Random Access Memory (RAM) can be classified as one of two types:
|Dynamic RAM (DRAM)||Dynamic RAM stores data using a single transistor for every bit of data (a 0 or a 1). To maintain the state of the transistor, dynamic RAM must continually supply power to the transistor; when the power is turned off, the data is lost.
|Static RAM (SRAM)||Static RAM stores data using four transistors for every bit of data. Static RAM does not require constant power to maintain the contents of memory.
All system memory used in personal computers is dynamic RAM. Individual DRAM chips are packaged onto a board that contains circuitry for reading and writing to the memory. You should be aware of the following standards for RAM:
|SDRAM (Synchronous Dynamic RAM)||SDRAM is synchronized with the system bus clock, allowing it to receive instructions in a continuous flow. New instructions can be received, even before the first instruction has finished executing.
|DDR (Double-Data Rate Synchronous Dynamic RAM)||DDR is a variation of the original SDRAM.
|DDR2||DDR2 doubles the data transfer rate of DDR, for four times the bandwidth of SDRAM.
|DDR3||DDR3 doubles the data transfer rate of DDR2, for eight times the bandwidth of SDRAM (twice that of DDR2).
|RDRAM (Rambus DRAM)||RDRAM is an alternative to DDR that was developed jointly with Intel.
Note: SDRAM, DDR, and RDRAM are no longer used in new motherboards, although you might encounter each as you support older systems. DDR3 will eventually replace DDR2, and will be eventually replaced by DDR4 or DDR5.
DDR increases the memory bandwidth by doubling the amount of data sent within a single clock cycle. Another method for increasing memory bandwidth is by providing multiple channels within the memory controller.
- Dual-channel systems use two memory controllers, while triple channel systems use three memory controllers. Each memory controller can communicate with one or more memory modules at the same time.
- To operate in dual-channel mode, install memory in pairs; to operate in triple-channel mode, install memory in sets of three.
- Dual-channel systems theoretically double the bandwidth. However, in practice, only a 5-15% increase is gained.
- Dual-channel and triple channel support is mainly a function of the motherboard (i.e. the memory controller), not the memory itself. DDR, DDR2, and DDR3 can all work in dual-channel systems (depending on the memory supported by the motherboard); a triple channel system can only use DDR3.
- The memory controller is in the Northbridge chip on the motherboard. Newer processors move the memory controller onto the processor chip, allowing the processor to communicate with RAM without going through the front-side bus.
Memory comes in various form factors (or packages), with the form factor determining the number of pins and the size of the memory module. Generic form factor labels that you should be familiar with are:
|SIMM||A SIMM (single in-line memory module) has pins on both sides of the module, but the pins are redundant on both sides.
|DIMM||A DIMM (dual in-line memory module) has pins on both sides of the module, with each pin being unique.
|SO-DIMM||A SO-DIMM (small outline dual in-line memory module) is a smaller DIMM used in laptops. RDRAM and DDR/2/3 are packaged into DIMMs, with each specification having a unique number of pins and notch position.|
|RIMM||A RIMM (Rambus in-line memory module) is a memory module used by the RDRAM specifications.