Intel Pentium III 1266S socket: Socket 370

What the Pentium III 1266S is

The Intel Pentium III 1266S is a 1266 MHz single-core server processor from July 2001. It runs one P6 Tualatin-S core at 1266 MHz with no turbo, so 1 core gives 1 thread. Cache is 512 KB of full-speed L2 with 32 KB of L1 on a 130 nm process. Power is rated at 30 W in a Socket 370 FC-PGA2 package with part numbers SL5LW and SL5QL on a 133 MHz front side bus with a 9.5x multiplier at 1.45 V.

A 9.5x half-ratio at the top of the server set gives the 1266S the lead among mid-2001 S parts, which is why it carried paired workgroup loads before the 1400S arrived. Intel placed the 1266S in July 2001 with desktop Tualatin 1000 through 1200 parts. The chip supports MMX, SSE and two-way SMP.

High server clocks with double cache suit late Socket 370 workgroup boards with ECC memory and PCI-X slots. The 30 W envelope keeps dual thermals workable with ducted air. That is why the 1266S shipped in paired database boxes.

Socket and motherboard compatibility

The 1266S fits Socket 370 late server boards with Tualatin-S support. It needs a 370-pin FC-PGA2 server board with a 133 MHz bus and a BIOS that lists Pentium III-S 1266S. The Socket vs chipset: what decides whether a CPU works guide covers pairing rules.

Late workgroup boards are the natural match. Boards listing 1266S accept one or two chips with 30 W coolers each. Early S boards with small VRM stages may need a revision check before the faster string boots.

Memory, PCIe and cooling

Memory is PC133 ECC SDRAM through the board chipset, matched to the board DIMM sockets. The CPU has no memory controller, so the chipset sets ECC paths. Late ServerWorks boards commonly run interleaved ECC pairs for dual 1266S chips.

A 133 MHz front side bus ties each CPU to the chipset, while PCI-X hangs off the chipset. No PCIe lanes originate on the dies. Cooling uses Socket 370 lug clips with 30 W server coolers and front-to-back ducts.

Upgrade paths

On the same socket, a move to a Pentium III 1400 such as Intel Pentium III 1400 shifts to desktop 256 KB cache at higher clocks when single-thread speed matters more than SMP cache. The 1400 drops SMP paths, so dual boards lose the second chip. A Pentium III 1133S such as Intel Pentium III 1133S drops 133 MHz at similar power.

A move to Pentium III-S 1400S keeps server cache with higher clocks on late boards. The Socket 370 page lists the server range for planning.