RAM

                   Different Types of RAM

 

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.

 RAM is considered “random access” because you can access any memory cell directly if you know the row and column that intersect at that cell. RAM is made in electronic chips made of so called semiconductor material, just like processors and many other types of chips. In RAM, transistors make up the individual storage cells which can each “remember” an amount of data, for example, 1 or 4 bits – as long as the PC is switched on. Physically, RAM consists of small electronic chips which are mounted in modules (small printed circuit boards). The modules are installed in the PC’s motherboard using sockets – there are typically 2, 3 or 4 of these.

There are two basic types of RAM :

(i) Dynamic Ram

(ii) Static RAM

 

Dynamic RAM : loses its stored information in a very short time (for milli sec.) even when power supply is on. D-RAM’s are cheaper & lower.

Similar to a microprocessor chip is an Integrated Circuit (IC) made of millions of transistors and capacitors.

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.

 

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.

 

In static RAM, a form of flipflop 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 speedsensitive cache, while dynamic RAM forms the larger system RAM space.

 

Some other RAMS are :

 

(a) EDO (Extended Data Output) RAM : In an EDO RAMs, any memory location can be accessed. Stores 256 bytes of data information into latches. The latches hold next 256 bytes of information so that in most programs, which are sequentially executed, the data are available without wait states.

 

(b) SDRAM (Synchronous DRAMS), SGRAMs (Synchronous Graphic RAMs) These RAM chips use the same clock rate as CPUuses. They transfer data when the CPU expects them to be ready.

 

(c) DDR-SDRAM (Double Data Rate – SDRAM) : This RAM transfers data on both edges of the clock. Therefore the transfer rate of the data becomes doubles.

 

ROM : Read only memory: Its non volatile memory, ie, the information stored in it, is not lost even if the power supply goes off. It’s used for the permanent storage of information. It also posses random access property. Information can not be written into a ROM by the users/programmers. In other words the contents of ROMs are decided by the manufactures.

 

The following types of ROMs an listed below :

 

(i) PROM : It’s programmable ROM. Its contents are decided by the user. The user can store permanent programs, data etc in a PROM. The data is fed into it using a PROM programs.

 

(ii) EPROM : An EPROM is an erasable PROM. The stored data in EPROM’s can be erased by exposing it to UV light for about 20 min. It’s not easy to erase it because the EPROM IC has to be removed from the computer and exposed to UV light. The entire data is erased and not selected portions by the user. EPROM’s are cheap and reliable.

 

(iii) EEPROM (Electrically Erasable PROM) : The chip can be erased & reprogrammed on the board easily byte by byte. It can be erased with in a few milliseconds. There is a limit on the number of times the EEPROM’s can be reprogrammed, i.e.; usually around 10,000 times.

 

Flash Memory : Its an electrically erasable & programmable permanent type memory. It uses one transistor memory all resulting in high packing density, low power consumption, lower cost & higher reliability. Its used in all power, digital cameras, MP3 players etc.

 

 

Chapter : Memory & Input/Output Unit
• Computer Fundamental