Intel Pentium III 866 socket: Socket 370

What the Pentium III 866 is

The Intel Pentium III 866 is an 866 MHz single-core desktop processor from June 2001. It runs one P6 Coppermine T core at 866 MHz with no turbo, so 1 core gives 1 thread. Cache is 256 KB of full-speed L2 with 32 KB of L1 on a 180 nm process. Power is rated at 38 W in a Socket 370 FC-PGA2 package with part numbers SL5QE and SL5HG on a 133 MHz front side bus with a 6.5x multiplier at 1.75 V.

A half-ratio lift over the 800 at the same 38 W keeps cooler choice identical, which is why the 866 suited buyers who wanted a small speed gain with no thermal change. Intel placed the Coppermine T 866 in June 2001 with the 800, 933, 1000 and 1133 parts. The chip supports MMX and SSE with Advanced Transfer Cache.

Balanced clocks with a full cache suit late Socket 370 office desktops with SDRAM and discrete AGP cards. The 6.5x ratio keeps the 133 MHz bus in spec. That is why the 866 shipped in mainstream business towers.

Socket and motherboard compatibility

The 866 fits Socket 370 late desktops with Coppermine T support. It needs a 370-pin FC-PGA2 board with a 133 MHz bus and a BIOS that lists Coppermine T parts. The Socket vs chipset: what decides whether a CPU works guide covers pairing rules.

Mainstream SDRAM towers are the natural match. Boards listing Pentium III 866 Coppermine T accept the chip with a 38 W cooler. Because the 866 shares power with the 800, swaps between the two rarely need a new cooler, but the BIOS string must still match.

Memory, PCIe and cooling

Memory is PC100 and PC133 SDRAM through the board chipset, matched to the board DIMM sockets. The CPU has no memory controller, so the chipset sets the type and speed. Office 815 boards commonly run PC133 pairs for 133 MHz parts.

A 133 MHz front side bus ties the CPU to the chipset, while AGP and PCI hang off the chipset. No PCIe lanes originate on the die. Cooling uses the Socket 370 lug clip on the FC-PGA2 spreader with a cooler rated for 38 W.

Upgrade paths

On the same socket, a move to a Pentium III 933 such as Intel Pentium III 933 keeps the board and memory when the support list names it. The 933 adds 67 MHz at lower 27 W, so thermals improve while clocks rise. A Pentium III 800 on the same list drops 66 MHz at equal power.

A move to Tualatin 1000 needs a Tualatin-compatible board with a BIOS update. The Socket 370 page lists the Coppermine T to Tualatin path for planning.