AMD Athlon 64 FX-55 socket: Socket 939

What the Athlon 64 FX-55 is

The AMD Athlon 64 FX-55 is a 2.6 GHz unlocked single-core enthusiast processor from October 2004. It runs one K8 core at 2.6 GHz with no turbo, so 1 core gives 1 thread. Cache is 1 MB of L2 with 64 KB of L1 instruction and 64 KB of L1 data cache on a 130 nm process. Power is rated at 104 W in a Socket 939 package with part number ADAFX55DEI5AS.

Clawhammer marks the second FX base after SledgeHammer, moving the FX line from Socket 940 registered memory to Socket 939 unbuffered memory. The FX-55 uses a 200 MHz system clock with a 13x unlocked multiplier at 1.50 V and a 1000 MHz HyperTransport link. AMD sold it at 827 dollars as the fastest widely available single core for the socket at launch. The chip supports MMX, SSE, SSE2, Enhanced 3DNow, NX bit and AMD64 with CoolnQuiet control.

Unbuffered memory is the practical change from the Socket 940 FX-53. Socket 939 boards take standard desktop DDR modules in two channels, while Socket 940 FX boards need registered modules. That is why the FX-55 fits mainstream enthusiast towers with AGP or PCIe graphics, rather than server style boards.

Socket and motherboard compatibility

The FX-55 fits Socket 939 desktops with unbuffered DDR memory and a BIOS that lists FX-55 parts. It needs a 939-pin board with dual channel DDR slots, not a Socket 940 board with registered slots or a Socket AM2 board with DDR2 slots. The Socket vs chipset: what decides whether a CPU works guide covers pairing rules.

A Socket 939 enthusiast board with AGP or PCIe graphics is the natural match. Boards that run the Clawhammer FX-53 at 2.4 GHz normally accept the FX-55 at 2.6 GHz with a cooler rated for 104 W, since the socket and memory type stay flat. Socket 940 looks close in pin count yet needs registered memory and a different power plane, so cross fitting fails. Forcing packages can bend pins.

Memory, PCIe and cooling

Memory is unbuffered DDR through the on-chip controller in two channels. Boards take desktop DDR modules to 6.4 GB/s of bandwidth, with the controller setting timing. Registered server modules are not needed and may not boot on desktop 939 boards.

No PCIe lanes originate on the die in the modern sense. A 1000 MHz HyperTransport link ties the CPU to the chipset, while AGP or PCIe slots hang off the chipset for graphics. Cooling uses the AMD retention frame with a heatsink rated for 104 W. The unlocked multiplier suits overclocking on boards that expose the setting, with stability set by voltage, cooling and memory timing.

Upgrade paths

On Socket 939 the faster unlocked step is the FX-57 at 2.8 GHz with 1 MB on the San Diego base, or the FX-60 at 2.6 GHz with two cores on the Toledo base for threaded tasks. The Clawhammer FX-53 at 2.4 GHz is the direct slower sibling on the same socket and memory. Beyond Socket 939 a move to Socket AM2 needs a new board with DDR2 memory, with Windsor FX-62 parts as the AM2 successor. The FX-55 lifts with the lever, so swaps need no solder work.