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

[Bryan J. Smith]

On Thu, 2005-08-04 at 05:16 +0000, Dale Barber wrote:
> Hey Bryan, thanks again.
> I don't understand why 4 bit wide chips or 8 bit wide chips wide is so 
> significant to the controller here. Surely by conncecting two 4 bit wide 
> chips side by side on a sodimm, the controller basically sees an 8 bit wide 
> memory device?

Not when it results in extra tri-state buffers and logic.  ;->

> And why is registered memory necessary in this case...I thought it was only 
> needed in the case of too much electrical load on the data and address buses 
> when more than 3 sodimms are connected to the controller? The 64MB memory on 
> my Toshiba motherboard is not registered memory so you suggest that a 
> registered sodimm wouldn't work with it?

There is a bit width total of 128-bit, double the 64-bit DIMM, which
requires extra logic on-DIMM.  This adds buffer and states which is not
ideal for Synchronous timing.

> Interestingly, the ASUS L8400 C notebook uses the same i440BX chipset and it 
> will accept a 256MB 16 chip sodimm, giving a maximum memory of 320MB or 384 
> MB, depending on whether there is 64MB or 128MB on the motherboard 
> respectively.

Are you sure it is the i440BX and not the i440GX or possibly the i815?

*OR* are you sure it's not a 256MB EDO SO-DIMM?

Many early SDRAM chipsets also supported EDO DIMMs at double the size because
there is not the synchronous issues in using more ICs/widths.  This includes
the i440BX.

> This leads me to suspect a BIOS limitation! Do you think Toshiba could write 
> a limit into their BIOS to limit the maximum size of the sodimm to 128MB or 
> the maximum total memory to 192MB? Have you heard of this being done before 
> in the BIOS? I know that desktop/server motherboards are a lot more 
> flexible, but maybe laptop manufacturers don't want to offer this kind of 
> flexibility in their BIOS!


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