CPU All Mac Pro systems were available with one or two central processing units (CPU) with options giving 2, 4, 6, 8, or 12 cores. As an example, the 8-core standard configuration Mac Pro 2010 uses two 4-core Intel E5620 Xeon CPUs at 2.4 GHz, but could be configured with two 6-core Intel Xeon X5670 CPUs at 2.93 GHz. The 2006–2008 models use the LGA 771 socket, while the Early 2009 and later use the LGA 1366 socket, meaning either can be removed and replaced with compatible 64-bit Intel Xeon CPUs. A 64-bit EFI firmware was not introduced until the MacPro3,1, earlier models can only operate as 32-bit despite having 64-bit Xeon processors, however this only applies to the EFI side of the System, as the Mac boots everything else in BIOS Compatibility mode, and operating systems can take advantage of full 64 bit support. The newer LGA 1366 sockets utilize Intel's QuickPath Interconnect (QPI) integrated into the CPU in lieu of an independent system bus; this means the "bus" frequency is relative to the CPU chipset, and upgrading a CPU is not bottlenecked by the computer's existing architecture. Memory The original Mac Pro's main memory uses 667 MHz DDR2 ECC FB-DIMMs; the early 2008 model uses 800 MHz ECC DDR2 FB-DIMMS, the 2009 and onward Mac Pro use 1066 MHz DDR3 ECC DIMMs for the standard models, and 1333 MHz DDR3 ECC DIMMs for systems configured with 2.66 GHz or faster CPUs. In the original and 2008 models, these modules are installed in pairs, one each on two riser cards. The cards have 4 DIMM slots each, allowing a total of 32 GB (1 GB = 10243 B) of memory (8 × 4 GB) to be installed. Notably, due to its FB-DIMM architecture, installing more RAM in the Mac Pro will improve its memory bandwidth, but may also increase its memory latency. With a simple installation of a single FB-DIMM, the peak bandwidth is 8000 MB/s (1 MB = 10002 B), but this can increase to 16000 MB/s by installing two FB-DIMMs, one on each of the two buses, which is the default configuration from Apple. While electrically the FB-DIMMs are standard, for pre-2009 Mac Pro models Apple specifies larger-than-normal heatsinks on the memory modules. Problems have been reported by users who have used third party RAM with normal size FB-DIMM heatsinks. (see notes below). 2009 and later Mac