[Pc_Support] Stupid RAM Questions

[Bryan J. Smith]

Bruce Metcalf <Bruce.Metcalf at figzu.com> wrote:
> Somebody here, it doesn't matter who, wrote:
>   >matched 1GiB DIMMs with fairly good timings
>   >(2.5-3-3-6) in a DDR400

That was me.  ;->

> Would one of you be so kind as to point me to a web site
> where I can find the definitions of those timings?

Here's a good, technical introduction:  
  http://www.pcguide.com/art/sdramTiming-c.html  

Timings aka "latency" _vary_ based on the synchronous clock
used.

For example, a typical "Value" PC3200 DDR Synchronous Dynamic
RAM (SDRAM) has timings of 3-3-3-8.  A typical "Value" PC4200
DDR2 SDRAM has timings of 4-4-4-12.  The higher the value,
the _worse_ they are for the _same_ sychronous clock.  Just
because you have a 

Understand that even though a PC3200 (3.2GBps = 64-bit *
400MHz -- 200MHz double data rate, DDR) SDRAM means a 2.5ns
(nanosecond -- 1/1,000,000,000 of a second) synchronous
timing, you *NEVER* get that type of performance out of DRAM.

CPU caches, high-speed/enterprise networking switches and
select disk controllers use Static RAM (SRAM) instead of
DRAM.  It has greatly reduced latency and power consumption,
but it is also much largerly footprint and cost.  SRAM is a
clocked boolean logic (CBL) circuit, whereas DRAM is a leaky
cell.

> Especially favored would be a site that helped me select
> the most appropriate memory specs for upgrading my systems.

Now that's _different_.
Timing is typically just for performance.

In fact, synchronous clocks are NOT the only thing you have
to be worried about when upgrading DRAM.  DRAM IC technology
levels are also important, the chip/width configuration,
etc...

In a nutshell, your chipset defines what IC sizes/technology
are supported -- before you even look at synchronous clocks. 
E.g., the Intel i440LX/BX used _different_ 256MiB
PC66/100/133 SDRAM DIMM configurations (forget even timing)
than the i810/815 chipsets.


-- 
Bryan J. Smith     Professional, Technical Annoyance
b.j.smith at ieee.org      http://thebs413.blogspot.com
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*** Speed doesn't kill, difference in speed does ***