All About Random access memory(RAM) Part 1

 

Types of random access memory

There are generally two broad categories of random access memory:

• DRAM memories (Dynamic Random Access Module), which are inexpensive. They are used essentially for the computer's main memory
• SRAM memories (Static Random Access Module), which are fast and costly. SRAM memories are used in particular for the processor's cache memory

Operation of the random access memory

The random access memory comprises hundreds of thousands of small capacitors that store loads. When loaded, the logical state of the capacitor is equal to 1, otherwise it is 0, meaning that each capacitor represents one memory bit.

Given that the capacitors become discharged they must be constantly recharged (the exact term is refresh) at regular intervals, known as the refresh cycle. DRAM memories for example require refresh cycles of around 15 nanoseconds (ns).

Each capacitor is coupled with a transistor (MOS-type) enabling "recovery" or amendment of the status of the capacitor. These transistors are arranged in the form of a table (matrix) thus we access a memory box (also called memory point) via a line and a column.

Each memory point is thus characterised by an address which corresponds to a row number and a column number. This access is not instant and the access time period is known as latency time. Consequently, time required for access to data in the memory is equal to cycle time plus latency time.

Thus, for a DRAM memory, access time is 60 nanoseconds (35ns cycle time and 25ns latency time). On a computer, the cycle time corresponds to the opposite of the clock frequency; for example, for a computer with frequency of 200 MHz, cycle time is 5 ns (1/200*106)).

Consequently a computer with high frequency using memories with access time much longer than the processor cycle time must perform wait states to access the memory. For a computer with frequency of 200 MHz using DRAM memories (and access time of 60ns), there are 11 wait states for a transfer cycle. The computer's performance decreases as the number of wait states increases, therefore we recommend the use of faster memories.

RAM module formats

There are many type of random access memory. They exist in the form of memory modules that can be plugged into the mother board.

Early memories existed in the form of chips called DIP (Dual Inline Package). Nowadays, memories generally exist in the form of modules, which are cards that can be plugged into connectors for this purpose. There are generally three types of RAM module:

• modules in SIMM format (Single Inline Memory Module): these are printed circuit boards with one side equipped with memory chips. There are two types of SIMM modules, according to the number of connectors:
  • SIMM modules with 30 connectors (dimensions are 89x13mm) are 8-bit memories with which first-generation PCs were equipped (286, 386). 
   
  
  
  • SIMM modules with 72 connectors (dimensions are 108x25mm) are memories able to store 32 bits of data simultaneously. These memories are found on PCs from the 386DX to the first Pentiums. On the latter, the processor works with a 64-bit data bus; this is why these computers must be equipped with two SIMM modules. 30-pin modules cannot be installed on 72-connector positions because a notch (at the centre of the connectors) would prevent it from being plugged in.  
  
  
  
  
  
  
  
  • modules in DIMM format (Dual Inline Memory Module) are 64-bit memories, which explains why they do not need pairing. DIMM modules have memory chips on both sides of the printed circuit board and also have 84 connectors on each side, giving them a total of 168 pins. In addition to having larger dimensions than SIMM modules (130x25mm), these modules have a second notch to avoid confusion.

 

 It may be interesting to note that the DIMM connectors have been enhanced to make insertion easier, thanks to levers located either side of the connector.

Smaller modules also exist; they are known as SO DIMM (Small Outline DIMM), designed for portable computers. SO DIMM modules have only 144 pins for 64-bit memories and 77 pins for 32-bit memories.

• modules in RIMM format (Rambus Inline Memory Module, also called RD-RAM or DRD-RAM) are 64-bit memories developed by Rambus. They have 184 pins. These modules have two locating notches to avoid risk of confusion with the previous modules.

Given their high transfer speed, RIMM modules have a thermal film which is supposed to improve heat transfer.

As for DIMMs, smaller modules also exist; they are known as SO RIMM (Small Outline RIMM), designed for portable computers. SO RIMM modules have only 160 pins.