4 DDR2 DIMMs versus 2

[Basilisk]

When I bought a mother-board several years ago, I recall comments (in the manuals and in reviews) that running with 4 DIMMs automatically dropped the maximum RAM access speed.

I'm now thinking of buying a BIOSTAR TForce TF7050-M2 with an AMD 5600+, and wondering what "price" I might pay for starting with 2 DIMMs and upgrading to 4 later: I have a pair of PC2-8500's (DDR2-1066's) [2x1GB] and would feel a bit silly if they were treated as, say, PC2-5300 after adding two more 8500's.

If someone has a link to a technical reference on this -- or solid knowledge -- I'd appreciate it. I don't seem to be Google'izing productively on this issue!

 

[lopri]

Let me apologize first because my reply isn't based on solid knowledge.

What you recall from the comments a few years back is still valid today (in varying degree per chipset, per mobo). It's about how well a memory controller handles memory. In theory and in practice, 4 DIMMs will always be more stressful to a memory controller than 2 DIMMs, other things being equal. Also in practice, how well BIOS is programmed to handle more RAM matters a lot. Poorly written BIOS will not even let you run 4 sticks of DIMM. The usual symptoms include, but are not limited to):

1. General instability
2. Poorer overclocking compared to when 2 sticks are used
3. Requiring more voltages (for CPU, NB, or memory itself)
4. Degraded performance (need to loosen timings/frequencies) -> but the performance loss stemming from this is quite trivial in a grand scheme of the things
5. Shortened life-span

When a certain RAM is advertised as DDR2-1066, it doesn't mean that it will automatically run at that speed when you first boot the system. Both on AMD/Intel platform, the supported speed by the CPU/chipset and the memory's SPD (which can be quite different from advertised specs), as well as the BIOS will determine the speed of memory. Rated PC8500 usually means that the vender will guarantee the said frequency, along with the timings and voltages.

Thankfully, there is this thing called 'dividers' and many BIOS will let you configure your memory subsystem. In generall, on Intel platform you will want at least 1:1 or higher (RAM speed vs FSB). On AMD side, as long as you can run your memory close to DDR2-800 you should be OK.

To answer your question more directly: You probably won't have to lower the frequency of your DIMMs by from going 2 sticks to 4 sticks, but it's most likely that you will have to run 2T instead of 1T with 4 sticks of DIMM, on AM2 platform. You will lose a tiny bit of performance, but it's nothing to lose your sleep over.

 

[Basilisk]

Thanks for your reply. The Nerd-Factor in me would like to find a performance-study of various boards' using 2 and 4 DIMM memories, but all I've found has been one passing reference to going from 1T to 2T with 4 sticks.

I wonder if your 5 points boils down to "increased capacitance and drain requires compensating V increases and/or slackened timings". No offense intended: it's just that some of your points seem to overlap or view the same issue from alternate directions.

Re "how well BIOS is programmed to handle more RAM": It's an indicator of my ignorance of current H/W that I'd supposed the only direct control an Athlon BIOS had over the timings was by altering the voltages -- that the BIOS was largely stuffing timing values into the Athlon's own timing registers. Now that I think about it, there are probably multiple, inscrutable registers and the BIOS is translating "4-4-4-12" timings into a slew of them -- and could well make a cock-up of the permutations and combinations.

With AMD the timings must be on-board the CPU, right? Isn't that where the memory-access control h/w is? So for any one board to really have disadvantageous memory accessing they'd have to screw up loading the registers or indirectly mess things up through increased "wire" lengths/capacitance/coupling/resistance/whatever).

Blah blah blah... all noise and no content from me this morning. Must get another coffee.