RAM (Random Access Memory) is the primary memory used in a computer. Its individual memory cells can be accessed in any sequence, and therefore it is called the random access memory. RAMs are divided in to two categories as Static RAM (SRAM) and Dynamic RAM (DRAM). SRAM uses transistors to store a single bit of data and it does not need to be periodically refreshed. DRAM uses a separate capacitor to store each bit of data and it needs to be periodically refreshed to maintain the charge in the capacitors.
What is Static RAM (SRAM)?
SRAM is a type of RAM and it is a volatile memory, which looses its data when the power is turned off. In a SRAM, each bit that stores data is made up of four or six transistors that make up a flip-flop. There are additional transistors that are used to control read and write accesses of storage cells. Even though typical SRAMs use six transistors to store each bit, there are SRAMs that use eight, ten or more transistors to store a single bit. When the number of transistors is reduced, the size of the memory cell decreases. Each SRAM cell can be in three different states called read, write and standby. A cell is in the reading state when data has been requested and it is in writing state when the data in the cell is modified. The cell is in the standby state when it is idling.
What is Dynamic RAM (DRAM)?
DRAM is also a volatile memory that uses separate capacitors to store each bit. Capacitors when not charged represent the value 0 of a bit and when charged represent the value 1. Since the capacitors discharge with time, they need to be refreshed periodically to maintain the values stored in them. Each memory cell in a DRAM consists of a capacitor and a transistor and these cells are arranged in a square array. DRAMS are widely used for main memories in personal computers and game stations since they are cheaper. DRAMs are manufactured as integrated circuits (ICs) that come in plastic packages with metal pins that could be connected in to busses. Currently there are DRAMs in the market that are manufactured as plug-in modules, which are easier to handle. Single In-line Pin Package (SIPP), Single In-line Memory Module (SIMM) and Dual In-line Memory Module (DIMM) are some examples of such modules.
What is the difference between Static RAM and Dynamic RAM?
Even though both SRAMs and DRAMs are volatile memories, they have some important differences. Since the DRAM requires a single capacitor and a transistor for each memory cell, it is much simpler in the structure than the SRAM, which uses six transistors for each memory cell. On the other hand, due to the use of capacitors, DRAM requires to be refreshed periodically as opposed to the SRAM. DRAMs are less expensive and slower than SRAMs. Therefore they are used for the large main memory of personal computers, workstations, etc., while SRAM are used for the smaller and faster cache memory.