AMD Athlon 64 FX-51 socket: Socket 940

What the Athlon 64 FX-51 is

The AMD Athlon 64 FX-51 is a 2.2 GHz unlocked single-core enthusiast processor from September 2003. It runs one K8 core at 2.2 GHz with no turbo, so 1 core gives 1 thread. Cache is 1 MB of L2 on a 130 nm process. Power is rated at 89 W in a Socket 940 package with part numbers ADAFX51CEP5AK and ADAFX51CEP5AT.

SledgeHammer FX opens the 64-bit enthusiast line on Socket 940 with registered memory like a server, before Clawhammer FX parts moved to Socket 939 with unbuffered memory. The FX-51 uses a 200 MHz system clock with an 11x unlocked multiplier at 1.50 V and an 800 MHz HyperTransport link. AMD sold it at 733 dollars in September 2003 as the first unlocked K8 single core. The chip supports MMX, SSE, SSE2, Enhanced 3DNow, NX bit and AMD64.

Registered memory defines the board. Socket 940 FX boards take server style registered DDR modules, while later Socket 939 FX boards take desktop unbuffered modules. That is why the FX-51 suits workstation towers with server memory paths, rather than mainstream enthusiast towers with standard DDR slots.

Socket and motherboard compatibility

The FX-51 fits Socket 940 desktops and workstations with registered DDR memory and FX-51 BIOS support. It needs a 940-pin board with server memory slots. The Socket vs chipset: what decides whether a CPU works guide covers pairing rules.

Workstation towers with registered DDR are the natural match. Boards that run Opteron 100 series SledgeHammer parts normally accept the FX-51 with the same 89 W cooling, since the socket and memory type match. Socket 939 boards need unbuffered memory instead.

Memory, PCIe and cooling

Memory is registered DDR through the on-chip controller. Only registered ECC modules boot on Socket 940 boards. The 1 MB L2 keeps working data on die at 2.2 GHz.

An 800 MHz HyperTransport link ties the CPU to the chipset, while AGP or PCIe slots hang off the chipset. No PCIe lanes originate on the die. Cooling uses the Socket 940 retention frame with an 89 W heatsink. The unlocked multiplier suits overclocking where boards expose the setting.

Upgrade paths

On Socket 940 the step is the Athlon 64 FX-53 SledgeHammer at 2.4 GHz with the same socket and memory. Socket 939 offers Clawhammer FX-53 parts with unbuffered memory for desktop towers. Beyond these K8 single cores a move to dual-core Toledo parts needs a new socket. The FX-51 lifts with the lever, so swaps need no solder work.