AMD Athlon II X4 638 socket: Socket FM1

What the Athlon II X4 638 is

AMD Athlon II X4 638 is a quad core Llano processor that AMD released in February 2012 for socketed desktops. It uses the Stars K10 core design on a 32 nm process, running at 2.7 GHz with no turbo. The cache is 4 MB L2 (1 MB per core, no L3). Power is 65 W. There are no integrated graphics on this Athlon variant, so a separate card is required for any display output.

The four cores carry 1 MB of L2 each with no L3, and threads equal cores at 4. Each core keeps its own floating point unit, which keeps single thread behavior predictable across office apps. The chip is a Llano die with the graphics block disabled, which is why it needs FM1 rather than AM3.

At 65 W the Athlon II X4 638 fits slim towers with stock cooling. The 2.7 GHz fixed clock suits office multitasking without tuning. Boards pair it with DDR3 DIMMs and SATA SSDs. The How to find your CPU socket type guide shows how to confirm the socket before ordering parts.

Socket and motherboard compatibility

The Athlon II X4 638 fits Socket FM1 on A55 and A75 boards. The 905-contact socket accepts Llano APUs and Athlon derivatives, but BIOS support still decides each model. DDR3 modules match stock operation without tuning.

Power delivery suits the 65 W rating. A board that lists Llano Athlon support is the safe choice. The CPU cooler compatibility by socket guide explains the 48 by 96 mm pattern shared with AM2, AM3 and FM2. Socket vs chipset: what decides whether a CPU works covers AMD pairing rules.

Memory, PCIe and cooling

Memory support is ddr3 up to 1866 mt/s, two channels across two channels. Boards commonly offer four DIMM slots on ATX and two on micro ATX. Two matched modules give the rated bandwidth.

PCI Express is PCIe 2.0 from the APU for one graphics slot with 16 lanes, since every Athlon FM1 build needs a separate card. A mainstream card of the era pairs without waste. SATA and USB come from the Hudson controller hub.

Cooling follows the 65 W rating. The stock cooler suits office loads, while a compact tower helps sustained tasks. The 48 by 96 mm mounting pattern carries over from AM2 and AM3, so an older AMD cooler often fits.

Upgrade paths

Within Socket FM1 the Athlon II X4 638 sits in the Athlon quad core group without graphics. A same socket move can add clocks or an APU with graphics, for example AMD Athlon II X4 641. The board stays, while memory and cooler often carry over.

A platform move means a new socket. AMD moved mainstream desktops to Socket FM2 with Piledriver cores and a different pin count. That move needs a new board while DDR3 memory often carries over. Keep the FM1 board for secondary duty where it still runs well.