The Gateway to Computer Science Excellence
First time here? Checkout the FAQ!
+4 votes
A $32$-$bit$ wide main memory unit with a capacity of $1$ $GB$ is built using $256$$M$ $\times$ $4-bit$ DRAM chips. The number of rows of memory cells in the DRAM chip is $2^{14}$. The time taken to perform one refresh operation is $50$ $nanoseconds$. The refresh period is $2$ milliseconds. The percentage (rounded to the closest integer) of the time available for performing the memory read/write operations in the main memory unit is_____.
asked in CO & Architecture by Boss (18k points)
edited by | 2.1k views
Flying over head :p

At start it seems simple RAM concept extended but after that it went out of track for me :p

Can't even guess the answer as it is NAT Question
here it was clearly mentioned that questions on RAM wouldn't be there !!!


To prevent this, external circuitry periodically reads each cell and rewrites it, restoring the charge on the capacitor to its original level.

Refer ->

Read/Write is happening as a part of refresh operation then why is it asked for separately ?

One more doubt why all the rows can not be refreshed in one cycle ?

@VS, @Subarna Das, @Ashwin Kulkarni, @ habedo007 and @Digvijay Pandey ji. What is your opinion ?



Refreshing does not employ the normal memory operations (read and write cycles) used to access data, but specialized cycles called refresh cycles which are generated by separate counter circuits in the memory circuitry and interspersed between normal memory accesses

1 Answer

+22 votes
Best answer

One refresh operation takes $50ns$.
Total number of rows $= 2^{14}$

Total time to refresh all Rows $= 2^{14}\times 50\; ns = 819200 \;ns =  0.819200\;ms$
The Refresh Period is $2ms.$

$\%$ Time spent in refresh  $= \frac{Total\ time\ to\ Refresh\ all\ Rows}{Refresh\ period}*100
                                             $= \frac{0.8192ms}{2.0ms}*100$ $= 40.96\%$

$\%$ Time spent in Read/Write $= 100 - 40.96 = 59.04\%$

$= 59\%$ (Rounded to the closest Integer)


answered by Veteran (54.9k points)
edited by
Great explanation :)
All the chips are refreshed simultaneously?

@Peach yes. Take a look at this question for better understanding:

It is said that "refresh period is 2 millisecond" ie the rows get refreshed every 2 seconds, why is the time take for refresh included in the period. Please correct my interpretation about period.
In 2msec, the time given to refresh is 2^14*50 nsec and rest will be for read and write.We are just talking about time spent on read/write in 2msec only.

Related questions

Quick search syntax
tags tag:apple
author user:martin
title title:apple
content content:apple
exclude -tag:apple
force match +apple
views views:100
score score:10
answers answers:2
is accepted isaccepted:true
is closed isclosed:true

37,162 questions
44,732 answers
43,805 users