Choosing a Xeon for the LGA 771 to 775 mod

Page 2 settled what your board can take in principle: its chipset, the bus speed it is rated for, and whether its BIOS knows 45 nm processors. This page turns that into a choice of chip. We don't keep a list of "tested motherboards", and we don't think one would help you much. Two boards with the same name can sit on very different BIOS releases, and an OEM board built on a given chipset can behave nothing like the retail version. What carries over from one board to the next is the short list of things a Xeon has to match: bus speed, power class, stepping, and the microcode that goes with that stepping. Get those four right and the model almost picks itself.

The specifications below come from Intel's product data as compiled in Wikipedia's list of Core-based Xeon processors. The CPUID values match the update files in Intel's public microcode repository.

The Xeon families that fit

Every chip that takes the adapter sticker uses the LGA 771 package, which has the same 37.5 mm square footprint as LGA 775. Four families are worth knowing, from two process generations.

Harpertown: E5400, X5400 and L5400 (45 nm, four cores)

This is the family most people mean when they talk about the mod. Harpertown is the dual-processor server sibling of the Core 2 Quad Q9000 series: two 45 nm dies in one package, 12 MB of L2 cache in total, SSE4.1 and VT-x. The letter in front of the number tells you the power class. E parts are rated at 80 W, X parts at 120 W (150 W for the two fastest 1600 FSB models), and L parts at 50 W.

These are the 1333 FSB models, the ones that line up with most LGA 775 boards:

For most boards the E5450 is the sensible top pick. It has the Q9650's clock, multiplier and cache with a lower rating, 80 W against 95 W. The X5450 runs at exactly the same 3.00 GHz and adds nothing but 40 W of rated heat, so skip it. The X5460 and X5470 are the faster choices when the board and the cooler have room for 120 W. One notch down, the E5440 matches the Q9550. The E5420 runs at the Q9300's clock with twice its cache. For a small case or a board with modest power delivery, look at the L5420 and its L-series siblings. Avoid the E5405 unless you want a chip that never clocks down: it is the one model in the family without Enhanced SpeedStep (EIST).

Two groups sit outside the table. The E5462, E5472, X5472, X5482 and X5492 run on a 1600 MT/s bus with lower multipliers, and the L5408 is a 40 W part on a 1066 bus. Both only make sense on a board with matching bus support, which the FSB section below explains.

Wolfdale-DP: E5200, X5200 and L5200 (45 nm, two cores)

Same generation, but a single die with two cores and 6 MB of L2. These are the server relatives of the Core 2 Duo E8000 series. The E5240 (3.00 GHz, x9, 65 W) lines up with the E8400. The X5260 (3.33 GHz, x10, 80 W) lines up with the E8600. The X5270 reaches 3.50 GHz (x10.5) at 80 W. It came only in the E0 stepping and has no desktop twin, which makes it the highest-clocked chip in either 45 nm family. The X5272 is the 1600 FSB version at 3.40 GHz. The low-power L5215, L5238, L5240 and L5248 are rated from 20 W to 55 W, though the L5238 and L5240 lack EIST.

A dual-core Xeon makes sense in two cases. One is a board whose support list includes 45 nm dual cores but no quads. The other is a machine that mostly runs single-threaded software, where 3.50 GHz on two cores beats 2.83 GHz on four.

Clovertown (5300) and Woodcrest (5100): the 65 nm parts

Clovertown is the 65 nm quad, a relative of the Kentsfield Core 2 Quad (Q6600 era), and it shares Kentsfield's B3 and G0 steppings. Woodcrest is the 65 nm dual core, the 5110 to 5160. Both fit the sticker, and both are weaker picks for three reasons:

• They need more power for less speed. An E5345 runs 2.33 GHz at 80 W, the same rating an E5450 carries at 3.00 GHz, and the X5355 needs 120 W to reach 2.66 GHz.
• They have less cache (8 MB on the quads, 4 MB on the duals) and no SSE4.1, so software built to require SSE4.1 will not run at all.
• Their voltage window on the standard models is 1.0 to 1.5 V, against 0.85 to 1.35 V for the 45 nm chips.

They earn a place in one situation: a board whose BIOS supports 65 nm Core 2 Quads but never got 45 nm support. Even then, check page 2 first, because the chipset may be the real ceiling. On a board limited to a 1066 bus, the E5310 and E5320 run at the same bus speed as a Q6600. Treat them as a stopgap, not a goal.

What to match against your board

Bus speed: 1333 or 1600

The front-side bus (FSB) links the CPU to the chipset. Its rating in MT/s counts four transfers per clock, so 1333 MT/s is a 333 MHz clock, 1600 is 400 MHz and 1066 is 266 MHz. Core clock is bus clock times multiplier, and on these Xeons you cannot raise the multiplier. That makes the bus speed decide the result.

