Question

1. Draw a memory hierarchy diagram.
2. Briefly describe the different types of memory in the memory hierarchy in terms of
performance, access time and cost.
3. Describe the followings regarding cache:
a. Block placement
b. Block identification
c. Block replacement
d. Write strategy

Answer 1

Answer) The Memory hierarchy diagram is shown below.

The Memory in the above diagram is divided into 5 types.

1) CPU Registers(Level 0)- The Registers memory is present in the Processor that is in the CPU. Since they are present in the CPU they have least access time and thus providing the best performance and considered as the fastest memory. They are very expensive and therefore comes in smaller sizes in KB(kilobytes).

2) Cache Memory(Level 1)- This memory stores that data or information which is frequently called or accessed by the CPU. It also has less access time but more than Level 0. It is also very expensive because it is a fast memory and therefore comes in smaller MB(megabytes).

3) Main Memory(Level 2)- This memory can directly communicate with the CPU through an O/I (Input/Output) Module. It is less expensive than cache memory and comes in larger sizes in GB. It uses static RAM and thus has good access time.

4) Magnetic Disk(Level 3)- These type of memory is also known as secondary memory and it is usually used to store the backups. It is used to store large files of data which is less used. It is cheaper than Main memory and thus comes in TB. Its access time is also large as compared to the above levels.

5) Magnetic Tap(Level 4)- Magnetic Taps are used to store files removable files. These are the cheapest and comes in very large sizes 1-20TB. They have poor performance and large access time.

Block Placement in Cache Memory- The Cache memory holds the recently or frequently used data by the CPU. A Block of data can't be randomly placed in the CPU cache memory. Placement policies are defined which specifies which data blocks should be placed in the cache memory. According to this policy, those data blocks are placed in cache memory which are frequently used by CPU.

Block identification in Cache Memory- The blocks are identified using the help of block number. The hashing function is used to tells where the block is present in the cache memory.

Block Replacement In Cache Memory-. When the cache is full there is a requirement of blocks to be replaced. Replacement algorithm is used for replacing the data block. For example in the least Recently used replacement algorithm that block is removed which is unused for a longer period of time as compared to other blocks.

Write Strategy in Cache Memory -  There are many writing strategies and choosing the right one can make a difference in response time and load on the database. Write through cache is a writing strategy in which data is firstly written to cache memory and then to the database. But in Write around data is directly written to the database.