associative mapping

Question

if the main memory is of 8Kbyte and the cache memory 2K word .it uses associative mapping .then each word of cache memory shall be .................................

ans::::: 16 bits how????????????????????

Comments

I am getting 32 bits.. :/

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how ?

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It's fully associative mapping. Each line in cache can be mapped to any.
Since main memory is 8KB and cache is 2Kw, 1w must be 4B = 32 bits.

Is this approach correct?

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this is wrong.

in fully associative memory, the main memory block can map to any of the cache lines, but it does not mean that cache should be of the same size as the main memory, otherwise, what is a use of cache

actually, I think we cannot find word size from a given information

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 joshi_nitish Sir please check this.. high probability of getting wrong..I am not comfortable with this word thing..

Kindly point out the mistake..

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why you put 11-b in the first part of an address?

in fully associative cache address look like,

tag

offset

in place of tag bits you have placed 11-b(bits to represent lines), this is wrong.

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Oh yes.. sorry..!! I totally confused it ! Thank you :)

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@joshi_nitish this question is incomplete????

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think so.

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joshi_nitish Sir I understand that this problem has insufficient information..

But the thing which you said that the memory block size is not equal to size of cache line.. but I have read something different.

From main memory's respect: 

The physical address is divided into two parts- byte offset and no. of blocks in main memory.

From cache memory's respect:

The physical address is divided into the same byte offset and the rest part depends on the type of mapping.

The total no. Of bits required to indentify the block no. in main memory = tag bits+(__) 

Where (__) may be set bits, cache line bits or nothing depending on the mapping.

Please clarify. Thank you.

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@MiNiPanda,

i was saying there that, size of main memory(entire main memory size) need not be equal to size of cache memory(entire cache memory size), i was not saying that block size should not be equal to line size.

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Ooh! Sorry I misunderstood then..

Thank you :)

Answer 1

it is clear that it cant be 21 bits or 20 bits because in these cases the cache size will be nearly equal to main memory. so remaining option are 11 bits and 16 bits. here i will go with 16 bits because in case of 11 bits if memory is byte addressable as we know 5 bits will be wasted and also the memory size should be in power of 2. i already have checked the answer in answer key it is 16 bits.