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Computer organization Notes

RAM-- Random Access Memory

Random Access Memory (RAM) is the best known form of Computer Memory. The Read and write (R/W) memory of a computer is called RAM. The User can write information to it and read information from it. With Ram any location can be reached in a fixed (and short) amount of time after specifying its address. The RAM is a volatile memory, it means information written to it can be accessed as long as power is on. As soon as the power is off, it can not be accessed. so this mean RAM computer memory essentially empty.RAM holds data and processing instructions temporarily until the CPU needs it.
The main memory is the central storage unit in a computer system. It is a relatively large and fast memory used to store programs and data during the computer operation. The principal technology used for the main memory is based on semiconductor integrated circuits. Integrated circuit RAM chips are available in two possible operating modes, static and dynamic. The static RAM consists essentially of internal flip-flops that store the binary information. The stored information remains valid as long as power is applied to the unit. The dynamic RAM stores the binary information in the form of electric charges that are applied to capacitors. The capacitors are provided inside the chip by MOS transistors. The stored charge on the capacitors tend to discharge with time and the capacitors must be periodically recharged by refreshing the dynamic memory. Refreshing is done by cycling through the words every few milliseconds to restore the decaying charge. The dynamic RAM offers reduced power consumption and larger storage capacity in a single memory chip. The static RAM is easier to use and has shorter read and write cycles.
There are two basic types of RAM:
(i) Dynamic Ram
(ii) Static RAM
Dynamic RAM: Dynamic memory cells store a data bit in a small capacitor rather than in a latch. It loses its stored information in a very short time (for milli sec.) even when power supply is on. D-RAM’s are cheaper & lower. In the most common form of computer memory, Dynamic Memory Cell, represents a single bit of data. The capacitor holds the bit of information – a 0 or a 1. The transistor acts as a switch that lets the control circuitry on the memory chip read the capacitor or change its state. A capacitor is like a small bucket that is able to store electrons. To store a 1 in the memory cell, the bucket is filled with electrons. To store a 0, it is emptied. The problem with the capacitor’s bucket is that it has a leak. In a matter of a few milliseconds a full bucket becomes empty. Therefore, for dynamic memory to work, either the CPU or the Memory Controller has to come along and recharge all of the capacitors holding it before they discharge. To do this, the memory controller reads the memory and then writes it right back. This refresh operation happens automatically thousands of times per second. This refresh operation is where dynamic RAM gets its name. Dynamic RAM has to be dynamically refreshed all of the time or it forgets what it is holding. The downside of all of this refreshing is that it takes time and slows down the memory.
Advantage of this type of cell is that it is very simple, allowing very large memory array to be constructed on a chip at a very low cost per bit. The disadvantage is that the storage capacitor cannot hold its charge over an extended period of time so data will be lose unless its refreshed periodically.
Static RAM uses a completely different technology. S-RAM 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. They store information in Hip-Hope. Static Ram has longer life and used as cache memory.
In static RAM, a form of flip flop holds each bit of memory. A flip-flop for a memory cell takes four or six transistors along with some wiring, but never has to be refreshed. This makes static RAM significantly faster than dynamic RAM. However, because it has more parts, a static memory cell takes up a lot more space on a chip than a dynamic memory cell. Therefore, you get less memory per chip, and that makes static RAM a lot more expensive. Static RAM is fast and expensive, and dynamic RAM is less expensive and slower. Static RAM is used to create the CPU’s speed sensitive cache, while dynamic RAM forms the larger system RAM space.

Difference Between SRAM and DRAM

Static RAM Dynamic RAM
It Consists of Flip-Flops. It Consists of Capacitors.
It works on Electricity Principle. It works on Charge Principle.
It Consumes More Power. It Consumes Less Power.
It is Faster. It is slower than Static RAM.
It does not require continuous refreshment. It requires continuous refreshment.
Its storage capacity is less. It is having more storage capacity.
It is easy to implement. It is difficult to Implement.
It is having longer read and write cycles. It is having shorter read and write cycles.
No need to read and write cells periodically. Here cells be read and write periodically.
It is more expensive. It is less expensive.
SRAM are used for the smaller and faster cache memory they are used for the large main memory of personal computers, workstations, etc.,

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