The Socket G3 Memory Extender (G3MX) was a planned

Advanced Micro Devices Advanced Micro Devices, Inc. (AMD) is an American multinational corporation and technology company headquartered in Santa Clara, California and maintains significant operations in Austin, Texas. AMD is a Information technology, hardware and F ...

' solution to the problem of connecting large amounts of memory to a single microprocessor. The G3MX was expected to be available on AMD 800S series chipset for server market starting from 2009, but was officially cancelled together with the cancellation of Socket G3 in early 2008.PC Watch report
retrieved August 20, 2008 Electrical limitations preclude connecting more than 2 unbuffered

DDR SDRAM Double Data Rate Synchronous Dynamic Random-Access Memory (DDR SDRAM) is a double data rate (DDR) synchronous dynamic random-access memory (SDRAM) class of memory integrated circuits used in computers. DDR SDRAM, also retroactively called DDR ...

DIMM A DIMM (Dual In-line Memory Module) is a popular type of memory module used in computers. It is a printed circuit board with one or both sides (front and back) holding DRAM chips and pins. The vast majority of DIMMs are manufactured in compl ...

s or 4 buffered DIMMs to a single shared bus. It is also impractical to manufacture a single chip with more than two DDR memory buses (channels). Thus, it is impossible to connect more than 8 DIMMs to a single chip. This is typically the per-processor limitation as well. The obvious solution is to use a narrower, higher-speed bus to interface to memory, and to implement it as a point-to-point link, daisy-chaining additional modules. However,

Intel Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California, and Delaware General Corporation Law, incorporated in Delaware. Intel designs, manufactures, and sells computer compo ...

have made two attempts at this, neither hugely successful: *

RDRAM Rambus DRAM (RDRAM), and its successors Concurrent Rambus DRAM (CRDRAM) and Direct Rambus DRAM (DRDRAM), are types of synchronous dynamic random-access memory (SDRAM) developed by Rambus from the 1990s through to the early 2000s. The third-generati ...

implements the bus on a DRAM chip. However, the high-speed circuitry increased power consumption, and the daisy-chaining caused significantly higher

memory latency ''Memory latency'' is the time (the latency) between initiating a request for a byte or word in memory until it is retrieved by a processor. If the data are not in the processor's cache, it takes longer to obtain them, as the processor will ha ...

. Because it is difficult to implement high-speed circuitry on the same semiconductor process, costs were high. *

FB-DIMM A Fully Buffered DIMM (FB-DIMM) is a type of memory module used in computer systems. It is designed to improve memory performance and capacity by allowing multiple memory modules to be each connected to the memory controller using a serial interfa ...

s add a separate memory controller chip to each memory DIMM. This "advanced memory buffer" (also known by the abbreviation ''AMB'') provides the necessary high-speed circuitry. However, the same power and heat problems have arisen. AMD's answer to this is the G3MX chip. This is very similar to the AMB, but is intended to be placed on the

motherboard A motherboard, also called a mainboard, a system board, a logic board, and informally a mobo (see #Nomenclature, "Nomenclature" section), is the main printed circuit board (PCB) in general-purpose computers and other expandable systems. It ho ...

, not on the DIMM. It can connect to multiple DIMMs but, to minimize latency, is not designed to be daisy-chained. The G3MX has an asymmetrical link to the processor, to match typical memory usage patterns. 20 differential signals supply read data to the processor, and 13 differential signals receive commands and write data. This totals 66 pins, less than half of what is required for a DDR2 or DDR3 interface. Thus, a processor can easily have 4 G3MX memory interfaces, each with 4 buffered DIMMs attached, allowing up to 16 DIMMs to feed one processor.

References

* * * Also mentions rumors that Intel is working on a similar on-motherboard "AMB2". {{reflist AMD products X86 memory management