[Pc_Support] Re: ATT: Bryan Smith - Ram compatibility with i440BX in ToshibaSatellite 4090XCDT

[Bryan J. Smith]

On Thu, 2005-08-04 at 04:18 +0000, Dale Barber wrote:
> Did you mean "the i440BX is not capable of 256MByte technology"....because 
> the Micron 256MByte sodimm that I tried uses 128Mbit chips (=16Mx8)  and it 
> has 16 of them, giving 16 x 16M x 8 = 256MB  ? So why doesn't this work?

Actually, 128Mbit (16MB @ 8-bit) IC -> 256MByte technology.  The whole
terminology gets a bit ambiguous.

The _only_ 16M chips you can use _must_ be 4-bit ICs, resulting in 8MB
ICs.  In a 32 chip "registered" 128-bit configuration (36 chip for
ECC/144-bit), that will be a 256MB Registered DIMM.

BTW, just because a chipset supports a particular technology size / IC
width doesn't mean it will work in all combinations.  You'd have to
design a memory controller to understand the exponential increase in
transistor count and, ultimately, additional wait state prevents a lot
of flexibility.

> I've been told that the limitation with the i440BX was its 4K refresh cycle, 
> and only the 256MByte sodimms configured as 16 x 16M x 8 had this...all 
> other 256MByte modules require an 8K refresh cycle and hence definitely 
> won't work!

That could be true, I honestly don't know.

I just know the IC size, width and chip arrangement configurations that
the i440BX supports.  The only 256MB modules supported are 32/36-chip
with 4-bit wide ICs in a registered configuration.

The "main problem" is that most 256MB modules in the i8xx series have IC
that are _not_ 4-bit, but 8-bit wide.  No i8xx chipset supported 4-bit
wide ICs -- leading to a "no man's land" of conflicting 256MB DIMMs.  In
a nutshell, the more "commodity" i8xx 256MB was far more commonplace
than the "registered" i440BX 256MB DIMM, hence why they are hard to
find.

Especially for notebook SO-DIMMs.


-- 
Bryan J. Smith     b.j.smith at ieee.org     http://thebs413.blogspot.com
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