recommendations for a 64-bit laptop with ECC memory?

[D. Hugh Redelmeier]

| From: Globe Trotter <itsme_410 at yahoo.com>

| Dell's Latitude D830 was suggested here,
| but it turns out that the memory it comes with is non-ECC, but DDR2. I don't
| know much about DDR2, but shouldn't ECC memory be important for me since I will
| be using it for scientific calculations? I am looking at a laptop with 4GB RAM.

[The following needs fact-checking.  It was more or less true a few
years ago but there may have been more recent developments.  Please 
correct any mistakes you notice.]

ECC may be a good idea, but the mass market seems to not want to use
it.

Most ECC memory is used in servers and most servers use "registered"
memory (i.e. memory with buffer logic to allow better fan-out).  So
ECC seems to be limited to registered memory and I suspect that no
current notebook uses registered memory.

Besides, ECC is trickier than it used to be.  Most ECC circuits assume
that the failure of each bit in a word is an independant event.  That
used to be the case when DRAM chips were one bit wide.  That is no
longer the case: each chip supplies a bunch of bits in each word and
so the failure of all those bits would be correlated.  The simple
Hamming code ECC mechanisms implemented in some chipsets cannot handle
this (other chipsets don't even implement Hamming code).

This Wikipedia article points at an interesting description of some of
these issues in a server context.  Ten years old!
  http://en.wikipedia.org/wiki/Chipkill

Summary: if you want real ECC in current hardware (i.e. with a
"Chipkill"-like property), you need to buy a moderately serious
server.  Many lesser servers will support ECC but without Chipkill.


PS: I just looked around on the Dell.ca site and found unbuffered
(i.e. not registered) DDR2 with ECC:
  http://accessories.dell.com/sna/productdetail.aspx?c=ca&l=en&s=dhs&cs=cadhs1&sku=A0454028
It seems to work in the following Dell systems:
    Dell Precision Workstation 370
    Dell Precision Workstation 390
    PowerEdge 800 System
    PowerEdge SC420 System
    PowerEdge SC430 system
    Precision WorkStation 370
    Precision WorkStation 390

In the old days you could tell ECC (parity) RAM modules by the fact
that there were 9 memory chips on it.  This has 8 visible chips.

(ECC and parity are sometimes mixed together in memory module terminology.  
Modules that supported parity usually had 9 bits per byte, the extra bit
being for parity.  Once memory transfers were done in units of 8 bytes, it 
was easy to gang the 8 parity bits together to provide ECC.)