[KLUG Hardware] Re: Half as much memory as it says -- i440BX only supports 128Mb (8Mb/IC) technology

[Bryan J. Smith]

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On Sun, 2002-12-01 at 19:47, Robert G. Brown wrote:
> Pity it doesn't work out that way. The BIOS only detects 128MB on each DI=
MM!
> I install two, and I get 256 MB. However, that seems to be quite a waste,=
 to
> say the least. A couple of good friends advised I run memtest86 on this=20
> configuration, so I have, and it all runs flawlessly, hour after hour of
> clean testing, but all at 128 MB per DIMM.
> The motherboard has an AMI BIOS, and memtest86 tells me it is based on an=
=20
> i440x chipset. The box is a Goldstar LG6L, web searches have been unrevea=
ling,=20
> A couple of people have raised the notion that this board and chipset sim=
ply=20
> can't adress more than 128 MB per DIMM slot. THe possible seller has had =
even
> worse symptoms, perhaps due to older hardware.

I get at least one Email a week on this from people who've found my
postings in ELUG-HW (a list no longer run) and PC_Support (a list I
moderate) archives c/o a Google search.

To start, understand there are three variables in DIMM sizing:
 - IC Technology (today:  64Mb, 128Mb, 256Mb, 512Mb)
 - IC Width (4-bit, 8-bit, 16-bit, 32-bit)
 - IC Number (4, 8/9, 16/18, 32/36 -- optional ECC is the 2nd number)

The i440BX only supports 128Mb (8Mb/IC) SDRAM technology.  To get 256MB
on a DIMM for the i440BX, the configuration must be 8Mb x 4-bit x 32 ICs
(36 ICs for ECC) "registered."  You can usually find them from
"liquidation" or "used" memory resellers.

Most "retail" 256MB DIMMs sold today are 16Mb x 8-bit x 16 ICs, or 16Mb
x 16-bit x 8 ICs.  Why?  Because that is what the Intel i815 and i845
support.  They don't support 4-bit SDRAM ICs, and do not support 32 (or
36 for ECC) "chip selects" in their memory controllers.  So your i440BX
chipset can only address the first 128MB on each DIMM, because the
memory controller doesn't support the "most significant bit" on the IC.

The opposite can happen if you put a 256MB "registered" DIMM in a newer
i810, i815 or i845 system (as well as first-gen Athlon chipsets, like
the AMD750 and ViA KX133).  It will only address the first 16 chips, and
not see the next 16 because it doesn't have the "chip selects" for
them.  Coincidently, I just had someone Email me regarding this today: =20
http://lists.leap-cf.org/pipermail/pc_support/2002-December/002466.html=20

DIMMs are normally 2 x 32-bit banks (64-bit), but 32/36 chips of 4-bit
widthwhich is actually 4 x 32-bit "banks" (128-bit) [usually] requiring
the "registered" buffering and extra chip selection logic (I say
"usually" because some AMD/ViA/SiS chipsets handle "unregistered" ones
files, although they aren't always as stable -- forget 3 DIMMs ;-).  No
newer Intel SDRAM chipset supports them _except_ the E75xx-series which
is simply OEM'd from ServerWorks (formerly Reliance Computer
Corporation, RCC).

*** SO HOW DO I TELL WHAT I SHOULD USE??? ***

When in doubt, _never_ trust the mainboard manual _nor_ even a computer
technician.  Most don't understand why.  You won't even find it on a
vendor's site, nor do those "high density" or other "descriptions"
really explain it.  Again, you must _know_ the "3 variables" above for
your DIMM module, and compare it to your chipset.

*** SO HOW DO I KNOW WHAT CHIPSET SUPPORTS WHAT??? ***

_Rread_ Intel (as well as ViA, SiS and AMD) _engineering_ spec sheets
avaliable on their site.  After awhile, you learn all these details.=20
Yes, there's a _lot_ of engineering babble throughout, but if you look
up the memory support, the details are there.  The memory technology
supported as well as the IC widths, the chip selects, etc...

I've detailed this in no less than 2 dozen posts on my local "ELUG-HW"
(ELUG Hardware") and "PC_Support" (LEAP-related) in Orlando.  Here's a
couple from early last year to start you off ... =20
http://www.zepa.net/hypermail/elug/hardware/2001/02/0008.html =20

I you pull up "Google" and search for either "+ELUG-HW" or "+PC_Support"
with your memory keywords, you'll probably find a good post I made.

-- Bryan J. Smith, BSECE
   Intimate with memory controller and addressing logic design

P.S.  I don't hold the CompTIA A+ in a high regard for 2 reasons:

  1)  They don't ask questions as deep as this (I think they should),
      and
  2)  I scored perfect on both sections of their 2001 Adaptive Exam
      (it wasn't able to ask a "hard enough" question ;-)

BTW, CompTIA did ask me to become a SME (Subject Matter Expert) for the
A+.  Unfortunately, I found their "terms" to be a bit restrictive (i.e.
I couldn't write study guides, which killed 90% of my incentive).

--=20
Bryan J. Smith, E.I.            Contact Info:  http://thebs.org
A+/i-Net+/Linux+/Network+/Server+ CCNA CIWA CNA SCSA/SCWSE/SCNA
---------------------------------------------------------------
Anything that needs to worry about coherency should be on the
system interconnect and treated like a CPU.  So why oh why does
the AGP bus exist for the GPU, which it is clearly a processor
that needs to worry about coherency too?  Furthermore, it would
benefit from the throughput of the system interconnect as well.
HyperTransport is the answer today, but Intel is not listening.


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