Computer Architecture

Memory organization

Types of Memory

Cache Memory• Serves as a buffer for frequently accessed data

• Small High Cost

RAM (Main Memory)• Stores programs and data that the computer needs when executing a

program

• Dynamic RAM (DRAM) • Uses Tiny Capacitors

• Needs to be recharged every few milliseconds to keep the stored data

• Static RAM (SRAM)• Holds its data as long as the power is on

• D Flip Flop

Types of Memory (Cont.) ROM

• Stores critical information necessary to operate the system.

• Hardwired can not be programmed

Programmable Read Only Memory (PROM)• Can be programmed once using appropriate equipment

Erasable PROM (EPROM) • Can be programmed with special tool

• It has to be totally erased to be reprogrammed

Electrical Erasable PROM (EEPROM) • No special tools required

• Can erase a portion

Memory Hierarchy

The idea • Hide the slower memory behind the fast memory

• Cost and performance play major roles in selecting the memory.

Hit Vs. Miss Hit

• The requested data resides in a given level of memory.

Miss• The requested data is not found in the given level of memory

Hit rate• The percentage of memory accesses found in a given level of

memory.

Miss rate• The percentage of memory accesses not found in a given level of

memory.

Hit Vs. Miss (Cont.)

Hit time• The time required to access the requested information in a given level of

memory.

Miss penalty• The time required to process a miss,

• Replacing a block in an upper level of memory,

• The additional time to deliver the requested data to the processor.

Miss Scenario

The processor sends a request to the cache for location X• if found cache hit

• If not try next level

When the location is found load the whole block into the cache • Hoping that the processor will access one of the neighbor

locations next.

• One miss may lead to multiple hits Locality Can we compute the average access time based on this

memory Hierarchy?

Average Access Time Assume a memory hierarchy with three levels (L1, L2, and L3)

What is the memory average access time?

h1 hit at L1 (1-h1) miss at L1t1 L1 access time

h2 hit at L2(1-h2) miss at L2t2 L2 access time

h3 hit at L3=100%(1-h3) miss at L3t3 L3 access time

Locality of Reference One miss may lead to multiple hits Locality

Temporal locality• Recently accessed items tend to be accessed again in the near future.

Spatial locality• when a given address has been referenced, it is most likely that

addresses near it will be referenced within a short period of time. (for example, as in arrays or loops).

Sequential locality part of the spatial locality• Instructions tend to be accessed sequentially.

Cache memory

Cache • Stores recently used data closer to the CPU

Your home is the cache and the main memory is the grocery store • Buy what is most probably to be needed in the coming week

How a processor can know which block(s) to bring to the cache?• No way to know but can benefit from the locality concept

Impact of Temporal Locality

Assume that:• A loop instruction that is executed n times

• The request data created a cache miss requires tm to load the requested block from the main memory to the cache

• tc is the cache access time

What is the average access time?

n tavg

What does it mean?

Impact of Spatial Locality

Assume that: • m elements are requested due to spatial locality.

• The request data created a cache miss that requires tm to load the requested block from the main memory to the cache

• tc is the cache access time

What is the average access time?

What does it mean?

m tavg

Cache Mapping Schemes

Cache Mapping Schemes Cache memory is smaller than the main memory

Only few blocks can be loaded at the cache

The cache does not use the same memory addresses

Which block in the cache is equivalent to which block in the memory? • The processor uses Memory Management Unit (MMU) to convert the

requested memory address to a cache address

Direct Mapping

Assigns cache mappings using a modular approach

j = i mod n j cache block number i memory block number n number of cache blocks

Memory

Cache

Example Given M memory blocks to be mapped to 10 cache blocks, show

the direct mapping scheme?

How do you know which block is

currently in the cache?

Direct Mapping (Cont.) Bits in the main memory address are divided into three fields.

Word identifies specific word in the block

Block identifies a unique block in the cache

Tag identifies which block from the main memory currently in the cache

Example Consider, for example, the case of a main memory consisting of 4K

blocks, a cache memory consisting of 128 blocks, and a block size of 16 words. Show the direct mapping and the main memory address format?

Tag

Example (Cont.)

Direct Mapping

Advantage

• Easy

• Does not require any search technique to find a block in cache

• Replacement is a straight forward

Disadvantages• Many blocks in MM are mapped to the same cache block

• We may have others empty in the cache

• Poor cache utilization

Group Activity

Consider, the case of a main memory consisting of 4K blocks, a cache memory consisting of 8 blocks, and a block size of 4 words. Show the direct mapping and the main memory address format?

Group Activity

Given the following direct mapping chart, what is the cache and memory location required by the following addresses:

31 126 3

3 2 2

Fully Associative Mapping Allowing any memory block to be placed anywhere in the

cache

A search technique is required to find the block number in the tag field

Example

We have a main memory with 214 words , a cache with 16 blocks , and blocks is 8 words. How many tag & word fields bits?

Word field requires 3 bits

Tag field requires 11 bits 214 /8 = 2048 blocks

Which MM block in the cache?

Naïve Method: • Tag fields are associated with each cache block

• Compare tag field with tag entry in cache to check for hit.

CAM (Content Addressable Memory)• Words can be fetched on the basis of their contents, rather than on the basis

of their addresses or locations.

• For example:

• Find the addresses of all “Smiths” in Dallas.

Fully Associative Mapping

Advantages • Flexibility

• Utilizing the cache

Disadvantage• Required tag search

• Associative search Parallel search

• Might require extra hardware unit to do the search

• Requires a replacement strategy if the cache is full

• Expensive

N-way Set Associative Mapping Combines direct and fully associative mapping The cache is divided into a set of blocks All sets are the same size

Main memory blocks are mapped to a specific set based on :

s = i mod S

• s specific to which block i mapped

• S total number of sets

Any coming block is assigned to any cache block inside the set

N-way Set Associative Mapping

Tag field uniquely identifies the targeted block within the determined set.

Word field identifies the element (word) within the block that is requested by the processor.

Set field identifies the set

N-way Set Associative Mapping

Group Activity

Compute the three parameters (Word, Set, and Tag) for a memory system having the following specification: • Size of the main memory is 4K blocks,

• Size of the cache is 128 blocks,

• The block size is 16 words.

Assume that the system uses 4-way set-associative mapping.

Answer

N-way Set Associative Mapping

Advantages:• Moderate utilization to the cache

Disadvantage • Still needs a tag search inside the set

If the cache is full and there is a need for block replacement ,

Which one to replace?

Cache Replacement Policies Random

• Simple

• Requires random generator

First In First Out (FIFO)• Replace the block that has been in the cache the longest

• Requires keeping track of the block lifetime

Least Recently Used (LRU) • Replace the one that has been used the least

• Requires keeping track of the block history

Cache Replacement Policies (Cont.)

Most Recently Used (MRU) • Replace the one that has been used the most

• Requires keeping track of the block history

Optimal • Hypothetical

• Must know the future

Example Consider the case of a 4X8 two-dimensional array of numbers, A.

Assume that each number in the array occupies one word and that the array elements are stored column-major order in the main memory from location 1000 to location 1031. The cache consists of eight blocks each consisting of just two words. Assume also that whenever needed, LRU replacement policy is used. We would like to examine the changes in the cache if each of the direct mapping techniques is used as the following sequence of requests for the array elements are made by the processor:

Array elements in the main memory

Conclusion

16 cache miss No single hit 12 replacements Only 4 cache blocks are used

Group Activity

Do the same in case of fully and 4-way set associative mappings ?