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Sunday 15 March 2015

A Cache Memory

Today we feel to revise what we know about cache memory. A cache is a memory device that improves performance of the processor by transparently storing data such that future requests for that data can be served faster. The data that is stored within a cache might be values that have been computed earlier or duplicates of original values that are stored elsewhere.

Access to cache can result in either one of the following: cache miss or cache hit.Cache hit means that the requested data is contained in the cache and cache miss means data is not found there in cache.On cache hit processor takes data from cache itself for processing.On cache miss the data is fetched from the original memory location.Cache memories are volatile and small in storage size.Since the storage size is small the address decoding takes less time and hence caches are faster then normal physical memories(RAM's) in computers.

As I said the data is stored transparently in cache.This means that the user who is requesting data from the cache need not know whether data is stored in cache or system memory.It is handled by the processor.The word cache means "conceal" in French.

A simple cache contains three fields.

1. An index which is local to the cache.
2. A tag which is the index with reference to the main memory.This will let the processor know the location in main memory where an exact copy of data is stored.
3. Data, which is actual data needed by the processor.

When processor needs some data from the memory it first checks in cache.It sees all the tag fields in the cache to see whether same data is available in cache.If the tag is found then the corresponding data is taken.Otherwise a cache miss error is asserted and the main memory is accessed.Also the cache memory is updated with the recent memory access.This is called cache update on cache miss.

During a cache update if the cache is full, then it has to delete a row.This is decided on a cache replacement algorithm.Some algorithms are:

1. LRU - Least recently used data is replaced.
2. MRU - Most recently used data is replaced.
3. Random replacement - Simple, used in ARM processors.
4. Belady's Algorithm - discards the data which may not be used for the longest time in future.Not perfectly implementable in practice.

The average memory access time of a cache enabled system can be calculated using the hit and miss ratio of a cache.

Average memory access time = (Time_cache * Hit_counts ) + ( (Time_cache + Time_mm) * Miss_counts)

Time_cache and Time_mm is the time needed to access a location for cache and main memory respectively.
Hit_counts and Miss_counts are the hit and miss probabilities.

There are two types cache writing: write back(copy-back) and write through.

When the data at a particular memory location is updated then this data must be written back to cache.If the data is updated only in the cache then it is called write back.If the updating of data happens both in cache and main memory then it is called write through.Write through keeps the cache and memory synchronized.In the write back operation since the cache data is not same as the main memory data it is marked as "dirty" data.These dirty data will be written back into main memory when the particular data is cleared from the cache.If a miss happens in a write-back cache it may sometimes require two memory accesses to service : one to first write the dirty location to memory and then another to read the new location from memory.

The main memory locations may be altered without proper updating in cache by peripherals using DMA or by a multi core processor.This results in a out of date data in cache.These type of data is called "stale" data.To solve these stale data problems we have to use cache coherence protocols between the cache managers to keep the data consistent.

All caches are CAM(content accessible memory).And for efficiency we have to scan all the memory contents in one cycle.This requires parallel hardware.Also higher the memory size the more is the memory access time.

Let us see now,how a cache is made.Say we have a 32 bit main memory in our system and the cache chip size is 4 Kb.Also say each line in cache stores 32 bytes so that there are totally 128 lines.Each line in cache have two fields. Address(4 bytes) and Data.The address is further divided into two fields- Tag(27 bits) and offset(5 bits for indexing a particular byte among the data).Remember that the tag contain the MSB 27 bits of the address here.These kind of caches are called Fully associative caches.Since the tag is 27 bits(relatively long) it takes more time to read data from Fully associative cache.Also more hardware circuit is required for parallel reading of tags from the CAM type cache.So they are expensive but more efficient.

Fully associative cache

Another type of cache architecture is known as direct mapped cache.In this the address is divided into three fields named tag(20 bits),index(7 bits used as an index the 128 lines in cache) and offset(5 bits).The problem with this type of cache is that the cache is less efficient since the main memory cannot be copied to any line in cache as in fully associative cache.This is because the addresses with the same index will be mapped to the same line in cache.But the cache access time is less here.In certain situations you may get a cache miss for almost every access.So they are cheap but less efficient.

Direct mapped Cache
Another type of cache is called set associative cache which has the advantages of both direct mapped and fully associative caches.These are again subdivided based on the number of bits in the index field.

2. 2 way set associative cache - In this type of cache we have two group of lines,each containing 64 lines.The cache has the same number of fields as direct mapped cache but tag has 21 bits and index has 6 bits here.

2. 4 way set associative cache - Here we have 4 groups each contains 32 lines.index has 5 bits and tag has 22 bits.

2-way and 4-way set associative caches

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