Random Access memory (RAM)

Because of the demands made by increasingly powerful operating systems and software, system RAM requirements have accelerated dramatically over time. 

For instance, at the turn of the millennium a typical computer may have only 128Mb of RAM in total, but in 2007 computers commonly ship with 2Gb of RAM installed, and may include graphics cards with their own additional 512Mb of RAM and more.

RAM

• Changes constantly as the system Operates
• Providing temporary storage for all data required by the Operating System (OS) and software
• Directly Accessible
• viewed as a collection of memory location in which program instructions and data are stored
• Each memory location is referenced by a unique address assigned to it. 
• The main memory of the computer is faster than external memory. The contents of each memory location are directly accessed using the address of location.
• Reading and writing of data is done electronically without any mechanical movement. 
• Volatile memory (data loss when power cut off)
• The program which is currently being executed and data which is currently being manipulated are transferred from external memory to RAM.

There are two types of RAM which are:

• Static RAM (SRAM)
• Dynamic RAM (DRAM). 

Dynamic Random Access Memory(DRAM)

• Store each bit in a separate capacitor within a integrated circuit
• Volatile
• Cheaper than Static Random Access memory(SRAM)
• Use less power
• Bigger storage capacity than SRAM
• Slower access time than SRAM
• Tendency to discharge

The capacitor can be either charged or discharged; these two states are taken to represent the two values of a bit, conventionally called 0 and 1.

Since capacitors leak charge, the information eventually fades unless the capacitor charge is refreshed periodically.Because of this refresh requirement, it is a dynamic memory as opposed to SRAM and other static memory.

A few improvements have been done on DRAM to improve its performance from the aspect of access time with different method and names, such as EDRAM, CDRAM, SDRAM and RDRAM.

 Figure 4: 1GB of SDRAM mounted in a person computer

   

Static Random Access Memory(SRAM)

Made from flip-flop – a logic circuit which retains the information stored in it as long as there is enough power to run the device.

• Type of semiconductor memory. 
• it does not need to be periodically refreshed because it uses bistable latching circuitry to store each bit. 
• Faster access time than DRAM
• However it was expensive than DRAM
•  Storage capacitor also small
• required 6 transistor to store 1 bit

Related articles:
Reading and writing operation of SRAM
Reading and writing operation of DRAM