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SDRAM vs DDR: what are the differences?

Memory or random access memory (RAM), comes in different types. The differences are due to the function and technology of the memory and other computer hardware. If you have ever wondered what DDR RAM and SDRAMM are, and the differences between them, keep reading to learn more about SDRAM and the different DDR types.

Memory generations

Two Crucial RAM memory modules in place on a motherboard

Memory standards are controlled by JEDEC, the Joint Electron Device Engineering Council, an independent semiconductor engineering trade organization and standardization body. As each new generation of memory is developed, this body controls the standards of the generation.

Each generation of memory is marked by speed and frequency increases and power consumption decreases. Because computer hardware is all connected and interdependent, this leads to speed increases in other components, too. For more information about computer hardware, read here.

SDRAM

Synchronous dynamic random access memory (SDRAM) was developed in response to increased speed in other computer components. As other computer components increased their speed, memory speed also needed to increase. Double data rate (DDR), was developed, and the previous technology became known as single data rate, or SDR. DDR was both faster and used less energy than SDR. DDR memory transfers data to the processor on both the rising and falling edges of the clock signal.

SDRAM was developed in 1988 in response to increased speed in other computer components. Previously, memory had to be asynchronous, that is, it operated independently of the processor. Synchronous memory synchronizes the memory module's responses with the system bus and the timing of the CPU.

By synchronizing itself with the CPU, the memory module knows the exact clock cycle, and the CPU does not have to wait between memory accesses. SDRAM can only read/write one time per clock cycle.

DDR

DDR was the next generation following SDRAM and was introduced in 2000. It achieved greater bandwidth and speed than previous single data rate memory. DDR transfers data to the processor on both the rising and falling edges of the clock signal, so twice per cycle. A clock signal is made up of both a downbeat and an upbeat. Using both beats to transfer data makes double data rate memory significantly faster than single data-rate memory, which used only one edge of the clock signal to transfer data.

DDR transfers two bits of data per clock cycle from the memory array to the internal input/output buffer. This is called 2-bit prefetch. DDR transfer rates are usually between 266MT/s and 400MT/s.

Double data-rate is different than dual-channel memory. Learn about dual-channel memory here.

DDR2

DDR2 was introduced in 2003 and operates external data twice as fast as DDR due to an improved bus signal. DDR2 operates on the same internal clock speed as DDR, however the transfer rates are faster due to the improved input/output bus signal. DDR2 has a 4-bit prefetch, twice that of DDR. DDR2 can reach 533MT/s to 800MT/s.

DDR3

In 2007, DDR3 brought about a reduction in power consumption, roughly 40% compared to DDR2 and double the prefetch data to 8-bits. This reduce usage allows for lower operating currents and voltages. DDR operates are about 2.5 V and DDR2 averages about 1.8 V, with DDR3 the voltage is reduced to 1.5 V. DDR3 has transfer rates between 800MT/s and 1600MT/s.

DDR4

DDR4 is the latest generation (2014) of double data-rate random access memory. It has the lowest operating voltage of 1.2 V and has higher transfer rates than previous generations. DDR4 introduced bank groups to avoid having a prefetch of 16, which is not desirable. With bank groups, each group can execute 8-bits of data independently from the other. This means DDR4 can process multiple data requests within a clock cycle.

DDR4 transfer rates are continually rising, DDR4 modules can reach speeds 5100MT/s and even higher when overclocked. Crucial Ballistix MAX modules broke numerous overclocking world records in 2020.

SDRAM

DDR

DDR2

DDR3

DDR4

Prefetch

1 - Bit

2 - Bit

4 - Bit

8 - Bit

Bit per Bank

Date Rate (M

100 - 166

266 - 400

533 - 800

1066 - 1600

2133 - 5100

Transfer Rate GB/s

0.8 - 1.3

2.1 - 3.2

4.2 - 6.4

8.5 - 14.9

17 - 21.3

Voltage (V)

3.3

2.5 - 2.6

1.8

1.35 - 1.5

1.2

Memory is NOT backwards compatible

One of the reasons for industry-wide standardization in memory is that computer makers need to know the electrical parameters and physical shape of the memory that can be installed in their computers. Because the electrical parameters are different for each generation of memory, the physical shape of the memory changes to prevent the wrong memory from being installed in a computer. Therefore, it is not a question of choosing between SDRAM vs DDR, as computers can use one generation of memory.. SDRAM and DDR generations are not directly interchangeable. Your system will only work with the appropriate RAM.

To determine the right kind of memory for your computer, use the Crucial® Advisor™ tool or System Scanner tool. These will help you determine which memory modules are compatible with your computer, along with options for your speed requirements and budget.

 

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