Mobile PCI Express Module (MXM)

A Mobile PCI Express Module (MXM) is an interconnect standard for GPUs (MXM Graphics Modules) in laptops using PCI Express created by MXM-SIG. The goal was to create a non-proprietary, industry standard socket, so one could easily upgrade the graphics processor in a laptop, without having to buy a whole new system or relying on proprietary vendor upgrades.

A common misconception about MXM is that a certain model graphics card (e.g. Nvidia 9800M GT) "is MXM 2.1", and therefore any notebook with a 9800M GT fully implements MXM 2.1. However, this is incorrect. While Nvidia defines a lot of MXM specifications, they do NOT manufacture or design MXM cards themselves, which mostly consist of a PCBwith vRAM and an Nvidia or ATI GPU core. Therefore any model of GPU can be manufactured in MXM, but a laptop released with any particular graphics card model may or may not implement MXM regardless. This is because its the decision of the ODM whether or not to implement MXM, not Nvidia's or ATI's.

2nd generation configurations (MXM 3)

Smaller graphics modules can be inserted into larger slots. Heatsink mounting remains the same for type A and B modules.

MXM Type	Width	Length	Module Compatibility	Thermal Compatibility	Max. Power	GPU memory bus
MXM-A	82mm	70mm	A	A	55W	64-bit or 128-bit
MXM-B	82mm	105mm	A, B	A, B	100W	256-bit

資料來源: http://en.wikipedia.org/wiki/Mobile_PCI_Express_Module