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Some ECC-enabled boards and processors are able to support unbuffered (unregistered) ECC, but will also work with non-ECC memory; system firmware enables ECC functionality if ECC memory is installed. ECC may lower memory performance by around 2–3 percent on some systems, depending on the application and implementation, due to the additional ...
Two types of DIMMs: a 168-pin SDRAM module (top) and a 184-pin DDR SDRAM module (bottom). The SDRAM module has two notches (rectangular cuts or incisions) on the bottom edge, while the DDR1 SDRAM module has one. Also, each module has eight RAM chips, but the lower one has an unoccupied space for the ninth chip; this space is occupied in ECC DIMMs.
For example, 64 would indicate a 64-bit data width, as is found on non-ECC DIMMs common in SDR and DDR1–4 families of RAM. A memory of width of 72 would indicate an ECC module, with 8 extra bits in the data width for the error-correcting code syndrome. (The ECC syndrome allows single-bit errors to be corrected).
In other words, LR-DIMMs buffer both control and data lines while keeping the parallel nature of all signals. As a result, LR-DIMMs provide large overall maximum memory capacities, while avoiding the performance and power consumption problems of FB-DIMMs, induced by the required conversion between serial and parallel signal forms. [7] [9
Modern DIMMs can for example feature one rank (single rank), two ranks (dual rank), four ranks (quad rank), or eight ranks (octal rank). [ citation needed ] There is only a little difference between a dual rank UDIMM and two single-rank UDIMMs in the same memory channel, other than that the DRAMs reside on different PCBs .
In computing, serial presence detect (SPD) is a standardized way to automatically access information about a memory module.Earlier 72-pin SIMMs included five pins that provided five bits of parallel presence detect (PPD) data, but the 168-pin DIMM standard changed to a serial presence detect to encode more information.
An equivalent system from Sun Microsystems is called Extended ECC, while equivalent systems from HP are called Advanced ECC [3] and Chipspare. A similar system from Intel, called Lockstep memory, provides double-device data correction (DDDC) functionality. [4]
Since then errors have become less visible as simple parity RAM has fallen out of use; either they are invisible as they are not detected, or they are corrected invisibly with ECC RAM. Modern RAM is believed, with much justification, to be reliable, and error-detecting RAM has largely fallen out of use for non-critical applications.