RAM

'RAM' is the common name for a computer's system memory. RAM stands for Random Access Memory. It is based on memory chips that store information as electrical charges in capacitors. This memory is typically DRAM - "Dynamic Random Access Memory". The capacitors slowly leak charge and therefore they require to be refreshed periodically to maintain the stored data. If powered off, then the data is lost - hence the term "dynamic" as opposed to "static" for SRAM which does not require refreshing.

Almost all memory designs follow the standards established by the JEDEC (Joint Electronic Devices Engineering Council).

RAM comes in the form of memory modules, which are small circuit boards with memory chips. These modules can be plugged into the memory slots of a motherboard. The memory modules communicate through the Northbridge chip and Front-Side-Bus with the CPU.

Types of RAM

Memory chips come in module packages called SIMM (Single In-line Memory Module) or DIMM (Dual In-line Memory Module). The main difference is that SIMMs have a 32-bit data path and DIMMs offer a 64-bit data path. SIMMs have been obsolete for many years now. Memory for notebooks typically comes in the form of SO-DIMM (Small Outline Dual In-line Memory Module), which are smaller to fit in notebooks that typically have very limited space.

SDRAM
Following up on the original DRAM in personal computers was SDRAM (Synchronous Dynamic Random Access Memory). In this type of memory instructions are synchronized with the computer's system bus. SDRAM modules commonly have 168 pins. SDRAM memory is not used anymore in new personal computers since 2007 and are superceded by DDR SDRAM.

DDR SDRAM
The current type of RAM in most system memory for personal computers is DDR SDRAM, which stands for Double Data Rate Synchronous Dynamic Random Access Memory. There are now three generations of DDR SDRAM with increasing speed performances:

• DDR SDRAM (also known as DDR1)
  Compared to its predecessor SDRAM, DDR1 has a double data rate, which in effect doubles the minimum read or write unit. DDR1 DIMMs commonly have 184 pins.
• DDR2 SDRAM (also known as DDR2)
  This type of memory again doubles the data rate of DDR1, meaning a fourfold data rate of SDRAM. DDR2 DIMMs commonly have 240 pins.
• DDR3 SDRAM (also known as DDR3)
  This type of memory doubles the data rate of DDR2, resulting a eightfold data rate of SDRAM. DDR3 DIMMs commonly have 240 pins.

 

Courtesy of the French Energy Agency ADEME, Future Electronics project. Expanded by VHK.