bel 15 memory systems

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Memory Systems

Magnetic Memory Semiconductor memory

Two classes memory

A digital processor generally requires a facility for storinginformation. The part of a digital processor, which providesthis storage facility is called the memory.

A ferromagnetic material can be magnetized in a particular

direction by the application of suitable magnetizing force. The

ability of a magnetic material to store information in two

different states, has resulted magnetic memory.

Recent advances in semiconductor technology have led to

economical and reliable MSI and LSI semiconductor memory

chips that are much smaller in size and much faster in

operation than conventional magnetic memories.S. Kal, IIT-Kharagpur


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Random Access Memory

(RAM)

Read Only Memory

(ROM)

Memory

Transistor flip-flops (either bipolar or MOS) stores 1 bit of

information and is referred to a memory cell.

Faster operation is obtained with a bipolar memory, but greater

packing density and therefore reduced size and cost as well as

lower power requirements are characteristics of MOS memory.

S. Kal, IIT-Kharagpur


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Memory ParametersMemory cell: An electrical circuit or device used to store a

single binary digit (bit) is known as a memory cell. Charged

capacitors, flip flop, single spot on magnetic tape or disk are

some examples of memory cell.

Word: A set of binary cells is physically grouped together to

form a unit called a word. A register consisting of eight flip

flops can be considered to be a memory that is storing an 8-bit

word. The word size in modern computers varies from 4 to 64

bits. A group of 8 bits is known as a byte. Word size can be

expressed in bytes as well as in bits. A word size of 16 bits isalso a word of 2 byte.



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Density or Capacity: The number of bits that can be stored in a

particular memory system is known as its capacity or density. Aparticular memory has greater density than other means that it canstore more bits in the same amount of space. For example, amemory of 16 twenty-bit word means the capacity of the memoryis 320 bits. It is common practice to refer memory capacity in K.The designation 1K represents 210 =1024. The number of words in

a memory is often a multiple of 1024. Thus a memory that has astorage capacity of 2K v 20 comprises of 2048 (2 v 1024) words of20 bits, which means the capacity of the memory unit is 40,960(2048 v 20) bits, In the recent times larger memories are designedas M or meg . 1M equal 1000K, which means 1M = 220 =1,048,576.

Address: The location in the memory system where a word is to bestored is called the address of the word. Similarly one has tospecify its address to retrieve a word from memory. Addresses aregenerally expressed as a binary number. Each word in a memoryhas an address code, which refers to a particular location in amemory.

Memory Parameters



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Read Operation: The read operation, sometimes known asfetch operation, is an operation whereby the binary wordstored in a particular memory location (address) is sensed andthen transferred to another device. If a word stored in somelocation is to be read then its address is entered in memoryaddress register (MAR). When the read signal is activated bythe control unit, the number is copied into memory bufferregister (MBR). Copying process, which leaves the wordundisturbed in its location is called non-destructive read out. Itmay be destructive also. It all depends upon the device usedfor a binary cell in the memory. Flip-flop binary cells, forexample, are non-destructive. Reading from binary cells made

with magnetic cores is destructive. In this case, whatever isread from the selected memory cell must be written back topreserve its contents.

Memory Parameters



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Write Operation: When a new word is placed into a particularmemory location, it is referred to as write or store operation. Inorder to write a word in a particular location, It is placed initiallyin memory buffer register or data register and the address ofthe location is entered in the memory address register. Thewrite signal is then initiated and the new word is stored into thememory location replacing the previously stored data.

Access Time: It is the time interval between the initiation of aread signal and the availability of the required word in thememory buffer register. Access time is a measure of a memorydevices operating speed. The method of accessing memorydepends on the particular device used for binary cell.

Write Time: The time interval between the initiation of a writesignal and the storing information in the specified address(location) in the memory is termed as write time. If readinginformation from the memory is destructive, the total timerequired for read and rewrite is called the memory cycle time.

Memory Parameters



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Simple Block

Diagram of a 8 v 4

bit Memory System

A Binary Memory

Cell with RS Flip-Flop



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Random Access Memory (RAM) A RAM has an addressing structure that allows either

reading from or writing into the memory. RAM uses where

frequent data change is required.

