Image of memory module

Computer memory has evolved since the first x86 PC, s became popular. New memory designs and standards have been developed enabling larger sizes and better performance.

The list below is of the most popular types of memory used in the x86 Personal Computer.

Memory type: SIMM

Image of simm

What does SIMM stand for ?
Answer: Signal In-line Memory Module

  • Approximate dimensions: W 107mm x D 5mm x H 25mm
  • Number of pins: 72-pin
  • Module sizes available: 1MB, 2MB,4MB,8MB,16MB,32MB,64MB,128MB
Image of memory socket

Image of 72pin simm socket

This type of memory was used in 486 and Pentium class computers.

Memory type: SDRAM

Image of SDRAM

What does SDRAM stand for ?
Answer: Synchronous Dynamic Random Access Memory

  • Approximate dimensions: W 133mm x D 3mm x H 28mm
  • Number of pins: 168-pin
  • Standards available: PC66,PC100 and PC133
  • Module sizes available: 32MB ,64MB,128MB,256MB,512MB

Used in the Intel Pentium class,AMD K5 and K6 model computers.

Memory type: DDR SDRAM

Image of DDR-SDRAM.jpg

What does DDR SDRAM stand for ?
Answer: Double Data Rate Synchronous Dynamic Random Access Memory

  • Approximate dimensions: W 133mm x D 4mm x H 28mm
  • Number of pins: 184-pin
  • Standards available: DDR-200,DDR-266,DDR-333 and DDR-400
  • Module sizes available: 64MB,128MB ,256MB, 512MB, 1GB
Image of memory socket

Image of DDR-SDRAM socket

DDR SDRAM is found in the Intel Pentium II , Pentium III, Celeron, AMD Athlon, Duron and Sempron computers.


DDR SDRAM has now been superseded by DDR2 and then by DDR3. To see a comparison between DDR, DDR2 and DDR3 click the infographic below.

DDR memory comparison image

Another type of memory called RDRAM was used in Intel computers in the late 90,s. It never became popular due to its high cost and RDRAM (Rambus DRAM) faded away as DDR SDRAM technology improved.