[Pc_Support] Memory Technology and Chip Select -- WAS: Rules-of-thumb on upgrading components for an older mainboard ...

[William A. Mahaffey III]

Bryan J. Smith wrote:

> William wrote:
>
>> Fair enough. By RAM mass I mean total amount of RAM,
>
>
> The size of RAM means nothing.  It only means more address lines on
> the ICs, or a bigger decoder for the CS required.
>
> Now if you mean number of DIMMs, that's different.  But it has zilch
> to do with the amount of RAM in GBs.
>
>> by speed I mean MHz that the RAM operates at.
>
>
> You mean the synchronize clock used for burst rates.  DRAM itself, on
> reads, is _damn_slow_, like 20MHz (50ns).  One you start bursting the
> read (or during writes), that's the timing between bits -- not so much
> how "fast" the DRAM is.
>
> So the synchronous clock is more of an interconnect limitationt than 
> anything.
>
>> Current DDR has limits on combinations of total RAM & speed, 2 GB (2 X 1
>> GB, I think) at DDR400, 4 GB at DDR 333 (even with DDR400-rated RAM),
>> does DDR2 lift these limits ?
>
>
> Again, has zilch to do with the amount of RAM in GBs.
> It has to do with the number of DIMMs.
> That's because of signal quality over the interconnect, not the SDRAM ICs.


Hmmmmm, OK, I wasn't clear on that point. But the net result is (or 
seems to be) .... a 2 GB limit for DDR400 ? i.e. either 2 X 1 GB DIMMs 
or 4 X 512 GB DIMMs, at least w/ current DIMM  types ? I haven't seen or 
heard of any DDR400 boards with higher limits (at least not S939 boards) 
....

>
>> Yeah, I am *quite* clear about dual channel. SGI used to have 8-way
>> interleaved RAM back in the 72-pin DIMM days.
>
>
> First off, interleaved is not quite the same either, although it's
> commonly used when you have a narrow channel but a wider, total memory
> path.


<snip>


Yeah, same for SGI (& Cray before them), 8 X 32 bit/DIMM for 256 bit bus 
width on SGI Power-challenges, 32 X 32-bit/DIMM for 1024 bit Cray bus 
width .... they just called it 'N-way interleaved' (4-way or 8-way for 
SGI, no name for Cray), not 'N-channel' .... different name, same net 
result.


-- 

	William A. Mahaffey III

 ----------------------------------------------------------------------

	"The M1 Garand is without doubt the finest implement of war
	 ever devised by man."
                           -- Gen. George S. Patton Jr.