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[ The PC Guide | Systems and Components Reference Guide | System Memory | Memory Speed, Access and Timing ]

Interleaving

Interleaving is an advanced technique used by high-end motherboards/chipsets to improve memory performance. Memory interleaving increases bandwidth by allowing simultaneous access to more than one chunk of memory. This improves performance because the processor can transfer more information to/from memory in the same amount of time, and helps alleviate the processor-memory bottleneck that is a major limiting factor in overall performance.

Interleaving works by dividing the system memory into multiple blocks. The most common numbers are two or four, called two-way or four-way interleaving, respectively. Each block of memory is accessed using different sets of control lines, which are merged together on the memory bus. When a read or write is begun to one block, a read or write to other blocks can be overlapped with the first one. The more blocks, the more that overlapping can be done. As an analogy, consider eating a plate of food with a fork. Two-way interleaving would mean dividing the food onto two plates and eating with both hands, using two forks. (Four-way interleaving would require two more hands. :^) ) Remember that here the processor is doing the "eating" and it is much faster than the forks (memory) "feeding" it (unlike a person, whose hands are generally faster.)

In order to get the best performance from this type of memory system, consecutive memory addresses are spread over the different blocks of memory. In other words, if you have 4 blocks of interleaved memory, the system doesn't fill the first block, and then the second and so on. It uses all 4 blocks, spreading the memory around so that the interleaving can be exploited.

Interleaving is an advanced technique that is not generally supported by most PC motherboards, most likely due to cost. It is most helpful on high-end systems, especially servers, that have to process a great deal of information quickly. The Intel Orion chipset is one that does support memory interleaving.

Note: This is not the same as the concept of interleaving related to hard disks.

Next: DRAM Technologies


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