[Autogenerated] we talked about p t e s,
p d s and the like here's what that looks like for x 64 we have a bunch of
bits here notice that there are some bits here which are more important than
others and would like to talk about them here.
So first thing is that we have the valid bit so here's the valid bit
and the valid bait indicates whether that entire entry really should be
examined and looked at by the processor.
If the valid bit is one then what we see here is the
thing that the processor is looking at.
If the value is zero,
the CPU doesn't make forward progress but instead raises a page
fault exception and then we have the PFM.
The PFM is the page frame number.
That's essentially the value that we need to multiply by the size of a
page to see where the next item is in physical memory.
For pT for example, these 36 bits,
if you add the other 12 It's coming from the office,
that's exactly 48 bits pointing to some address in physical memory which is
also the maximum that is supported by those processors,
not just in the future address space but also in the physical address space,
no more than 256 terabytes.
And then you might recall,
we mentioned the fact that there are large pages also that are supported.
So what is the benefit of a large page?
From a hardware perspective?
The benefit is that instead of having a four k page,
you now have two megabyte page size so that you don't need
page tables and a single entry can map an entire two
megabyte range in physical memory.
I'll discuss some of the downsides of large page files in
the last module in this course however,
the benefit is mostly with the translation.
Look aside buffer cache.
The tea will be cash because now a single entry in the cache maps two
megabytes in size instead of just for kilobytes.
And so the large page beat here tells the CPU not to move further,
but consider the rest of what remains in the bits for
the non used address as the offset.
So why is the page size?
Two megabytes.
Let me go back to the previous slide for a moment
and see the full translation here.
You'll notice that if we dismissed with the page tables
then we get 12 bits plus nine bits.
21 beaten, 21 bits is exactly two megabyte in size.
So this is where these large page sizes coming from
and for a huge page by the way,
which are also mentioned supported by intel
processors is sized with one gigabyte.
So again,
where can we find one gigabyte here And now it's perhaps obvious
that one gigabyte is dismissing with the page.
Director is also and we get 12 beats plus nine beats plus nine bits.
That's exactly 30 bit, which is exactly one gigabyte in size.
So with a huge page we have even a better usage of the translation.
Look aside buffer, of course,
there is a downside which I'll discuss in the last module in this course,
so that would be the large page size bit.
You can see that there are some reserved bits here,
which means that there's still room to increase that scheme and with PML
level five with the new skin that intel supports today.
If you add nine more bits,
there's still a little bit of free bits there in order to
even support more memory in the future.