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Types of RANDOM ACCESS MEMORY (RAM)

Learn: What are the types of RAM (Random Access Memory), how they work and what are the difference between SRAM and DRAM?

There are two basic types of RAM as given below:

1. Dynamic RAM (DRAM)
2. Static RAM (SRAM)

1) Dynamic RAM

As the name suggests, it is dynamic in nature/operation. The stored information, DATA is lost in a very short time, it can be in order of milliseconds depending upon the design and specifications and internal structure. It is a cheaper and slower memory device. It is made up using semiconductor device technology. Millions of pn- junction transistors and capacitors are used in making the Dynamic Random Access Memory. It requires periodically refresh as the capacitors get discharged.

Operation of DRAM

DRAM is an Integrated Circuit (IC) and the most basic form of computer memory, Inside the DRAM, Dynamic Memory Cell, represents a single bit of data. The capacitor holds the bit of information – a 0 or a 1. The charged state of capacitor is taken as binary“1”(one) and the discharged state as “0”(zero). Due to leakage of capacitors, refresh is required. The pn -junction transistors act as a switch. This transistor switch is used by the control circuitry on the memory chip to read the capacitor or change physical state of charging/ discharging. Capacitor stores the charge. When it is fully charged it is treated as binary“1”(high level) in the memory cell. When there is no charge stored in capacitor, it is treated as binary“0”(low level). It’s discharging rate is very high and it takes few milliseconds.

For DRAM to work, it is to be recharged before they get discharged. Instructions are written so that the memory controller reads the memory and then writes it right back by charging. Refreshing operation is performed automatically thousands of times per second. The DRAM loses the data if it does not get refresh operation. Refresh operation is an important and essential task for the successful operation of Dynamic Random Access memory (DRAM).

2) Static RAM

Static RAM is a memory device in which the information or the data stored in it remains protected unless and until the power supply is removed. It is costlier than the DRAM and it consumes more power. It has higher working speed than DRAM.

SRAM made up of different technology than that of DRAM. This retains stored information only as long as the power supply is on. Static RAM’s are costlier and consume more power. They have higher speed than D-RAMs.

Operation of SRAM

A Flip Flop is used in static RAM, which holds each bit of memory that is binary 1 or 0. It does not require refresh operation. A number of transistors are used in flip-flop for a Memory Cell which never has to be refreshed. This is the main reason why static RAM is significantly faster than dynamic RAM.

SRAM has more parts; its static memory cell takes up a lot of more space on a chip than it takes in a dynamic memory cell. During design a Memory chip Engineer has to optimize the space significantly to have a better memory. Because of more parts in SRAM, one get less memory per chip and that makes static RAM a lot more expensive than DRAM. Hence, Static RAM is fast and expensive; on the other hand, Dynamic RAM is less expensive and slower.

Similarity between SRAM and DRAM is that both lose data once the power is removed from the device. However, DRAM essentially needs to be refreshed periodically. The key difference between SRAM and DRAM is the technology that is being used for holding information or the data. SRAM makes use of latches (flip-flops) in order to store data (transistor circuit), whereas DRAM uses capacitors for storing bits in the form of charge (bits: binary“1” for full charge and “0” for no charge). High speed complementary Metal Oxide semiconductor (CMOS) technology is the basis for SRAM construction, whereas Dynamic RAM uses Special DRAM processes for construction. Static RAMs have a complex internal structure than in comparison to DRAMs.