
Y
TLB
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Real-Time SRAMManagement
CLASSIFY ALL CODE & DATA:
• MUST ALWAYS REMAIN CACH
• MUST NEVER BE CACHED
• EXHIBITS PERIODIC LOCALIT(i.e. loop code & data)
FOR PERIODIC ITEMS:
• Add code at beginning to set up
• Add code at end to unmap TLBand write out any dirty values
PAGE 94
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ES
aceaging
nce
xecution
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Real-Time SRAMManagement
RESULTS:
• VM-style extending of SRAM spinto DRAM space via demand-p
• PROACTIVE demand-paging,not REACTIVE demand-paging
• Deterministic memory performafor all references
• Slight overhead in code size & e
PAGE 95
CACHES FOR
ESory
S:
ormanceent)
ACES
r ...
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
Summary
UNIFORM MEMORY SPACES:
• Provide Orthogonal Look at Mem
• Cache Architectures ExhibitNon-Deterministic Performance
NON-UNIFORM MEMORY SPACE
• Non-Orthogonal Memory Map
• Caches Offer Deterministic Perf(at the Price of Explicit Managem
TREND IS TOWARD UNIFORM SP
• Easier to Program & Compile fo
PAGE 96
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ES
UNIVE
RES:
aches
TION
ast ...
blj/talks/
REAL-TIMEEMBEDDED
SYSTEMS
Bruce Jacob
University ofMaryland
C Summer ’99
RSITY OF MARYLAND
Summary, cont’d
REAL-TIME CACHE ARCHITECTU
• Really Big, Highly Associative C
• Software-Managed Caches
• Virtual Addressing w/ RT-TLB
• Real-Time SRAM Management
VIRTUAL MEMORY:
• Nice Programming Paradigm
• Separates NAMING from LOCA
• Like Tang ®, Not Just for Breakf
slides at http://www.ece.umd.edu/~
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