Dual-channel architecture

Dual-channel architecture describes a technology that theoretically doubles data throughput from RAM to the memory controller. Dual-channel-enabled memory controllers utilize two 64-bit data channels, resulting in a total bandwidth of 128-bits, to move data from RAM to the CPU.

Operation

In order to achieve this, two or more DDR/DDR2 SDRAM memory modules must be installed into matching banks, which are usually color coded on the motherboard. These separate channels allow each memory module access to the memory controller, increasing throughput bandwidth. It is not required that identical modules be used, but this is often recommended for best compatibility for dual channel operation. It is possible to use a single-sided module of 512 MBRAM size is specified using binary meanings for K (1024¹ instead of 1000¹), M (1024² instead of 1000²), G (1024³ instead of 1000³), ... ] and a double-sided module of 512 MB in dual-channel configuration, but how fast and stable it is depends on the memory controller.

If the motherboard has two pairs of differently colored DIMM sockets (the colors indicate which bank they belong to, bank 0 or bank 1), then one can place a matched pair of memory modules in bank 0, but a different-capacity pair of modules in bank 1, as long as they are of the same speed. Using this scheme, a pair of 1 GB memory modules in bank 0 and a pair of matched 512 MB modules in bank 1 would be acceptable for dual-channel operation.cite web
author = Infineon Technologies North America Corporation and Kingston Technology Company, Inc.
month = September | year = 2003
url = http://www.kingston.com/newtech/MKF_520DDRwhitepaper.pdf
title = "Intel Dual-Channel DDR Memory Architecture White Paper", Rev. 1.0
format = PDF, 1021 KB
publisher = Kingston Technology
accessdate = 2007-09-06]

Modules rated at different speeds can be run in dual-channel mode, although the motherboard will then run all memory modules at the speed of the slowest module. Some motherboards, however, have compatibility issues with certain brands or models of memory when attempting to use them in dual-channel mode. For this reason, it is generally advised to use identical pairs of memory modules, which is why most memory manufacturers now sell "kits" of matched-pair DIMMs. Several motherboard manufacturers only support configurations where a "matched pair" of modules are used. A matching pair needs to match in:

* Capacity (e.g. 1024 MB). Certain Intel chipsets support different capacity chips in what they call Flex Mode: the capacity that can be matched is run in dual-channel, while the remainder runs in single-channel.
* Speed (e.g. PC5300). If speed is not the same, the lower speed of the two modules will be used. Likewise, the higher latency of the two modules will be used.
* Number of chips and sides (e.g. 2 sides with 4 chips on each side).

Dual-channel architecture is a technology embraced by motherboard manufacturers and does not apply to memory modules. In other words, any matched pair of memory modules may support single- and dual-channel operation, provided the motherboard supports this architecture.

Purpose

Dual-channel technology was created to address the issue of bottlenecks. Increased processor speed and performance requires other, less prominent components to keep pace. In the case of dual channel design, the intended target is the memory controller, which regulates data flow between the CPU and system memory (RAM). The memory controller determines the types and speeds of RAM as well as the maximum size of each individual memory module and the overall memory capacity of the system. When the memory is unable to keep up with the processor, however, a bottleneck occurs, leaving the CPU with nothing to process. Under the single-channel architecture, any CPU with a bus speed greater than the memory speed would be susceptible to this bottleneck effect.

The dual-channel configuration alleviates the problem by doubling the amount of available memory bandwidth. Instead of a single memory channel, a second parallel channel is added. With two channels working simultaneously, the bottleneck is reduced. Rather than wait for memory technology to improve, dual-channel architecture simply takes the existing RAM technology and improves the method in which it is handled. While the actual implementation differs between Intel and AMD motherboards, the basic theory stands.

Performance

Tom's Hardware found little significant difference between single-channel and dual-channel configurations in synthetic and gaming benchmarks (using a "modern" system setup). Dual channel at best might give a 5% speed increase in memory intensive tasks, not even close to the two times that "dual" suggests [ [http://www.tomshardware.com/reviews/PARALLEL-PROCESSING,1705-15.html Parallel Processing, Part 2: RAM and HDD | Tom's Hardware ] ] .

It should be noted that Tom's Hardware had a discrete graphics comparison. The difference is significant (~25%) for any 3D apps when using integrated graphics.

See also

* List of device bandwidths

References