RAM Facts

Random access memory (RAM) is important. The speed a PC can access memory contributes to its reliability and processing speed.

This lesson covers the following topics:

RAM types
Dynamic RAM (DRAM)
Memory form factors
Error Correction Code (ECC) RAM

RAM Types

There are two types of RAM. The following table describes the two types.

TypeDescription

Static RAM
(SRAM)

SRAM stores data using four transistors for every bit of data. SRAM does not require constant power to maintain the contents of memory.

SRAM is more complex and less dense (has a lower storage capacity) than DRAM.
SRAM is faster and requires less power than DRAM.
Regular SRAM requires periodic power to maintain the state of memory.
- The rate of refresh is less than with DRAM.
- Non-volatile SRAM (nvSRAM) can maintain memory contents with the power turned off.
SRAM is typically used in cache memory, such as CPU cache, hard disk cache, and cache in networking devices.

DRAM

DRAM stores data using a single transistor for every bit of data (a 0 or a 1). To maintain the state of the transistor, DRAM 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 (a high storage capacity).
Because of its simplicity, DRAM is relatively inexpensive.
DRAM is used in the main system memory on a computer.

DRAM

All system memory used in personal computers is DRAM. Individual DRAM chips are packaged onto a board that contains circuitry for reading and writing to the memory. Double-Data Rate Synchronous Dynamic RAM (DDR SDRAM) is a new generation from the original synchronous Dynamic RAM (SDRAM).

You should be aware of the following standards for DRAM.

		HardwareStandardDescription
DDR is no longer used in modern motherboards, although you might encounter DDR memory in older systems.	DDR (DDR1)	DDR is a new generation from the original synchronous DRAM (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. At the same frequency, DDR has twice the bandwidth of SDRAM. DDR operates at 2.5 volts with standard spec bus frequencies between 100-200 MHz. DDR memory has a single notch, right-center. DDR memory has 184 pins and two notches. Older SDRAMs have 168 pins.
	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. Includes a buffer between the data bus and the memory. Operates at 1.8 volts with standard spec bus frequencies between 200-533 MHz. The internal memory frequency is half that of the bus frequency (100-266 MHz). DDR2 memory differs from DDR memory as follows: The notch is slightly right-center, closer to the middle. It has 240 pins. Notice that the pins are smaller because they have to fit in the same space as the DDR memory.
	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. Operates at 1.5 volts with standard spec bus frequencies between 400-1000 MHz. The internal memory frequency is one-fourth that of the bus frequency (100-266 MHz). DDR3 memory has a single notch left-center. Like DDR2, DDR3 has 240 pins.
	DDR4	DDR4 doubles the data transfer rate of DDR3 for ten times the bandwidth of SDRAM. DDR4: DDR4 accepts eight consecutive 64-bit words per bus clock cycle. Operates at 1.2 volts at standard spec bus frequencies between 1600-3200 MHz. Reduces the demand for power. Is not compatible with earlier types of RAM. DDR4 differs in signaling voltages, physical interface, etc. Allows (theoretically) for DIMMs up to 512 GB in capacity, compared to the DDR3's theoretical maximum of 128 GB per DIMM. DDR4 memory has a single notch slightly right of center. DDR4 has 288 pins.
DDR5 is often found in high-end gaming PCs' and laptops.	DDR5	DDR5 doubles the peak memory speeds and greatly increases memory size over DD4. DDR5 is a completely overhauled architecture compared to DDR4. DDR5 modules can incorporate on-board voltage regulators to reach higher speeds. DDR5: Uses decision feedback equalization (DFE) to enable input/output (I/O) speed scalability for higher bandwidth and better performance. Uses new architecture and adds a power management integrated circuit (PMIC) on each chip to handle its power management instead of the motherboard, as in DDR4. Accepts two independent channels of eight consecutive 64-bit words per bus clock cycle. Operates at 1.1 volts at standard spec bus frequencies 3200-6400 MHz. Reduces the demand for power. Supports a speed of 51.2 GB/s per module with 2 memory channels per module. Octuples the maximum DIMM capacity from 64 GB to 512 GB with higher frequencies than DDR4. DDR5 memory has a single notch left-center. DDR5 still has 288 pins, but has changed the pinouts to accommodate new features.

Both SDRAM and DDR RAM have a synchronous interface. This means that it waits for a clock signal before responding to control inputs. This makes it synchronized with the computer's system bus.

Synchronization allows the memory chip to have a more complex pattern of operation than an asynchronous DRAM.
This is also why the speed of SDRAM and DDR RAM is rated in MHz instead of nanoseconds (ns).
Double Data Rate means that memory transfers data on both the rising and falling edges of the clock signal.

Memory Form Factors

Memory comes in various form factors (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 described in the following table.

		HardwareFormDescription
DDR DIMM	DIMM	A DIMM 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/2/3/4 are packaged into DIMMs. Each specification has a unique number of pins and notch position. DDR4 allows for DIMMs of up to 512 GB in capacity.
144-pin SODIMM 200-pin SODIMM	Small outline DIMM (SODIMM)	A SODIMM is a smaller DIMM used in laptops. SODIMMs are much smaller than other memory (perfect for notebook computers). Notice the notch slightly off center in the 144-pin SODIMM. 144-pin SODIMMs are used by SDRAM, DDR, and DDR2 memory. On the 200-pin SODIMM, notice that the notch is farther off center than the 144-pin SODIMM. Also notice the higher pin density. 200-pin SODIMMs are used by DDR2 and DDR3 memory.
	Universal DIMM (UniDIMM)	UniDIMM is a DIMM specification designed to carry DRAM chips. UniDIMMs can be populated with either DDR3 or DDR4 chips, but do not support additional memory control logic. Because of this, the computer's memory controller must support both DDR3 and DDR4 memory standards. UniDIMM: Is an upgrade to the current SODIMM standard. Allows mobile platform users to use both DDR3 and DDR4.

ECC RAM

ECC RAM is another kind of RAM memory you should know about.

HardwareFormDescription

ECC memory is for enterprise-level workloads and servers. The average PC user's motherboard probably doesn't support it.

ECC

ECC uses a code to detect and correct data corruption for critical databases. ECC:

Is used in industries like finance and cloud computing to protect against data corruption.
Helps reduce crashes and maximize system availability in multi-user server applications.
Is more expensive because of an extra memory chip that performs the error detection and correction.

Modules have nine chips instead of the usual eight.