FSM, Cache memory

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FSM, Cache memory

• Prof. Sin-Min Lee
• Department of Computer Science

CS147 Lecture 12
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The Five Classic Components of a Computer
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The Processor Picture
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Processor/Memory Bus
PCI Bus
I/O Busses
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Using T Flip Flop and JK Flip Flop

• log24 2, so 2 flip flops are needed to
  implement this FSA

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Step 1 - Translate diagram into StateTable

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Step 2 - Create maps for T and JK

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Step 3 - Determine T, J, and K equations

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Step 4 - Draw resulting diagram

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Implementing FSM with No Inputs Using D, T, and
JK Flip Flops

• Convert the diagram into a chart

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Implementing FSM with No Inputs Using D, T, and
JK Flip Flops Cont.

• For D and T Flip Flops

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Implementing FSM with No Inputs Using D, T, and
JK Flip Flops Cont.

• For JK Flip Flop

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Implementing FSM with No Inputs Using D, T, and
JK Flip Flops Cont.

• Final Implementation

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von NeumannArchitecturePrinceton
Memory
Address Pointer
Arithmetic Logic Unit (ALU)
Data/Instructions
Pc Pc 1
Program Counter
Featuring Deterministic Execution
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Cache Memory

• Physical memory is slow (more than 30 times
  slower than processor)
• Cache memory uses SRAM chips.
• Much faster
• Much expensive
• Situated closest to the processor
• Can be arranged hierarchically
• L1 cache is incorporated into processor
• L2 cache is outside

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Cache Memory

• This photo shows level 2 cache memory on the
  Processor board, beside the CPU

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Cache Memory- Three LevelsArchitecture
Memory Multi- Gigabytes Large and Slow 160 X
Cache Control Logic
2 Gigahertz Clock
8X
2X
16X L3 Cache Memory
L2 Cache Memory
L1 Cache Memory
32 Kilobytes
128 Kilobytes
16 Megabytes
Featuring Really Non-Deterministic Execution
Address Pointer
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Cache (1)

• Is the first level of memory hierarchy
  encountered once the address leaves the CPU
• Since the principle of locality applies, and
  taking advantage of locality to improve
  performance is so popular, the term cache is now
  applied whenever buffering is employed to reuse
  commonly occurring items
• We will study caches by trying to answer the four
  questions for the first level of the memory
  hierarchy

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Cache (2)

• Every address reference goes first to the cache
• if the desired address is not here, then we have
  a cache miss
• The contents are fetched from main memory into
  the indicated CPU register and the content is
  also saved into the cache memory
• If the desired data is in the cache, then we have
  a cache hit
• The desired data is brought from the cache, at
  very high speed (low access time)
• Most software exhibits temporal locality of
  access, meaning that it is likely that same
  address will be used again soon, and if so, the
  address will be found in the cache
• Transfers between main memory and cache occur at
  granularity of cache lines or cache blocks,
  around 32 or 64 bytes (rather than bytes or
  processor words). Burst transfers of this kind
  receive hardware support and exploit spatial
  locality of access to the cache (future access
  are often to address near to the previous one)

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Where can a block be placed in Cache? (1)

• Our cache has eight block frames and the main
  memory has 32 blocks

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Where can a block be placed in Cache? (2)

• Direct mapped Cache
• Each block has only one place where it can appear
  in the cache
• (Block Address) MOD (Number of blocks in cache)
• Fully associative Cache
• A block can be placed anywhere in the cache
• Set associative Cache
• A block can be placed in a restricted set of
  places into the cache
• A set is a group of blocks into the cache
• (Block Address) MOD (Number of sets in the cache)
• If there are n blocks in the cache, the placement
  is said to be n-way set associative

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How is a Block Found in the Cache?

• Caches have an address tag on each block frame
  that gives the block address. The tag is checked
  against the address coming from CPU
• All tags are searched in parallel since speed is
  critical
• Valid bit is appended to every tag to say whether
  this entry contains valid addresses or not
• Address fields
• Block address
• Tag compared against for a hit
• Index selects the set
• Block offset selects the desired data from the
  block
• Set associative cache
• Large index means large sets with few blocks per
  set
• With smaller index, the associativity increases
• Full associative cache index field is not
  existing

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Which Block should be Replaced on a Cache Miss?

• When a miss occurs, the cache controller must
  select a block to be replaced with the desired
  data
• Benefit of direct mapping is that the hardware
  decision is much simplified
• Two primary strategies for full and set
  associative caches
• Random candidate blocks are randomly selected
• Some systems generate pseudo random block
  numbers, to get reproducible behavior useful for
  debugging
• LRU (Last Recently Used) to reduce the chance
  that information that has been recently used will
  be needed again, the block replaced is the
  least-recently used one.
• Accesses to blocks are recorded to be able to
  implement LRU

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What Happens on a Write?

