Dwg: Article written and Venn diagram created.

2025-12-08T19:52:53Z

Article written and Venn diagram created.

New page

== Differences between DDR and SDRAM ==
'''Double Data Rate (DDR) SDRAM''' is a type of synchronous dynamic random-access memory that succeeded Single Data Rate (SDR) SDRAM.<ref name="ref1" /> Both are types of SDRAM, but the primary difference is that DDR memory transfers data on both the rising and falling edges of the clock signal, effectively doubling the data transfer rate without increasing the clock frequency.<ref name="ref2" /><ref name="ref3" /> This improvement allows for significantly higher bandwidth compared to its predecessor.<ref name="ref4" /> SDR SDRAM, which transfers data only once per clock cycle, was common in computers before 2002, after which DDR SDRAM became the mainstream technology.<ref name="ref2" />

Due to architectural and physical differences, DDR SDRAM and SDR SDRAM are not forward or backward compatible.<ref name="ref5" /> Motherboards and controllers are designed to support only one type of memory.<ref name="ref2" /> The memory modules are physically keyed with notches to prevent incorrect installation in an incompatible slot.

=== Comparison Table ===
{| class="wikitable"
|-
! Feature !! SDR SDRAM !! DDR SDRAM
|-
| '''Full Name''' || Single Data Rate Synchronous Dynamic Random-Access Memory || Double Data Rate Synchronous Dynamic Random-Access Memory
|-
| '''Data Transfers per Clock Cycle''' || 1 || 2
|-
| '''Clock Signal Edge Used''' || Rising edge only<ref name="ref3" /> || Rising and falling edges<ref name="ref3" />
|-
| '''Prefetch Buffer Size''' || 1n (1 data word per access) || 2n (2 data words per access)
|-
| '''Operating Voltage''' || 3.3 V || 2.5 V / 2.6 V
|-
| '''DIMM Module Pins''' || 168 pins<ref name="ref1" /> || 184 pins<ref name="ref1" />
|-
| '''DIMM Notches''' || Two || One
|}

[[File:Venn_diagram_Differences_between_DDR_versus_SDRAM_comparison.png|thumb|center|800px|alt=Venn diagram for Differences between DDR and SDRAM|Venn diagram comparing Differences between DDR and SDRAM]]

=== Technical Architecture ===
The key to DDR SDRAM's increased performance is its 2n prefetch architecture. The internal memory bus fetches two words of data from the memory array during a single clock cycle and places them in an I/O buffer. The memory module then transfers these two words of data consecutively on the rising and falling edges of the clock signal. In contrast, SDR SDRAM uses a 1n prefetch, transferring only one word of data per clock cycle.

This "double pumping" technique allows a DDR module to achieve twice the bandwidth of an SDR module running at the same core clock speed. For example, an SDR SDRAM module on a 133 MHz bus has a transfer rate of 133 MT/s (megatransfers per second), while a DDR SDRAM module on the same 133 MHz bus can achieve 266 MT/s.<ref name="ref1" />

=== Physical Characteristics ===
SDR SDRAM and DDR SDRAM modules have distinct physical differences that prevent them from being interchangeable. Standard desktop modules, known as DIMMs, have different pin counts and notch locations. An SDR SDRAM DIMM has 168 pins and two notches on its connecting edge.<ref name="ref5" /> A DDR SDRAM DIMM has 184 pins and only a single notch.<ref name="ref5" />

Additionally, DDR memory operates at a lower voltage than SDR memory. Standard DDR SDRAM modules run at 2.5 volts, compared to the 3.3 volts required for SDR SDRAM. This reduction in voltage results in lower power consumption.

== References ==
<references />

[[Category:Comparisons]]

Differences between DDR and SDRAM - Revision history

Dwg: Article written and Venn diagram created.