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Author: Stan Eisenstat
Subject: Re: [Cs323] Block address Index
Date: Wednesday, 07 Oct 2020, 21:45:55


    > Message Posted By: Unknown
    >
    > On B-13 of HP, the diagram divides Block address into the Tag portion and
    > the Index portion. Does the Index portion correspond to the line # from
    > the notes, which was calculated  by taking the hash of the block #? If so,
    > what is the use of the Block offset?

Yes.

This is a two-way associative cache with 2^9 pairs of
lines of size 64 = 2^6 bytes for a total of 64KB.  The
40-bit address is split into a
* 25-bit block number (= tag bits)
* 9-bit set number (= index)
* 6-bit offset
Our description of caches would have each line storing
the pair (block number, set number) as the BLOCK NUMBER
or TAG BITS.  But since each set of lines "knows" its
set number, it only needs to store the block number and
not the set number.

The offset bits are required to specify which bytes of
the block were accessed.

--Stan-
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