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| Please note: Do It Yourself articles and guides are intended for technically advanced users. Please review important cautionary information at the end of this page. Republished articles presented in the Do It Yourself section do not necessarily reflect the opinions or positions of AMD.
AMD64 Technology: Wired For DDR Memory Technology
"Texpert"
After the processor, system storage - or random access memory (RAM) - is arguably a PC’s most important component. It is the all-important intermediary between a system’s cache (up to 1,152KB on the AMD Athlon™ 64 FX-55 processor) and its mechanical hard drives, the slowest components in the storage hierarchy.
As an intercessor, RAM enables substantially more capacity than an “on-die” processor cache, and operates many times faster than a disk drive. Among other benefits, extra RAM lets you edit larger images and play more extreme, realistic games without sacrificing speed.
While more memory is always better, paying attention to each memory module's speed will help maximize performance and minimize the time needed to transfer information from one storage level to another. The AMD64 architecture is designed to optimize the most popular memory standards’ throughput, and thus guarantee blazing performance from a broad range of hardware components.
An Onboard Memory Controller
Why DDR?
The Best Memory for Your AMD64 Processor
Conclusion
An Onboard Memory Controller
No matter how far processor micro-architectures advance, their performance depends on the steady delivery of information to the core. Any stall in data flow results in processor idling.
Traditional processor designs communicate with connected memory devices via a memory controller housed in a complementary chipset's Northbridge. While this setup usefully ties memory compatibility to the chipset, it also adds performance-dragging latency to read and write transactions.
AMD64 technology introduces several bold architectural innovations, including extensions for 64-bit processing, Enhanced Virus Protection for Windows® XP SP2*, and a processor-die-integrated memory that interfaces directly with the processor itself, thus slashing access time and boosting throughput.
As an integral part of the processor, a well-designed memory controller is critical. It must provide enough bandwidth to satisfy tomorrow's demanding software applications, while supporting memory devices currently in mass production.
To this end, AMD equips the Socket 940 AMD Opteron™ processors, Socket 939 AMD Athlon 64 FX processors, and Socket 939 AMD Athlon 64 processors with a 128-bit memory interface. Socket 754 AMD Athlon 64 processors feature a 64-bit bus.
Because today's double data-rate (DDR) memory modules are 64-bits wide, processors with 128-bit buses actually benefit from dual-channel configurations – that is, running two like-sized memory modules in separate channels to realize the bus's potential.
As important as the memory bus's width is the speed at which it runs. All AMD Opteron processors and AMD Athlon 64 processors support up to DDR400 memory, which is becoming one of the most widely available memory specifications†. Running at 200 MHz (with an effective 400 MHz data rate), DDR400, also referred to as PC3200, delivers 3.2 GBps per channel. A dual-channel controller populated with DDR400 modules yields up to 6.4 GBps of bandwidth for its host processor.
Memory performance improves further with the multiprocessing AMD Opteron processor. Its crossbar architecture controls command and data flow between the processing core, integrated memory controller, and HyperTransport™ technology interface.
With coherent HyperTransport technology links enabling communication between connected processors, memory bandwidth scales for each additional CPU. This physical topology is referred to as “glueless” because all of the logic necessary to enable a highly scalable multiprocessing environment is contained within each processor, deemphasizing Northbridge functionality and standardizing processor performance.
*Enhanced Virus Protection will by default only protect the user's Windows® operating system. After properly installing the appropriate Windows release, users must enable the protection of their applications and associated files from memory buffer overrun attacks. Contact your application software vendor for information regarding use of the application in conjunction with Enhanced Virus Protection. AMD and Microsoft strongly recommend that users continue to use third party anti-virus software as part of their security strategy.
†All processors centering on AMD64 technology also include backwards compatibility for DDR200, DDR266, and DDR333 memory modules.
Why DDR?
