Consequently, JEDEC developed the Fully-Buffered DIMM (FB-DIMM) specification, a serial interface that eliminates the parallel stub-bus topology and allows higher memory bandwidth while maintaining or increasing memory capacity.
FB-DIMM architecture
The FB-DIMM architecture has serial links between the memory controller and the FB-DIMMs, which are connected in a daisy chain configuration (Figure 15). Relative to the memory controller, there are 10 outbound links and 14 inbound links, also known as southbound and northbound links, respectively. These serial links connect the memory controller to an advanced memory buffer (AMB) chip that resides on each FB-DIMM, creating a point-to-point architecture. The outbound links transmit commands and write data to the FB-DIMMs while the inbound links transmit read data back to the memory controller.
The clock signal is distributed over a different set of pins. In addition to communicating over the Outbound lanes, the memory controller communicates configuration information with each AMB over the SMBus. The AMB is an intelligent chip that manages serial communication with the memory controller and parallel communication with local DRAM devices. Each AMB receives signals (address, write data, and command information) through the outbound links and re-transmits the signal to the next FB-DIMM on the channel. Each AMB decodes the command data and ignores the commands that are targeted for a different DIMM. The targeted AMB performs a read or write operation to local DRAM devices through a parallel interface. In the case of a read operation, the AMB serializes data from the DRAM devices and transmits it to the memory controller through the inbound links.
Figure 15. Serial communication between daisy-chained FB-DIMMs on a single channel.
When using DDR2-667 DRAM on the FB-DIMM, the peak theoretical throughput of the inbound links is 5.4 GB/s. The peak theoretical throughput of the outbound links is half the amount of data as the inbound links, approximately 2.6 GB/s.