[ On Saturday, April 17, 1999 at 12:11:48 (-0400), Joseph Sarkes wrote: ]
> Subject: Alpha memory requirements
>
> I have some parity edo memory that I have occasionally used in
> my multia with good results. The parity edo memory that I have
> has 12 memory chips of all the same size though, rather than the 9
> that would work in an ecc parity application.  Only one bit out of
> 4 is used in the 4 parity bit chips. Ecc parity needs to access the 
> entire 72 bit wide word at once (36 bit simm pair), whereas the typical
> pc needs byte addressibility with parity per byte, and ability
> to write a single byte without doing a read-modify-write cycle.

That's kinda confusing stuff you wrote there....

Any computer that requires 36-bit SIMMs is going to be using full 4-bit
ECC for protection.  I've never heard of any intel-compatible chipsets
that use only one bit of 36-bit SIMMs for a parity bit, not to mention
but that doing single-bit parity on 32-bit data is a losing proposition
in the first place!

I'm not a true DRAM expert, but I spent quite a bit of time recently
trying to find some compatible memory to upgrade some DECserver 90TL
machines which require 36-bit memory with quad-CAS.  I found some, but
it's still not quite right and unfortunately I can't find the data sheet
for the original part so that I can discern the difference (probably
detect pins, but...).

Anyway, here's some hopefully factual information that won't further
confuse things:

The number of chips on the SIMM is usually meaningless unless you know
the configuration of the chip (i.e. how many bits wide it is, and so
on), whether all the chips are the same configuration, how the chips are
connected to the "Column Access Strobe" (CAS) lines, "Row Access Strobe"
(RAS) lines, etc.  For example the Samsung KMM5361000BK is a 16Mx36bit
72-pin FPM SIMM with four 16Mx4 and two 16Mx1 DRAM chips, and
quad-CAS/dual-RAS.  I.e. you can have 36-bits with only three chips
where two big 16-bit chips have two CAS lines each, and one shared 4-bit
chip has 4 CAS inputs.  As I understand it there are SIMMs which have
only one or two active CAS lines, but I don't know what systems use
them, or if they're common.

Then there's the number of supported "presence detect" pins and other
"optional" features.

Lastly there's "Extended Data Out" (EDO), which is an optional feature
of "Fast Page Mode" SIMMS.  As I understand it EDO is a function of the
DRAM chips, not the SIMM design (at least that's the way it appears from
the two data sheets I compared -- there was no wiring difference).  EDO
is also called "Hyper Page Mode" or "Hyper Page Read Cycle" in various
circles, and the timing diagrams show an entirely different read cycle
is used with a much faster cycle time.  In PC's you'll see reference to
timing cycles such as 5-2-2-2, or 6-3-3-3, etc.  There's an associated
"Hyper Page Write Cycle" too.

Supposedly all "current" major PC chipsets that support FPM also support
EDO, but I don't yet know too much about what the Digital Alpha
motherboard chipsets support (21172 in particular), though I'd bet you
can find data sheets for many of the "supported" SIMMs and if you can
also find data sheets for any SIMMs you have on hand then you should be
able to compare them and see if they're compatible.  Just because they
work the first time you try them doesn't mean they'll be reliable or
work for the long term (though it is quite likely they will).  Give the
way the EDO read cycle timings look I'd suspect that all EDO SIMMs that
are otherwise compatible will work in FPM systems too, but I may be out
to lunch here.

From what I see in the PC164 manual, the board definitely requires
quad-CAS SIMMs, and does not use the "presense detect" pins.
Interestingly the board lists a RAS3 pin (33) and RAS1 pin (45) that at
least one 16Mx4bit FPM SIMM data sheet I have list as NC; and it shows
+5 on pin 66 where the SIMM shows NC; and GND on 71 where the SIMM shows
NC.  There's also the interesting statement "The AlphaPC 164 implementes
the alternate memory mode for DRAM RAS and CAS control signals.
Alternate memory mode is explained in the Digital Semi. 21172 Core Logic
Tech. Ref. Manual."

As I recall there was an explicit list of supported SIMM part numbers on
the Alpha OEM hardware compatability list page, but it's been updated to
only include the 164SX and 164LX SDRAM DIMMS and I can't find the
previous version anywhere, and the product documents for the PC164 are
gone too.  If anyone has a copy of this list saved perhaps they could
post it.

Lots of info about PC RAM can be found here:

	http://www.tomshardware.com/guides/ram.html

One side issue -- when I first looked at Alpha ATX motherboards I
thought the PC164 (and the 21172 chipset in particular) used ECC
protected L3 Bcache, but in the manual it says it's just 128-bit wide
direct mapped synchronous SRAM (though they are supposedly 32Kx36bit).

Also, I note the PC164 lists the fan sensor as a "line that will drive a
signal when air-flow has stopped" whereas most PC fans with sensors
generate a tachometer signal, most often twice per revolution.  I
understand though that these boards will come with fans, so hopefully
this isn't an issue (or perhaps the manual is backwards?).

-- 
							Greg A. Woods

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