• Two basic options when writing to the cache
• Writhe through the information is written to
  both, the block in the cache an the block in the
  lower-level memory
• Write back the information is written only to
  the lock in the cache
• The modified block of cache is written back into
  the lower-level memory only when it is replaced
• To reduce the frequency of writing back blocks on
  replacement, an implementation feature called
  dirty bit is commonly used.
• This bit indicates whether a block is dirty (has
  been modified since loaded) or clean (not
  modified). If clean, no write back is involved

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There are three methods in block placement
Direct mapped if each block has only one place
it can appear in the cache, the cache is said to
be direct mapped. The mapping is usually (Block
address) MOD (Number of blocks in cache) Fully
Associative if a block can be placed anywhere
in the cache, the cache is said to be fully
associative. Set associative if a block can
be placed in a restricted set of places in the
cache, the cache is said to be set associative .
A set is a group of blocks in the cache. A block
is first mapped onto a set, and then the block
can be placed anywhere within that set. The set
is usually chosen by bit selection that is,
(Block address) MOD (Number of sets in cache)
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•                                                   
                                           

• A pictorial example for a cache with only 4
  blocks and a memory with only 16 blocks.

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• Direct mapped cache A block from main memory can
  go in exactly one place in the cache. This is
  called direct mapped because there is direct
  mapping from any block address in memory to a
  single location in the cache.

cache
Main memory
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• Fully associative cache A block from main
  memory can be placed in any location in the
  cache. This is called fully associative because a
  block in main memory may be associated with any
  entry in the cache.

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Memory/Cache Related Terms

• Set associative cache The middle range of
  designs between direct mapped cache and fully
  associative cache is called set-associative
  cache. In a n-way set-associative cache a block
  from main memory can go into n (n at least 2)
  locations in the cache.

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Locality of Reference

• If location X is access, it is very likely that
  location X1 will be accessed.
• Benefit of the cache with data blocks

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Current CPUs
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Replacing Data

• Initially all valid bits are set to 0
• As instructions and data are fetched from memory,
  the cache is filling and some data need to be
  replaced.
• Which ones?
• Direct mapping obvious

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Replacement Policies for Associative Cache

• FIFO - fills from top to bottom and goes back to
  top. (May store data in physical memory before
  replacing it)
• LRU replaces the least recently used data.
  Requires a counter.
• Random

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Replacement in Set-Associative Cache

• Which if n ways within the location to replace?
• FIFO
• Random
• LRU

Accessed locations are D, E, A
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Writing Data

• If the location is in the cache, the cached value
  and possibly the value in physical memory must
  be updated.
• If the location is not in the cache, it maybe
  loaded into the cache or not (write-allocate and
  write-noallocate)
• Two methodologies
• Write-through
• Physical memory always contains the correct value
• Write-back
• The value is written to physical memory only it
  is removed from the cache

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Cache Performance

• Cache hits and cache misses.
• Hit ratio is the percentage of memory accesses
  that are served from the cache
• Average memory access time
• TM h TC (1- h)TP

Tc 10 ns Tp 60 ns
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Page Replacement - FIFO

• FIFO is simple to implement
• When page in, place page id on end of list
• Evict page at head of list
• Might be good? Page to be evicted has been in
  memory the longest time
• But?
• Maybe it is being used
• We just dont know
• FIFO suffers from Beladys Anomaly fault rate
  may increase when there is more physical memory!

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FIFO vs. Optimal

• Reference string ordered list of pages accessed
  as process executes
• Ex. Reference String is A B C A B D A D B C B
• OPTIMAL
• A B C A B D A D B C B

System has 3 page frames
5 Faults
toss A or D
A B C D A B C
FIFO A B C A B D A D B C B
toss ?
7 faults
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Least Recently Used (LRU)

• Replace the page that has not been used for the
  longest time

3 Page Frames Reference String - A B C A B D A
D B C
LRU 5 faults
A B C A B D A D B C
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LRU

• Past experience may indicate future behavior
• Perfect LRU requires some form of timestamp to be
  associated with a PTE on every memory reference
  !!!
• Counter implementation
• Every page entry has a counter every time page
  is referenced through this entry, copy the clock
  into the counter.
• When a page needs to be changed, look at the
  counters to determine which are to change
• Stack implementation keep a stack of page
  numbers in a double link form
• Page referenced move it to the top
• No search for replacement