Buying a computer today is becoming a more complicated process where buyers are thrown technical terms like GHz, DDR, FSB, Dual-Channel and Hyper-Threading. Traditionally, the GHz number, or processor speeds, has always been the key factor in any computer purchase. As the GHz number increases, so does the computer performance. Today, the GHz number is no longer the sole criterion for determining system performance.
Dual-Channel Memory
Most modern motherboards now support dual-channel memory, which doubles the RAM’s data throughput. Enabling dual-channel simply means populating the memory slots in matching pairs.
Most dual-channel systems will have four memory DIMM sockets. Two sockets belong to channel A and the other two sockets to channel B. For the best dual-channel memory performance, you must identically “match” memory modules in DIMM sockets 0 of channel A and B. For example, plugging in matching 256MB DIMMs in both DIMM 0 slots and plugging in matching 512MB DIMMs in both DIMM 1 slots.
“Matching” modules means:
1. Both modules are the same capacity (e.g. both are 256MB)
2. Both modules are the same speed (e.g. both are PC2700)
3. Both have the same number of chips and module sides (e.g. both have the same number of chips on the module and both are single-sided.)
Dual-Channel Workflow and Bandwidth
Dual-channel memory utilizes two channels to feed data to the processor, thereby being able to deliver up to twice the data of the single channel. That’s because it is sending or receiving data from two memory module pairs at the same time.
To prevent the channel from being over-filled with data or to reverse the flow of data through the channel, there is a chip called the “Memory Controller” that handles all data transfers. The Memory Controller manages all movement of data between the processor and the memory modules. With dual-channel memory, the memory controller is able to move double the peak amount of data that it could normally move with single-channel memory. The memory industry generally describes memory performance in Peak Bandwidth in seconds.
To describe the maximum data throughput of memory, calculate its Peak Bandwidth. This calculation, expressed in number of Bytes per second (1 Byte = 8 bits where bits, represent either a ‘0’ or a ‘1’), gives a general idea of the performance of the memory. To calculate peak bandwidth use this formula:
Memory Speed x Number of bytes transferred per channel x Number of Channels.
For example PC3200, also known as DDR400, modules on a dual-channel motherboard, calculates as Peak Bandwidth = (400 MHz) x (8 Bytes) x (2 Channels) = 6400MB/s or 6.4GB/s.
Looking at the Processor’s Front-Side Bus
A processor’s link to the memory controller is called the processor’s front-side bus (FSB). The front-side bus determines how fast the processor can obtain data from the memory controller. To increase efficiency, the memory controller needs to send data as fast as the processor can receive it (and store it back into memory modules as fast as the processor can send the data out). Peak efficiency is only reached when the data throughput from the processor’s front-side bus matches the memory modules’ throughput.
Enabling Dual-Channel Memory
The majority of systems supporting dual-channel memory can be configured in either single channel or dual-channel memory mode. Keep in mind that even if a new computer or motherboard supports dual-channel DDR memory, this does not necessarily guarantee that both memory channels are being utilized in dual-channel mode. You cannot just plug multiple memory modules into their sockets to get dual channel memory. Users need to follow specific rules when adding memory modules to ensure that they get dual-channel memory performance. Otherwise, their system may fall back to single-channel memory mode.
Going from single to dual-channel PC3200 memory increases system performance by over 15%. Going from single-channel PC2700 to dual-channel PC3200 increases performance by over 25%. In addition, system performance actually increases by nearly 5% when 1GB of total memory is used instead of 512MB.
Conclusion
Dual-channel DDR memory systems offer a new level in system performance. Combining faster memory speeds with dual-channel memory platforms, creates new price/performance value levels. Dual-channel memory performance is only realized when matched memory modules are added in pairs and installed in the correct socket configurations. Otherwise, a system will revert to the lower-performing, single-channel memory mode. In future years, we can expect to see more dual-channel memory platforms requiring even faster memory than those available today.
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