Text 41335, 76 rader
Skriven 2008-01-02 07:42:44 av Jeff Guerdat (1:123/789.0)
  Kommentar till text 41330 av James Bradley (1:134/77.0)
Ärende: Graphics.
=================
James Bradley -> Jeff Guerdat wrote:

 JG>> It's easy to compute memory needs for basic (and that's a
 JG>> key) usage. Simply multiply the x and y pixel counts needed
 JG>> and store that.  Now, decide what color depth you want -
 JG>> usually 24 or 32 bit, simply because it's there and
 JG>> usable.  Divide the depth number of bits by 8 (32/8 = 4,

 JB> Is this because it uses Four, Eight-bit 'words'? (If that's the right
 JB> term?)

Yup, that's what I intended.  However, I goofed in the next step - it's 24 or
32 bits for ALL colors, not per color, so my math is off by a factor of 3.  The
formula is X x Y x Z where X is the number of pixels for the width of the
display, Y is the number of pixels for the height, and Z is the color depth
converted to bytes (32 bit color is 4 bytes, 24 bit color is 3 bytes).  My
mistake was saying that you then needed to multiply by the number of primary
colors when that's already in the color depth description.  Sorry for any
confusion.

 JG>> AGP's purpose was to allow direct access to system memory
 JG>> without involving the rest of the computer and that this is
 JG>> different from the low-end chipsets that MUST share system
 JG>> RAM) but that's usually slower and impacts how much memory
 JG>> is available to the system.  (I found that setting my "AGP
 JG>> aperture" in the BIOS to 64MB freed up ~300-400MB of usable
 JG>> RAM after I added my RAID card, which maps itself into the
 JG>> system at just below the 4GB limit so it can use it's
 JG>> onboard RAM, giving me a total of 3.5GB available during
 JG>> POST and 3.37GB inside XP.)

 JB> I have a AGP system that uses system RAM for the on-board vid adapter.
 JB> It's hardwired to use 4M. I'm suspecting, in your setting a 64M aperture
 JB> allowed a more congruous block of RAM to service your RAID cache, as it
 JB> obviously uses system RAM?

I'm not sure how the mapping works out, especially with AGP apertures and other
memory-mapped devices.  I know the RAID card has ~100MB on it but how much is
mapped into the system, I don't know.  Why the AGP aperture makes difference in
the way it does, I also don't know.  I just tried various aperture settings to
see what gave me the most usable system memory based on comments from the
Internet and a white paper from Sun on their Intel-based machines.  The
difference was not linear, where each increment changed available memory by the
same amount.  I do assume (silly me!) that I freed up a block of memory that
allowed the RAID controller to insert itself in a different location.

As an aside, I find it infuriating that not only is there the issue of low
(>1MB) memory causing fragmentation of the address space but there's also a
similar one at the 4GB limit on 32 bit machines.  64 bit doesn't have the
problem (when using a 64 bit OS).  "Standards" get put into place because
they're expedient for the moment, not because they address the future.  I stuck
4GB of RAM in to be able to use it.  Between the motherboard and OS designs,
I'm stuck with somewhat less...

 JB> I've always suspected with my system, that this aperture would tax its
 JB> low memory (128M minus 4M) situation, but your message has me second
 JB> guessing my assumption.

Turns out this assumption is basically incorrect although the situation could
arise to make it an issue.  I had to look it up; here's a link that describes
it fairly well:

http://www.techpowerup.com/articles/overclocking/vidcard/43

So, unless you've run the system out of video RAM, you're not using system RAM.
 The video subsystem simply has the ability to use that 4MB chunk but it's not
allocated until needed.

-- 
Jeff Guerdat

The real problem is not whether machines think, but whether men do.
                                                      B.F. Skinner

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