Ask Dan: 99 dollars per core?!

Date: 21 July 2007
Last modified 03-Dec-2011.


Is the Intel Core 2 Quad Q6600 that Aus PC Market have for sale for only $AU396 a real QUAD core CPU? For $396?

Why does the description say "w/8Mb Cache (4Mb per core)"? Shouldn't it be 2Mb per core?

Also, why is this CPU (and several others on the Aus PC site) "not recommended for use with 4 sticks of RAM"?

And why does it say "P35 boards recommended..."? I read in Intel's product brief (PDF here) that it is designed for i975X as well.

What's the big difference between P35 and i975X motherboards for this CPU?


Yes, it's a real quad core CPU, and yes, it only costs $AU396 (actually, the day after I put this piece up, the price dropped to $AU385 delivered!). That's 99 Australian dollars ($96.25 now!) per core, including delivery.

It wasn't always that way. When the Q6600 was new in January 2007, the thousand-unit wholesale price was $US851. Now it's well under half that price, retail! I'm not sure, but Intel's price cuts of the last week may be the fastest CPU discounting in history.

The Q6600, at 2.4GHz stock speed, is the base model in the range that also includes the Q6700 (2.66GHz) and the Core 2 Extreme QX6800 and QX6850 (2.93 and 3GHz, respectively).

The Extreme chips have unlocked multipliers. The 6600 and 6700 have a locked multiplier and so can only be overclocked by increasing the motherboard bus speed. Fortunately, these chips aren't the new breed with a 1333MHz stock bus speed; their bus is "only" 1066MHz by default, which on a motherboard that can handle 1333MHz-plus-a-bit turns out to give a decent amount of overclocking headroom.

All four of these chips use the "Kentsfield" core, which contains two physically separate processor dies, each of which is further divided into two independent CPUs. Each of the dies inside the Kentsfield is much the same as a whole Core 2 Duo chip, as you'd expect.

(Incidentally, as far as Microsoft is concerned, even a quad-core CPU only counts as one "processor" for licensing purposes. So even if you're running Windows XP Home or Windows Vista Home Basic or Premium, which all only allow you to have one "processor", you'll still be able to use all four cores in a Core 2 Quad.)

The Level 2 cache on Core 2 CPUs is called "Advanced Smart Cache", which for once is not just a new marketing label slapped on a previous technology. The cache is shared between the cores and can be dynamically allocated. If one core needs a lot of L2 cache and the other doesn't - for instance, if you're playing a game that only uses one core - all of the cache can be used by that one core.

The Kentsfield chips can do this too, but only between the cores on each physical die. The "4Mb per core" thing on the AusPC page therefore isn't actually correct - it should be "4Mb per die" - but it at least partially conveys the idea that the cache on the CPU is shared between cores, but is pre-divided into two 4Mb portions, with the cores on one die unable to access any of the cache on the other.

The recommendation against using four memory modules comes from the fact that a lot of people who buy certain CPUs are going to overclock them, and using more than two memory modules makes that harder to do. Some older Athlon 64 and Opteron processors, which have an on-board memory controller, actually ran the memory bus slower if you installed more than a couple of modules. No such issues exist with current Intel chips, but memory modules still use a chunk of the motherboard's regulator capacity, which can start running low when you're overclocking a quad core chip.

This applies particularly strongly to bus-speed overclocking, the only kind you can perform on a non-Extreme-Edition Core 2 Quad. Increasing the processor bus speed increases the speed of things other than the CPU, and also makes perfect timing more important; you often need to bump up CPU and RAM voltage to get higher speeds to work, and that increases power draw too.

Plenty of people are perfectly successfully getting solid overclocks out of quad-core Intel chips while using four memory modules, but if you're buying a new computer, you might as well pay the small price premium to get the same amount of memory in only two modules.

(Well, unless you want a 64-bit system with more than 4Gb of memory, which can give rise to its own problems; read about them here and here.)

The P35 chipset recommendation's much the same deal. Yes, an Intel quad core CPU ought to work fine at stock speed on various 975X and even 965 boards, with the usual disclaimers about BIOS support for newer processors - though the quads have been out for more than six months now, so recent BIOSes for lots of boards support them. But if you want to overclock, which a large segment of the cheap-Q6600 market (in particular) does, your best bet is a board based on the newer P35 chipset.

Aus PC Market are very aware of the special needs of this particular market, and have put together a handy-dandy package of a Q6600, Gigabyte GA-P35-DS3P motherboard and two 1Gb Corsair "XMS2-6400" memory modules (which certainly shouldn't be the limiting factor in your overclocking adventures...), all for $AU798 ($787, now!) delivered. Yes, considerably less than the CPU by itself used to cost, only months ago. Australian shoppers who'd like to snap one up can do so here!

If you've got more money burning a hole in your pocket you could buy the same mobo and CPU and substitute two 2Gb memory modules, for two or three hundred bucks more. The problem with that is that you're likely to not be able to use a large portion of the fourth gigabyte of the memory, as I explain in tiresome detail in my aforementioned piece about the "3Gb barrier".

The above motherboard, like pretty much every other modern motherboard, will run your RAM in double-speed dual channel mode if you install two or four paired memory modules. Like tons of other boards for some time now, though, it also supports Dual-Channel Asymmetric mode. That means that if you install three 1Gb memory modules, giving the optimum amount of useful RAM for most PCs today, you'll get full dual channel speed when you're accessing the single pair of modules, and only revert to single channel speed for the last unpaired one.

Since the speed difference between dual and single channel mode seldom amounts to much for real world tasks, Dual-Channel Asymmetric is very nearly exactly as fast as full dual channel. I'd be willing to go out on a limb and say that nobody in the world would be able to tell the difference without benchmarking software, or at least a stopwatch.

Going to three memory modules may still slightly lower your overclocking ceiling, though. An alternative that'll still work in Asymmetric mode is one 2Gb memory module and one 1Gb one. Then, it's possible that the inescapable mis-matched-ness of the dissimilar modules may cause a problem. But I still had to mention it, or I'd get letters.

Overall, even taking into account the cost of a new motherboard, the Q6600 is just amazing value for money. The Q6600, like every other CPU, will be replaced by even better value options as time passes - but those of us still living in July 2007 have just been given a very tempting upgrade opportunity.

As I said the other day, lots of people buying dual core CPUs don't really need them, but when they're under $AU200, why the heck not? The value proposition's similar for the Q6600 even if you don't intend to overclock. If you do intend to wind the bus speed up, a P35 board with two sticks of fast RAM ought to be OK a bit above 1400MHz, and most Q6600s (Aus PC Market obviously offers no guarantees...) seem to be willing to handle that roughly-30% overclock.

That'll give you four cores at about 3.2GHz, faster than the stock speed of the $AU1500 Core 2 Extreme QX6850.

From a sub-$AU400 processor.

Any further questions?

Australian shoppers can order Aus PC Market's Q6600, motherboard and RAM combo pack here.

If you'd like to buy the CPU by itself, it's here.