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

[Bryan J. Smith]

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.

> 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.

Secondly ...
- Intel uses shared bus for all memory
- RISC (and Athlon Socket-462) use a switch to multiple memory banks.
- AMD64 uses a partial mesh for interconnect, and _direct_ DDR channels
  (x2 in the case of Socket-939/940, and Socket-AM2/940 and F/LGA-1207)

The whole concept of "dual channel" is rather over-simplified, since
there are many different implementations with their own benefits and,
more importantly, lack of benefits.

On AMD64 Socket-939/940, you have a _direct_ 2 x 184-pin DDR SDRAM
trace set for _true_ 128-bit wide -- literally pin for pin to each
184-pin DIMM channel.  It's not just  merely "interleaved" between
multiple DIMMs in a single channel.

Socket-AM2/940 and Socket-F/1207 have _direct_ 2 x 200-pin DDR2 SDRAM
traces (yes, I know, DDR2 uses 240-pin on desktop, but it only needs
200-pin on SO-DIMM) for essentially the _same_, _true_ 128-bit wide
path.