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What's an AMB?
Today, as servers and workstations are continuously enhanced, memory buses connecting memory modules to servers/workstations need to be faster and larger in capacity.
To increase the number of memory modules, connection by means of a stub-bus is used for conventional unbuffered or registered memory modules. In this type of connection all modules are connected to a single channel, and the load accumulated each time modules are added. At the same time, the growing number of branches makes discontinuity of impedance on transmission lines complicated, and may exert a negative influence on the waveform of signals when high-speed buses operate.
The problem is that when modules are increased in stub-bus architecture, channel performance will decrease, and the use of high-speed memory does not have any significance. For instance, if DDR2-800 is used for DIMM, it is said that only one or two units of DIMM can be connected to a channel to make good use of high speed.
One solution is FB-DIMM, which overturns the concept of bus connection.
With FB-DIMM, modules are connected point-to-point. The host memory controller is connected to a memory module, and a memory module to a memory module one to one. The load of each channel becomes constant, and no branch is needed in this type of connection. Even when modules are added, the signal quality on each channel is kept constant, and operation of DIMM with high-speed memory is assured.
By using high-speed serial interfaces for connecting modules, the number of signals required for connecting FB-DIMM to the memory controller is dramatically reduced from the conventional 240 to only around 70. This allows complicated wiring on the main board to be greatly simplified. As the signals required for connecting to the memory controller are reduced, multi-channeling is achieved more easily, allowing further increases in capacity.
The AMB is a key device recently proposed for configuring FB-DIMM.
It has two serial links, one for upstream and the other for downstream, and a bus to DRAM on DIMM. Serial data from the host memory controller sent through the downstream serial link (southbound) is temporarily buffered, and then sent to DRAM on DIMM. The serial data contains the address, data and command information given to the DRAM, converted in the AMB and sent out to the DRAM bus.
The AMB writes in and reads out from DRAM as instructed by the memory controller. The read data is converted to serial data, and sent back to the memory controller on the upstream serial link (northbound).
The AMB also performs as a repeater between FB-DIMMs.
The AMB transfers information from the primary southbound link connected to the memory controller or the upper AMB to the lower AMB on the next DIMM through the secondary southbound link. The AMB receives information in the lower DIMM from the secondary northbound link, and after merging the information with information of its own, sends it to the upper AMB or memory controller through the primary northbound link. This forms a daisy chain among memory modules.
The AMB provides direct bus interfaces to DDR2 DRAM. With these interfaces, DDR2 DRAM can be mounted as the commodity memory on DIMM without the need to recognize a new channel architecture.
