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So, welcome back, today we are going to be
talking about how to go

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about building cache coherent systems, so,
we've

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just come off talking about consistency,
and

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memory consistency and now we're going to
start

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talking about, memory coherent systems and
talk

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about, sort of, the beginning protocols
when

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you go about building memory coherent
systems.

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Before that.

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Let's go back and review what we worked
what

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we're working on at the end of last
lecture.

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So at the end of last lecture, we were
talking about

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mutual exclusion and one of the we talked
about having test and

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set and specialized operations that can
do, that can give you

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mutual exclusion but you could also just
use basic loads and stores.

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And, we talked about using Decker's
algorithm

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and, one of the key insights into the

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Decker's algorithm is that you have this
shared

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turn variable here which between the 2
processes,

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which are trying to communicate, are
trying to lock the same, variable.

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Then we went on an expanded this to.
Multiple process mutual exclusion.

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And this is the sort of the moral
equivalent of

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a, going to the deli and try and take a
ticket.

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So you go to the deli, you take a little
ticket, and

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then someone goes and calls your ticket
number, and then you are served.

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This is how you can implement multiple
people trying to access one resource.

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but unlike in a deli where you have let's
say

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a, a number on the, on the wall which
ticks up.

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Or you have a, a person behind the deli
who calls your number.

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Or in a bakery where they call your
number.

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Instead here we need to do that in some
sort of distributing manner.

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So in the end process mutual exclusion.

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The person who actually finishes being
served wakes up the next person.

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And as I said, this is a little bit more

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complex but you can still do the same idea
here.

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excuse me, totally with loads and stores.