RAM Memory guide
Different types of RAM - SDRAM, DDR, DDR2 and RDRAM
RAM (Random Access Memory) comes in several different types, which are not compatible with each other, and a motherboard is compatible with one type of RAM only. For example, DDR Memory won't work with motherboards designed for only DDR2, SDRAM or RDRAM memory, and vice versa.
SDRAM
SDRAM stands for synchronous dynamic random access memory, so called because it synchronises with a clock signal and therefore the processor. Single data rate SDRAM is the oldest type of SDRAM, and is therefore commonly known simply as SDRAM rather than SDR SDRAM. While older PCs still use SDRAM it is no longer used in new systems, DDR or DDR2 being used instead. SDRAM has 168 pins.
DDR
Double data rate (DDR) SDRAM is an evolution of SDRAM with 184 pins. While SDRAM transfers data once per clock signal, DDR transfers data twice, almost doubling the data transfer rate.
DDR2
DDR2 introduces features and functions that go well beyond the DDR SDRAM specification. It has 240 pins and is able to operate at data rates much faster than DDR. Additionally, DDR2 adds a second port to transfer data packets to the CPU, doubling the transfer rate of DDR.
DDR3
DDR3 offers further improvement. The DDR3 standard offers modules up to 16 GB in size. Power consumption is lowered with approximately 30% compared to DDR2 (which gives longer battery life in laptops) but the biggest improvement compared to DDR2 is a higher bandwidth - double the speed compared to DDR2.
RDRAM
An alternative to DDR SDRAM is Rambus DRAM (or RDRAM). It uses double data rate in a similar way to DDR and has the same number of pins, and is used in some games consoles, but it is not common in PC systems.
Different types of memory modules - SIMM, DIMM, SODIMM
Memory modules come in different forms, and it's essential to get the right kind to fit your motherboard. A DIMM module won't fit into a SODIMM slot, and vice versa.
SIMM
SIMM stands for Single In-Line Memory Module. These memory modules are installed in pairs, and are the older type of module, having a 32-bit data path and only 72 pins.
DIMM
Newer systems use DIMMs - Dual In-Line Memory Modules. DIMMS have seperate electrical contacts on each side, as opposed to the SIMM's redundant contacts. DIMMs with their 64-bit data path can be installed individually and replace the matched pairs of SIMMs. The number of pins depends on the type of DIMM.
SODIMM
Small outline dual in-line memory modules (SODIMMs) are a smaller version of a DIMM, being about half the size and with either 72, 144 or 200 pins. They are therefore commonly found in laptop computers while regular DIMMs are found in PCs.
ECC and Flash memory
ECC memory
Error-correcting code memory is memory that tests the accuracy of data moving in and out, and is able to detect and correct some errors without user intervention. These small errors are normally caused by cosmic rays. ECC memory is not normally needed for home systems, but is commonly used in server computers.
Flash memory
Flash memory is a non-volatile form of memory - it doesn't need a power supply to maintain the data stored on it. While it has some applications within PCs, including the BIOS chip, it is more commonly found in devices such as USB memory, MP3 players, memory cards for digital cameras and video game consoles. Some manufacturers have also started replacing hard drives with flash memory.
