[dba-Tech] A few hardware/memory questions

John,

Indeed, it is very clear.  Thank you.  I had forgotten all the hard
lessons learned about memory usage back in the days of "conventional",
"expanded", and "extended" memory.  Very similar situation, methinks,
with 4 GB addressable rather than 1 MB.

Steve Erbach

On Tue, May 20, 2008 at 9:47 AM, jwcolby <jwcolby at colbyconsulting.com> wrote:
>  >Is there a performance boost with the mapped memory if you've got 4 GB
> installed?  If so then the reverse is true if you've only got 2 GB win
> no mapping?
>
> Hmm.. there is a performance boost IF you need more than 2 gigs of
> memory.  If not then it may never be used, though this is unlikely.
>
> If you have so much memory that Windows cannot map the video card and
> other things it needs to map, then it carves holes in the memory.  Does
> it take longer to access this memory?  I don't think so.  What happens
> is that the processor can "translate" memory requests, and does all of
> the time.
>
> The video card has some amount of memory physically on the card, little
> memory chips right on the card.  That may be 32 meg or it might be 512
> meg.  Let's assume for argument's sake that it has 512 megs.
>
> That memory has to be "mapped" somewhere within the total 4 gigs that
> the 32 bit address register can address.  So... Windows says "this 512
> megs will be physically located at the very upper 512 megs of the 4 gig
> address range".  It programs the card itself to "answer up" when it sees
> an address on the bus between 3.5 and 4 gigs.  In the meantime it
> "carves" a hole out of the memory such that the memory sticks no longer
> "answer up" between those addresses.  The memory now answers up between
> 0 and 3.5 gigs and the video card now answers up between 3.5 gig and 4
> gig.  There are actually other things that take MUCH smaller chunks as
> well, the video card is just the "worst offender".
>
> If you think about this though, what happens if you only have 2 gigs?
> The video still gets 3.5-4 gigs.  The memory gets 0-2 gigs.  There is a
> "hole" between 2 and 3.5 gigs but it is not there because Windows
> "carved it out" it is just there because there is no device occupying
> that space.
>