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Random Access Memory (RAM) can be classified as one of two types:

Type Description
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.
  • DRAM is simple to implement.
  • DRAM can have a very high density (i.e. high storage capacity).
  • Because of the simplicity, DRAM is relatively inexpensive.
  • DRAM is used in the main system memory on a computer.
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.
  • SRAM is more complex and less dense (i.e. lower storage capacity) than DRAM.
  • SRAM is faster and requires less power than DRAM.
  • Regular SRAM still requires periodic power to maintain the state of memory, but the rate of refresh is less than with DRAM. Non-volatile SRAM (nvSRAM) is able to maintain memory contents when the power is turned off.
  • SRAM is typically used in cache memory, such as CPU cache, hard disk cache, and cache in networking devices.

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:

Standard Description
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.
  • SDRAM accepts one command and one data set per clock cycle. For this reason, SDRAM is sometimes called single data rate synchronous DRAM (SDR SDRAM).
  • SDRAM can read or write 64-bits at a time, matching the width of the system bus. (The set of data transmitted together is called a word.)
  • A 64-bit word is stored across 8 DRAM chips, with each chip receiving 8-bits of data.
  • SDRAM operates at 3.3 volts (original SDRAM operated at 5 volts) at bus frequencies between 33-166 MHz.
DDR (Double-Data Rate Synchronous Dynamic RAM) DDR is a variation of the original SDRAM.
  • All variations of DDR are synchronized with the system clock and accept 64-bit words.
  • DDR accepts a single command and two consecutive data sets per bus clock cycle (double the data within the same time period).
  • Operating at the same frequency, DDR has twice the bandwidth of SDRAM.
  • DDR operates at 2.5 volts at bus frequencies between 100-200 MHz.
DDR2 DDR2 doubles the data transfer rate of DDR, for four times the bandwidth of SDRAM.
  • DDR2 accepts four consecutive 64-bit words per bus clock cycle.
  • DDR2 includes a buffer between the data bus and the memory.
  • DDR2 operates at 1.8 volts at bus frequencies between 200-533 MHz. The internal memory frequency is half that of the bus frequency (100-266 MHz).
DDR3 DDR3 doubles the data transfer rate of DDR2, for eight times the bandwidth of SDRAM (twice that of DDR2).
  • DDR3 accepts eight consecutive 64-bit words per bus clock cycle.
  • DDR3 operates at 1.5 volts at bus frequencies between 400-1000 MHz. The internal memory frequency is one-fourth that of the bus frequency (100-250 MHz).
RDRAM (Rambus DRAM) RDRAM is an alternative to DDR that was developed jointly with Intel.
  • RDRAM transfers data either 16- or 32-bits at a time.
  • RDRAM transfers two consecutive words in a single clock cycle.
  • RDRAM uses a memory controller on each memory chip instead of on the motherboard or CPU. Data must pass from one memory module to the next in line.
  • Continuity modules must be installed in unused memory slots.
  • RDRAM operates at 2.4 volts at 400-800 MHz. Because of the higher frequencies, RDRAM modules always have heat spreaders to dissipate heat.

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:

Form Description
SIMM A SIMM (single in-line memory module) has pins on both sides of the module, but the pins are redundant on both sides.
  • SIMMs were used with older memory modules (not SDRAM or DDR).
  • SIMMs had a 32-bit data path, so you had to install them in pairs for a 64-bit bus.
DIMM A DIMM (dual in-line memory module) has pins on both sides of the module, with each pin being unique.
  • DIMMs have a 64-bit data path that matches the system bus width.
  • RDRAM and DDR/2/3 are packaged into DIMMs, with each specification having a unique number of pins and notch position.
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.
  • A single channel RIMM has a 16-bit data path.
  • A dual channel RIMM has a 32-bit data path.