The arithmetic is easy to check by hand. An E5450 at 333 MHz x 9 gives 3.00 GHz. On a board that can only run 266 MHz, the same chip would give 266 x 9 = 2.40 GHz, if the board starts it at all. The 1600 parts suffer more on a slow bus because of their low multipliers. An E5472 on a 1333 board would give 333 x 7.5 = 2.50 GHz, no faster than an E5420. Buy a 1600 FSB Xeon only for a board whose documentation lists 1600 FSB processors.

Power: TDP against what the board was designed for

TDP (thermal design power) is the heat a cooler must remove at the rated clock. It also says something about current draw, which lands on the board's voltage regulator (VRM). Your board's CPU support list tells you the heaviest chip its maker signed off. If it lists the Q9650 (95 W), an 80 W E-series Xeon sits inside that envelope. If it also lists a 130 W Core 2 Extreme such as the QX9650, a 120 W X-series part is reasonable, provided your cooler is up to it. If the list stops at 65 W chips such as the Q9400S and Q9550S, or you are dealing with a compact OEM desktop, stay with the 50 W L-series.

Stepping and CPUID: C0 (10676) or E0 (1067A)

The stepping is the silicon revision, and the CPUID signature is the number the chip reports to software to identify its family, model and stepping. Both 45 nm Xeon families came in two steppings. C0 reports CPUID 10676 and E0 reports 1067A, the same values as the desktop Core 2 chips of those steppings. You can usually tell them apart from the sSpec code printed on the heat spreader. In Intel's sSpec data, as compiled on Wikipedia, every C0 5200 and 5400 sSpec starts with SLA (SLANQ for a C0 E5450) and every E0 sSpec starts with SLB (SLBBM for an E0 E5450).

This matters because of microcode, the small firmware patch the BIOS loads into the CPU at every start. The BIOS keeps a table of updates keyed by CPUID and by a second value, the platform ID. A Xeon can carry a different platform ID from the desktop chip with the same CPUID. So a BIOS that runs a Q9650 perfectly can still lack the entry an E5450 needs. The Xeon microcode guide explains how to check and fix that. Intel's public microcode package still carries updates for both CPUIDs, revision 60F for 10676 and A0B for 1067A, both dated September 2010.

A board that lists the Q9650 or the E8600 (both E0 only) almost certainly carries 1067A microcode for those desktop chips, which makes an E0 Xeon the closer match. Mod guides and forum threads tend to recommend E0 Xeons for that reason. That is owner experience rather than anything Intel documented for this mod, but the logic holds.

Decision table: from your board to a Xeon class

Read the CPU support list for your board's latest BIOS, find the row that matches the strongest chip on it, and start there.

For a quick first read on a particular combination, the LGA 771 to 775 compatibility checker flags bus, power and microcode questions. To compare a shortlist side by side, use the table of LGA 771 and LGA 775 quad-core chips.

Two hypothetical boards

Say you have a P45 board whose maker lists the Q9650 and the QX9650 from a certain BIOS version onward. Flash that version and the E5450 in E0 (SLBBM) is the straight swap: 333 MHz x 9 = 3.00 GHz, CPUID 1067A like the Q9650, and 80 W against the Q9650's 95 W. The X5460 also fits inside the envelope the QX9650 implies. Before you choose it, run the numbers: 3.16 / 3.00 is about 5 percent more clock, and 120 / 80 is 50 percent more rated heat. On a stock-style cooler, that trade is rarely worth it.

Now say you have a compact office desktop on a G41 board whose support list stops at 65 W chips. An X-series part is out on power alone. The L5420 at 2.50 GHz or the E0-only L5430 at 2.66 GHz stays well under the limit. Read page 2's notes on OEM boards before you order, because some of them check more than the support list shows.

Read the listing before you buy

• Look for the sSpec in the photos and match it to the table above. It tells you the model, the stepping and, through the stepping, the CPUID.
• Skip chips marked as engineering or qualification samples. Their sSpec codes start with Q, and they can be early steppings that no retail BIOS ever supported.
• Don't buy an LGA 771 chip by mistake when you need none. The Xeon X3350, X3360, X3370, X3380 and L3360 are LGA 775 parts that drop straight into a desktop board without a sticker. The X3323, X3353 and X3363 are LGA 771 single-processor parts, and page 2 explains how they differ from the 5400 series.
• Check the gold pads in the photos for deep scratches or corrosion. An LGA chip has no pins to bend, but a damaged pad is still a dead contact.

With a model and an sSpec that match your board, page 4 takes you through the BIOS update, the socket tabs and the sticker, in that order.