memory units

MEMORY UNITS

PRESENTED BYVANITHA KUMARI. R

MCA

INTRODUCTION The computer system essentially comprises

three important parts – input device, central processing unit (CPU) and the output device.

The CPU itself is made of three components namely, the arithmetic logic unit (ALU), memory unit, and the control unit.

In addition to these, auxiliary storage/secondary storage devices are used to store data and instructions on a long-term basis.

COMPUTER MEMORY

The Term Computer Memory is defined as one or more sets of chips that store Data/program instructions, either temporarily or permanently.

It is critical processing component in any computer. The PCs use several different types of memory.

MEMORY TYPES

Secondary Memory/Auxiliary Memory Primary Memory/Main Memory

-RAM*SRAM*DRAM

-ROM*PROM*EPROM*EEPROM

SECONDARY MEMORY The computer usually uses its input/output channels to access secondary storage and transfers the desired data using intermediate area in primary storage.

It is typically used as a supplement to main storage. Secondary storage does not lose the data when the device is powered down—it is non-volatile.

Per unit, it is typically also an order of magnitude less expensive than primary storage.

PRIMARY MEMORY Primary storage (or main memory or internal

memory), often referred to simply as memory, is the only one directly accessible to the CPU.

Main memory is directly or indirectly connected to the CPU via a memory bus. It is actually two buses an address bus and a data bus. It reads or writes the data itself using the data bus.

RAM RAM or Random Access Memory is the central

storage unit in a computer system. It is the place in a computer where the operating system, application programs and the data in current use are kept temporarily so that they can be accessed by the computer’s processor.

The data stored in RAM is lost if the power to the IC chip is disconnected. Random access memory, also called the Read/Write memory, is the temporary memory of a computer. It is said to be ‘volatile’ since its contents are accessible only as long as the computer is on.

Types Of RAMStatic RAM (SRAM)

The information remains stored as long as power is applied to the chip, unless the same location is written again.

Dynamic RAM (DRAM)

The data stored at each location must be periodically refreshed by reading it and the writing it back again, or else it disappears.

ROM ROM or Read Only Memory is a special type

of memory which can only be read and contents of which are not lost even when the computer is switched off.

It typically contains manufacturer’s instructions. Among other things, ROM also stores an initial program called the ‘bootstrap loader’ whose function is to start the computer software operating, once the power is turned on.

TYPES OF ROM PROM EPROM EEPROM

PROM Information once stored on the ROM or PROM chip

cannot be altered. One step up from the masked ROM is the PROM

(programmable ROM), which is purchased in an unprogrammed state.

The process of writing the data to the PROM involves a special piece of equipment called a device programmer.

The device programmer writes data to the device one word at a time by applying an electrical charge to the input pins of the chip. PROMs are also known as one-time programmable (OTP) devices.

EPROM EPROM (Erasable PROM) allows a user to

erase the information stored on the chip and reprogram it with new information.

An EPROM is programmed in exactly the same manner as a PROM. However, EPROMs can be erased and reprogrammed repeatedly.

To erase an EPROM, we can simply expose the device to a strong source of ultraviolet light. By doing this, we essentially reset the entire chip to its initial-unprogrammed-state.

EEPROM

EEPROMS are electrically-erasable-and-programmable. Internally, they are similar to EPROMs, but the erase operation is accomplished electrically, rather than by exposure to ultraviolet light.

Any byte within an EEPROM may be erased and rewritten. Once written, the new data will remain in the device forever-or at least until it is electrically erased.

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