On Wed, Mar 28, 2018 at 5:35 PM, D. Hugh Redelmeier via talk < [email protected]> wrote:
> | From: o1bigtenor via talk <[email protected]> > > | Not the OP but - - - - one issue I'm seeing - - - my main system (its > over > | 6 years old) > | has 8 slots for ram. Almost none of the newer mobos have that many. > | (Just one point for where newer isn't always 'better' (whatever that > means! > | - - grin).) > > My Sun 3/60 has 24 slots. But the largest memory module that it will > accept is 1MiB (note: not 1GiB). So more isn't always better :-) > > As memory systems get faster, fan-out and signal path length matter > more. So 4 is apparently the limit for "unbuffered" memory on one PC > memory bus these days. > > If you have multiple processors on a motherboard, they probably have a > separate bus for each processor so you get 4 slots per processor. > > If you have a big server, you use buffered memory (probably with ECC too) > and can have lots* of RAM sockets. With buffering, you lose speed but > gain fan-out. > > Complicating this is the market segmentation games that Intel is > playing. Apparently you pay a lot for a Xeon processor that has enough > address lines. AMD Epyc is smashing those limits so interesting > things might happen. > > This supports 16 LRDIMM DDR4 modules, each could be 32GiB (I think). > 2TiB in total (that's 16 x 128 GiB, so I'm missing something). > <http://b2b.gigabyte.com/Server-Motherboard/MZ31-AR0-rev-10> > > In the good old days, bulk dynamic RAM chips were 1-bit wide so if > your bus was 32-bits wide, you could drive 32 chips with no fan-out on > the data lines. Now RAM chips are typically 8-bits wide, modules are > 64-bit wide, and busses are 64-bit wide, so fan-out is a larger > problem. > > * "lots" is a technical term meaning more than four, but I don't know > how many > OK - - - if I had been referring to server boards I wouldn't have commented but as I was talking about desktops - - - well - - - my point still stands. IIRC when I last looked (a couple months now) all the desktop boards were topping out at 4 ram slots. Dee
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