Fully Buffered DIMM
Fully Buffered DIMMs (FB- DIMM or FBDIMM for short ) are DRAM modules in which the memory chips are not connected directly to the computer, but instead contain an additional component for control (buffer). They are seen as the predecessors of registered modules , both of which are primarily used for the server and workstation area. This property is used on DDR2 and DDR3 memory modules. In addition to the memory chips is an Advanced Memory Buffer (AMB) on the DIMM - PCB . This module establishes a connection to the memory controller via 24 pairs of lines, similar to the serial PCI Express technology. FB-DIMM technology can manage a maximum of eight modules per memory channel.
The serial interface technology of the AMB enables high performance and scalability. One advantage over parallel Registered DIMMs is the increase in memory capacity by approximately a factor of 24 and the memory bandwidth by approximately four times. Extensive data security features round off the concept. Disadvantages include increased power consumption and heat generation in the modules, primarily caused by the AMB. In addition, there is a slightly higher latency compared to conventional (parallel) DDR (2) -SDRAM, due to the error correction of the AMB chip.
Various methods provide increased security with regard to the error rate. Silent Data Corruption Failure in Time (SDC-FIT-Rate) is given by a factor of 1000 less than that of the overall system. ECC and CRC (via data and commands) are well-known standards. Supplemented by the transient bit error function, bit errors are recognized and data is transferred again. The path-through-path logic increases the reliable availability of the data. Faulty data lines are detected by means of bit lane fail correction and the data line is replaced by a functioning one (through internal rerouting of the signal line). The AMB maintains an error register in which faulty memory locations are noted; if there are too many, the diagnostic software informs the system administrator.
With the hot-add function, memory modules can be added during operation without first having to deactivate the memory bank. Since 2006 there have been the first ( motherboard ) chipsets that offer corresponding memory support. However, this must be supported by the operating system, otherwise the additional memory will not be recognized or used.
FBDIMMs have a different socket than the usual DRAM modules. With FBRAM, the coding notch is shifted 8 mm to the right of the center of the module. With the usual DRAM modules it is only 4 mm.