| From: Globe Trotter <itsme_410@xxxxxxxxx> | 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.)
