1. Explain why DRAM memory requires refreshing. 2. What are the differences betw
ID: 3604732 • Letter: 1
Question
1. Explain why DRAM memory requires refreshing. 2. What are the differences between write-back and write through? 3. Suppose a memory system has four banks, each of which has a latency of 200 ns and is pipelined to allow 8 operations to overlap execution. If each bank returns 4 bytes of data in response to a memory request, what is the peak throughput and peak bandwidth of this system? What is the bandwidth of a memory system that transfers 32 bits of data per request, has a latency of 30 ns per operation, and has a precharge time of 5 n:s between operations? 4. 5. Suppose a DRAM has a RAS-CAS delay of 40 ns, has a CAS-data delay of 25 ns, and requires a precharge time of 10 ns. If the DRAM does not support page mode and returns 4 bits of data from each memory reference, what is its latency for read operations, throughput (assume reads and writes take the same amount of time), and bandwidth? IF a DRAM has a 256 rows, and its refresh time is 10 ms, how often (on average) does a row refresh operation need to be done? 6. What are the hit and miss rates in the cache and the main memory if the processor executes a total of 1,500,000 memory references, , 1200,000 of which hit in the cache and 300,000 of which hit in the main memory 7.Explanation / Answer
Solution:
Q 1 and 2are solved, please repost others,
1)
DRAM requires refreshing because each memory is stored as the presence or absence of electric charge which gets weakened over time, so to avoid the loss of data the memory cells gets restored the charge by getting refreshed over a time interval.
2)
Difference between write back and write through:
write-back:
it is a caching technique in which the modifications made to the data by the user in the cache are not storing to the main memory until it is demanded. the data is written into the cache everytime a change occurs but not to the main memory. original data resides in main memory, cache memory has only a copy of the original data. so the modifications are not reflecting in the original data. there is a chance of losing data if the system crashes
when a data is updated in write-back mode, the data stored in the cache is known as fresh and the corresponding data ( which is not modified) stored in the main memory is known as stale. whenever a request from any application arrives in main memory cache controller will update the data in main memory before the application uses it.
write-back shows better performance than write-through because the number of writes into the main memory is low. it writes into the main memory only when it is required or in a regular interval of time. here latency time is low.
write-through:
it is a caching technique in which the modifications made to the data by the user is storing to the cache and main memory at the same time. cache memory allows fast retrieval of the data and main memory ensures that data will not be lost if a system crash occurs.
the number of writes is more here than write-back so it is a little bit time consuming than write-back. it reduces the risk of data loss. the exact copy of the data will be there in main memory.
I hope this helps, please let me know in case of any doubt. Thumbs up if this helped.
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