What Is DIMM?
DIMM stands for dual in-line memory module, and this comprises of a series of Dynamic Random Access Memory for Integrated Circuits. In plain language, it is a small circuit board that acts as a reservoir for memory chips and has a 64-bit path. DIMM’s have already began to uproot SIMM’S (for single in-line memory module) as the memory module of choice as Pentium chips from Intel begin to become more and more entrenched in the market. SIMM’s had 32-bit path as opposed to 64 for DIMM’s, and the former had to be installed in pairs I order to match them. This was the inefficiency that DIMM’s were designed to cure. Yet another difference between SIMM’s and DIMM’s is that the DIMM’s have separate electrical contacts on each side of the module, while the electrical contacts for SIMM’s are redundant. There are two notches at the bottom edge of 168-pin DIMM’s and the location of each notch determines the particular features of the module.
The first notch is the Dynamic Random Access Memory (DRAM) key position, and it represents the Reserved Future Use (or RFU) which is registered and unbuffered. The second notch is the voltage key position. DIMM’s capacity and timing are identified with Special Presence Detect (or SPD), an additional chip that contains information about the module type. There are DIMM’s that are referred to as the ECC DIMM’s, and these have extra data bits which can be used by the systems memory controller to correct and detect errors. There are a wide variety of ECC schemes available, but the most pervasive is the Single Error Correct, Double Error Detect (SECDED) which utilizes the 9th extra bit per byte.
The number of ranks on a DIMM is equivalent to the number of independent sets of DRAMs that can be set simultaneously for the full data bit-width of the DIMM to be driven on the bus. The physical layout of the DRAM chips on the DRAM on the DIMM is however, not necessarily related to the number of ranks. The layout of all DRAM on one side of the DIMM PCB vis-a-vis both sides is rendered single sided vis-à-vis double sided. The terms are not necessarily related to how DIMM’s are organized or accessed, and may cause confusion. For instance, on a single rank DIMM that has 64 data bits of I/O pins, there is only a single set of DRAMs turned on to drive a read, a receive, or a write on all 64-bits. In many electronic systems, memory controllers are designed to have access to the full data bus width of the memory module simultaneously.
Most of the DIMM’s are developed using “x4” memory chips or “x8” memory chips. "x4" or "x8" refers to the width of the data of the DRAM chips in bits. There are several form factors that are used in DIMMs. There are other foam factors which include the SO-DIMM, the Mini-DIMM and the VLP Mini-DIMM. The single data rates SDRAM DIMM’s commonly came into two main sizes, 1.7 and 1.5. DIMM’s are sold according to the clock speed, bus speeds and transfer rate.
For more information on DIMM,read this article.