Address is a series of binary digits used to specify thelocation of a word stored in memory. The time required to

write one word into memory or to read one word from

memory is known as access time.

In the read mode, data from the selected memory cell is

made available at the output. In the write mode, informationat the data input is written into (stored in) the selected cell.



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When high speed is required, the BJT is the RAM of choice (

access time ~ 35 ns), in contrast to the MOS RAM (access time

~ 400 ns ). On the other hand, cell density is high ( ~ 4096 ) in

MOS compared to that in bipolar ( ~1024 ).

A memory cell is simply a flip-flop. However some specialfeature is incorporated into it in order to facilitate addressing

the cell, writing into it, and reading from it.

The memory unit that loses its contents with a loss of

electrical power is known asv

olatile memory. SinceR

AM cellis made of flip-flops, a loss of power means a loss of data, and

thus RAM is volatile memory.

Random Access Memory (RAM)



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Internal Architecture of a Simple 16 X4 RAM Chip



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Static Memory (SRAM ) and Dynamic Memory ( DRAM )

Memory cells that are capable of storing binary information

indefinitely are known as static memory. For example, the BJT

or MOS flip-flops remain set or reset as long as power is

applied. A magnetic core remains set or reset even power is

removed.

A dynamic memory is composed of memory cells whose

contents tend to decay over a period of time ( ~ ms ); thus

their contents must be restored (refreshed) periodically. The

leaky capacitance associated with a MOSFET can be used to

store charge, and is the basic unit used to form a memory.

The need for extra timing signals and logic to periodically

refresh the dynamic memory is a disadvantage, but the higher

speeds, lower power dissipation, the increased cell density

outweighs the disadvantages.S. Kal, IIT-Kharagpur


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A Static NMOS Memory cell (SRAM) using NMOS

Transistor



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A Symbolic Representation of a DRAM Cell

S1,S2,S3,S4 represent four MOSFET switches and are

controlled by various address decoder outputs. CS is the

actual storage cell

To write data to the cell CS, decoder signal closes switch S1and S2 while keeping S3 and S4 open

To read data from the cell, S2,S3,S4 are closed and S1 is kept

openS. Kal, IIT-Kharagpur


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One Transistor NMOS DRAM Cell



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Read Only Memory ( ROM )

o A Read-Only Memory (ROM) is a memory device intended tostore information which is fixed. There is an initial operationduring which information is written in the memory andthereafter the memory is read only and is not again writteninto. Since the data are permanently stored in each cell, a loss

of power does not cause a loss of data, and thus a ROMprovides nonvolatile storage. For example, a look up tablethat stores the values of mathematical constants such astrigonometrical functions etc.

o Programmable Read Only Memory (PROM) : Memories which

allow the user to establish the store of information in thememory. Operation is essentially the same as that of a ROM,but the store data can be set in the memory by reading intothe PROM at the users convenience.



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o Erasable PROM ( EPROM) : An application in which thedata may change from time to time might call for the use ofan erasable PROM (EPROM). The data can be programmedinto the EPROM and can be erased and reprogrammed ifdesired. Such memory devices are ROM in the sense that

to change the stored information it is necessary tointerrupt the digital processing in which the memory isinvolved.

o Electrically Erasable PROM (EEPROM): The limitation ofEPROM are removal of the chip from the circuit for eraseand reprogram, unable to erase a single select addressetc. The limitations are removed by electrical erasabletechnique.

Read Only Memory ( ROM )



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An Array of

BipolarROM Cells

PROMS withFusable Links



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Cross Section Schematic of a MOS EPROM

( Quartz Window on EPROM Chip )S. Kal, IIT-Kharagpur


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Applications ofROM:

1. It can be used to provide the realization of an arbitrarytruth table.

2. ROMs are widely used in code conversion and inconnection with alphanumeric displays.

3. ROM is used to yield results that would otherwise beachieved by computation involving a sequence ofarithmetic operations.

Examples:

1. Main memory in a hand calculator RAM

2. Storing values of logarithms R

OM3. Storing prices of vegetable produce EPROM

4. Emergency stop procedures for an industrial mill now inthe design stage PROM


bel 15 memory systems

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