The Joint Electron Device Engineering Council (JEDEC), which is responsible for standardizing discrete semiconductor devices and integrated circuits, defines DDR Synchronous DRAM (SDRAM) as a high-speed complementary metal oxide semiconductor (CMOS) – that is, dynamic random-access memory internally configured as a quad-bank DRAM device.
Originally accepted at 3.3V, today's DDR modules are driven by 2.6V power supplies. And as indicated by its name, a DDR device is designed to transfer two data words per clock cycle, which is why modules running at 200 MHz are able to achieve effective data rates of 400 MHz and earn the name DDR400.
AMD pioneered the use of DDR memory with its 760 chipset in 2000. Because DDR memory is easier to manufacture and receives broad industry support, it continued to evolve, giving way eventually to DDR400.
Once thought of as an extremely aggressive setting, DDR400 now offers the best balance between raw frequency and low memory timings.
While the AMD64 architecture is designed to accommodate new memory standards as they emerge, DDR400 still offers a compelling level of performance unrivaled by other technologies.
The Best Memory for Your AMD64 Processor
Owning an AMD64 processor allows for any number of system configurations, from general computing on a mainstream budget to running 64-bit database software on a corporate server. That said, your PC’s performance and reliability depends largely on the memory you use.
There are only a few variables to weigh. To begin, you'll likely want to limit your search to DDR400 modules - the key to realizing an AMD64 processor's full potential. And you should understand some of the acronyms and numerical ratings involved.
For example, what does it mean when a DDR400 module is followed by the numbers 2-3-2-6? Well, those numbers, in that order, correspond to Column Access Strobe (CAS)-RAD to CAS Delay (tRCD)-RAS Precharge (tRP)-Active to Precharge Delay (tRAS), all of which affect a module's responsiveness in some way or another:
- CAS represents the latency between the time a column is requested from a memory page and that data's availability, primarily impacting the performance of sequential accesses. Lower numbers, corresponding to low latency, are better, and the lowest latency modules sport a CAS rating of 2.
- tRCD indicates the latency between the activation of a row and the accessibility of a column within that row. The tRCD rating does not impact memory accessed sequentially, but plays a role in random accesses, where one active row is deactivated and another made active.
- module's tRP rating is a measure of the time, in memory cycles, it takes to deactivate one row and select the next. This comes into play only when memory is accessed non-sequentially on another row.
- tRAS is the minimum number of cycles a row in a given bank must be active before it can be closed and another row in that same bank activated.
For each specification, lower latency ratings indicate better performance, but there is a catch – if you dip too low, more speed risks less stability. The fastest DDR400 modules currently run at 2-2-2-5, but most are more conservative.
Even with average DDR400 modules, the AMD64 architecture still offers a significant advantage over designs that communicate with system memory through a Northbridge.
Conclusion
It is easy to optimize your computing experience with the AMD64 architecture.
In addition to large caches, 64-bit processing, HyperTransport™ technology, and support for an array of multimedia instruction sets, AMD64 introduces an onboard DDR memory controller that minimizes access latency during time-sensitive operations, keeping the CPU fed with data.
And, because DDR memory is based on an industry standard and is evolutionarily in a manner similar to the SDRAM DIMMs of yesteryear, it is easier to manufacture and less expensive than other memory technologies.
Cautionary Statement
Activities and projects described herein may involve the use of tools and materials that may present health and safety hazards. These must be handled carefully and all tools and products should be used strictly according to manufacturers' precautions and instructions for the safe use of the respective tool or product. The techniques described herein may result in the voiding of manufacturers' warranties. The user assumes all risks associated with the techniques described in this article/guide. THIS INFORMATION IS PROVIDED “AS IS” WITH NO WARRANTY, EXPRESS OR IMPLIED. AMD ASSUMES NO RESPONSIBILITY FOR ANY ERRORS CONTAINED IN THIS ARTICLE/GUIDE AND HAS NO LIABILITY OR OBLIGATION FOR ANY DAMAGES ARISING FROM OR IN CONNECTION WITH THE USE OF THIS ARTICLE/GUIDE.
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