Understanding System Memory Types
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There are many different types of memory products available, and they aren't all compatible with each other or provide the same features, such as dual channel mode, registers and error checking. System memory support depends on your motherboard, therefore it is vital that you determine the memory type and speed support of your motherboard (or processor) before choosing the memory product to purchase. Newegg offers hundreds of RAM products and the following information will help you make an educated purchase for your personal computer.
Synchronous Dynamic Random Access Memory: SDRAM has a synchronous interface. It waits for a clock pulse before transferring data and is therefore synchronous with the computer system and processor. To illustrate, imagine a clock face with a minute hand. Every time the minute hand makes a complete revolution will be called a clock cycle. In that clock cycle – for example, at the 30-second mark – a clock pulse triggers the CPU, the computer system and the RAM to process information. In reality, clock cycles are much faster than 60 seconds. This design greatly improves performance over asynchronous DRAM. SDRAM, by itself, is not as popular as it once was, however, current RAM, like DDR3, build off of the SDRAM architecture.
Rambus Dynamic Random Access Memory is a type of synchronous DRAM created by the Rambus Corporation. RDRAM features an architecture designed to achieve high bandwidth, it is used in the Sony PlayStation 2, early Pentium 4 desktop systems and other applications. The XDR DRAM, RDRAM's successor, is used in IBM's Cell processor and Sony PlayStation 3. RDRAM is also mainly used for capacity expansion of old desktop systems and often come in the form of 184-pin RIMMs (Rambus Inline Memory Modules).
Double Data Rate SDRAM sends and receives data twice as often as common SDRAM. This is achieved by transferring data on both the rising edge and the falling edge of a clock cycle. To continue the illustration from earlier, instead of the clock pulse only triggering at the 30-second mark, it also triggers at the 60-second mark, effectively doubling the processes. DDR memory is replaced by DDR2 memory. DDR memory modules usually take the form of 184-pin DIMMs.
Second generation DDR memory provides greater performance with a larger bandwidth, which allows more information to be processed. DDR2 replaces DDR in the desktop DRAM market. DDR2 memory modules are 240-pin DIMMs. It also features a host of new technologies that you can read about in the DDR section of this guide.
Third generation DDR memory leaps greatly forward in data transfer rate and power management. DDR3 provides even higher bandwidth than DDR2. You can read about DDR3 in the DDR section of this guide.
Unbuffered Memory vs. Registered Memory
This topic will be discussed in greater detail later in this guide. For now, here’s some basic information: Almost all system memory in PCs today is unbuffered memory. With increasing system memory, stability and performance deterioration is inevitable since the memory controller has to address each memory chip on all modules directly. To solve this problem, systems with more RAM use registered memory instead, which contains registers as buffers to temporarily hold data for one clock cycle before it is transferred. This increases the reliability of high-speed data access to high density memory but sacrifices some performance. Registered memory modules are typically used only in servers and other mission-critical systems where it is extremely important that data is properly handled. Motherboard support is necessary as well.
ECC stands for Error Checking and Correction. Occasionally, the data held by memory becomes corrupted. ECC utilized by memory modules uses single bit error correction, which is capable of detecting and correcting single-bit errors. With error correction, as far as your processes are concerned, the error never happened. ECC will also detect two-bit and some multiple bit errors, but is unable to correct them. This feature needs a motherboard's support, and is usually applied in workstation and server products. You can read more about this topic in the ECC Memory section of this guide.