The Memory Controller: Ganged vs Unganged Mode
One of most important developments in the past 10 years has been how how memory speeds have improved. In a decade the speeds went from 133 Mhz SDR DIMM RAM to 1333 Mhz DDR3 DIMM RAM. However, CPU speeds have gotten even faster. Nevertheless, as fast as RAM speeds have improved there has been a latency that has not been overcome. This affects the speed of the CPU.
One way to improve the latency, (or reduce it) is with a process called granularity. Here, the process involves splitting one 128 bit channel in two 64 bit channels. This is "gang" mode.
In ganged mode, there is a 128 bit wide logical DIMM that maps the first 64 bits on the physical DDR channel A and the last 64 bit on DDR channel B. The physical address space, in other words, is interleaved between the two DIMMs in 64 bit steps
In unganged mode, each DCT can operate independently with its own 64 bit wide address space. So the processor can be programmed to interleave the single, physical address space on the two normalized address spaces associated with the two memory channels.
The position that AMD takes is to enable unganged mode to get the benefits from increased parallelism. Note that some CPU models, the 8 and 12 core Magny Cours G34 processors, can only use the unganged mode. This method is one of several that allow the CPU to have faster access to memory.
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