Internal MemoryInternal Memory

IntroductionIntroductionComputer memory exhibits different types

of technique, organization, performance, size and cost.

But, no technology is optimal in memory requirements for a computer system.

As a result, a computer equipped with a hierarchy of memory subsystems.

Some are internal to the system – RAM, ROM, CACHE

External memories are Hard disk, Optical and Pen drive etc.,

RAMRAMStorage cell unit consists of N cells each of

which can store 1 bit either 0 or 1.Every location of RAM accessed

independently. So, the access and cycle times can constant.

The address of memory location transferred through MAR using address bus.

The address then proceed by the address decoder.

Which select required location in storage unit.

RAM RAM contd.,contd.,

The contents of the selected location placed in IR through MDR processor register.

The R/W control line specifies the type of access to be performed.

Write operation, the word to be place in MDR and the address specified in MAR and then transferred to selected cell.

DDR RAM-Double Data Rate synchronous dynamic random access memory

RAMRAM

ROMROMData are written into a ROM when it is

manufactured.ROM is mask programmed by the

manufacturer in the factory with the contents ordered by the customers.

The contents are fixed by metal masks used during chip fabrication.

Once programmed, the contents cannot be erased.

Even a single bit wrongly programmed the ROM chip is useless.

ApplicationApplicationUsed to store control programs such as

micro program.Character generation, code conversion, etc.,

Boot screenBoot screen

PROM-Programmable ROMPROM-Programmable ROMPROM is a field programmable device.The customer buy a blank PROM and store

desired data using PROM programmer(burner).Programmability achieved by inserting a fuse at

point P.Before programmed, the memory contains all 0s.The user can insert 1 by burning out the fuse in the

particular cell using high current pulse.The PROM chip can be programmed only once

and its contents cannot be erased.PROM are flexible , faster and less expensive

because they can be programmed directly by the user.

Application-PROMApplication-PROMBoot program in micro computer

PROMPROM

PROMPROM

EPROM-Erase programmable EPROM-Erase programmable ROMROM

A rewritable chip that holds its contents without power.

Previous data can be erased and new data can be inserted

EPROM chips are written on an external programming device before being placed on the circuit board.

Capable of retaining stored information for a long time.

EPROM EPROM contd.,contd.,

Eraser requires breakup the charges trapped in the transistors of memory cell.[this is done by break the chip to ultraviolet light].

This reason EPROM packaged with transparent window.

Disadvantages:Entire EPROM is erased as a whole and

selective erasing is not possible.Should be removed from the chip for

reprogramming.

EPROMEPROM

EEPROM-Electrically Erased EEPROM-Electrically Erased PROMPROM

It can be both programmed and erased electrically( flashed back to Zero).

They do not need to removed when the chip content erasure.

Also, erase selected content in the chip.Erasing and programming dynamically

without removing the EEPROM from the circuit.

Disadvantages:Different voltages are required for erasing,

reading and writing the data.

ApplicationApplicationUsed as ROMCritical system setup information in a non-

volatile way.

EEPROMEEPROM

EEPROMEEPROM

EEPROMEEPROM

Flash MemoryFlash MemoryRecent kind of EEPROM is flash memory.Which is erased by ultraviolet light.EEPROM is byte erasable whereas, flash

memory is a block erasable.Like a EEPROM when erasing the contents

of flash memory no need to remove from the circuit.

In flash it is possible to read the contents of a single cell, but is only possible to write an entire block of cells.

Flash memory Flash memory contd.,contd.,

It need single power supply and consume less power.

Application:Cell phones, hand held devices, digital

camera, washing machine, AC, Tablet etc.NOTE: low power consumption of flash

memory makes it attractive for use in portable equipment that is battery driven

Flash CardFlash CardMount flash chips on a small card.That card simply plugged into a slot.64 MB can store 1 hour songs.Now a days, the device can equipped with

up to 32GB flash memory.

Speed, size and CostSpeed, size and CostSRAM chip is very fast memory in the

computer system. but it is expensive because their basic cells have 6 transistors.

Which impossible to packaging a very large number of cells onto a single chip.

Cost wise also expensive.Cache memory can be implemented based

on SRAM.Alternative is to use DRAM, which can have

same basic cells and thus much less expensive. But significantly slower.

Still it is small in size compared to magnetic drives.

Speed, size and CostSpeed, size and CostMain memory can be build with DRAM.Secondary storage- large memory

space with reasonable price but much slower than semiconductor memory unit.

conclusionconclusionHuge amount of cost-effective

storage can be provided by magnetic disk.

Main memory can be built with DRAM.

SRAM to be used in smaller unit where speed is of the essence.(Cache)

SPEEDSPEED

SPEEDSPEEDFastest access is to data held in

processor registers.There are often two levels of cache,

the primary cache inside processor refer to as L1. secondary cache placed outside referred as LEVEL 2(L2) usually SRAM chip.

Next level in the hierarchy is called main memory.

The typical main memory is about ten times slower than the access for the L1 cache.

SPEEDSPEEDThe bottom level in the hierarchy

is called the secondary memory such as magnetic disk.

Disk provide a huge amount of inexpensive storage. But they very slow.