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Layers of Cache

 

There are in fact many layers (Schichten / Ebenen) of cache in a modern PC. This does not even include looking at caches included on some peripherals, such as hard disks. Each layer is closer to the processor and faster than the layer below it. Each layer also caches the layers below it, due to its higher speed compared to the lower levels:

Level

Devices Cached

Level 1 Cache

Level 2 Cache, System RAM, Hard Disk / CD-ROM

Level 2 Cache

System RAM, Hard Disk / CD-ROM

System RAM

Hard Disk / CD-ROM

Hard Disk / CD-ROM

Internet

What happens in general terms is this. The processor requests a piece of information. The first place it looks is in the level 1 cache, since it is the fastest. If it finds it there (called a hit on the cache) it uses it with no performance delay. If not, it's a miss and the level 2 cache is searched. If it finds it there (level 2 "hit"), it is able to carry on with relatively little delay. Otherwise, it must request to read it from the system RAM. The system RAM may either have the information available or have to get it from the still slower hard disk or CD-ROM.In addition to reading operations from memory, the cache system is able to write to memory. There are two different ways that the cache can write, and this is referred to as the "write policy" of the cache.

·                     Write-Back Cache: The processor writes new data only to the cache, not to the main memory. Later, if new data needs to use the cache line where this data is stored, it is saved ("written back") to the system memory and then the line can be used by the new address. Write-back cache is faster but needs a more intelligent controller.

·                     Write-Through Cache: With this method, every time the processor writes to a cached memory location, both the cache and the underlying memory location are updated. This write policy is the safest but it is also slower as there are two write processes at a time.

1. True or false or not in the text?

1.      Each layer of cache caches the memory which is below it.

true

false

n.i.txt

2.      Level 1 cache is the fastest.

 

 

 

3.      When the CPU needs data it first looks in the RAM.

 

 

 

4.      If the Ram does not have the info. required it may take it from a        CD.

 

 

 

5.      A success in finding data is called a hit.

 

 

 

6.      L2 cache mainly caches the HDD or a CD ROM.

 

 

 

7.      If there is a write-back cache all data is written back to the RAM.

 

 

 

8.      Write-through cache is faster.

 

 

 

9.      The L1 cache is built in the CPU.

 

 

 

10.   The Internet is the slowest source of data mentioned in the above       table.

 

 

 

2. Questions

  1. Why is cache memory used?
  2. What can you say about the size of the memories mentioned.
  3. Compare the two write policies of cache (advantages and disadvantages)

3. Explain:      a. ATM

                          b. PDA

                          c. AGP

4. Please translate the bold-printed lines!