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334 Fundamentals of Computers NPP

Memory _o_moar

4.1 Memory Cell 4.1 _o_moar gob
A memory cell may be defined as a single `h EH$ {~Q> H$mo g§J«hrV H$aZo dmbr `w{º$ h¡ Ÿ&
bit storage device. A computer memory consists EH$ H$åß`yQ>a H$s _o_moar Eogo H$B© gobm| go ~Zr hmoVr h¡&
of large number of such cells. For example, in O¡go, AmYw{ZH$ {S>{OQ>b H$åß`yQ>am| _| EH$ pâbn-âbm°n
modern day digital computers a cell may be a gob H$m H$m`© H$aVm h¡ Š`m|{H$ BgHo$ AmCQ>nwQ> Q na
flip-flop which stores a bit at its output Q. If the EH$ {~Q> ‘1’ `m ‘0’ g§J«hrV H$s Om gH$Vr h¡Ÿ& `{X
voltage at Q is high, we say that Q = 1. If the
voltage at Q is low, we say that Q = 0. Thus, the pâbn-âbm°n Ho$ AmCQ>nwQ> na CÀM {d^d h¡ Vmo Q =
bit is stored at Q. Another example of cell is a 1 H$hm OmEJm AÝ`Wm Q = 0 H$hm OmEJm Ÿ& Xygam
MOSFET (Metal Oxide Semiconductor Field CXmhaU EH$ MOSFET gob h¡ Ÿ& {Og_| Amdoe Ho$ ê$n
Effect Transistor) which stores a bit in the form _| OmZH$mar g§J«hrV hmoVr h¡Ÿ& Amdoe H$s CnpñW{V ‘1’
of charge. Thus, a memory cell is a fundamental VWm AZwnpñW{V ‘0’ Xem©Vr h¡Ÿ& AV… `h _o_moar H$s EH$
unit of memory. _yb^yV BH$mB© h¡Ÿ&

Characteristics of Memory Cell _o_moar gob H$s {deofVmE±
1. Bistable cell: In modern digital computer, 1. ~mBñQ>o~b gob: My±{H$ AmYw{ZH$ H$åß`yQ>am| _| àË`oH$
each cell may have two stable states. Thus, gob _| Xmo pñWa AdñWmE± hmoVr h¡, Bg{bE BÝh|
it is called a bistable cell.
~mBñQ>o~b gob H$hVo h¢&
2. Volatile cell and Non-volatile cell: If the 2. dmînerb VWm Admînerb gob: `{X {dÚwV H$s
contents of cell are lost, when electric power Amny{V© ~ÝX hmo OmE Vmo dmînerb _o_moar H$s
goes off, it is called volatile cell. For OmZH$mar ZîQ> hmo OmVr h¡ Ÿ& O¡go, RAM H$s gob&
example, the cells in a RAM are volatile.
But if the information retains in a cell even `{X OmZH$mar ZîQ> Zht hmoVr h¡ Vmo Bgo Admînerb
if the power is turned off, it is called non- gob H$hVo h¢Ÿ& O¡go, ROM H$s _o_moar gob `m
volatile cell. For example, a cell in ROM or goH§$S>ar _o_moar H$s gob&
any magnetic memory.
3. Read time and write-time: The time taken 3. n‹T>Zo H$m g_` d {bIZo H$m g_`: {H$gr ^r gob
to perform read and write operation is a Ho$ {~Q> H$mo n‹T>Zo `m {bIZo _| bJm g_` AË`ÝV
very important property of a cell. When a _hËdnyU© hmoVm h¡Ÿ& O~ _o_moar go CPU {H$gr
CPU makes request for an information, OmZH$mar Ho$ {bE {ZdoXZ H$aVm h¡, Vmo Bgo àXmZ
memory takes time to supply this H$aZo _| _o_moar Hw$N> g_` bJmVr h¡Ÿ& _o_moar Ûmam
information. This time is called access time
of memory. This time affects the overall {bE JE Bg g_` H$mo EŠgog g_` H$hVo h¢ & Bggo
program execution speed of computer. The H$åß`yQ>a H$s àmoJ«m_ H$mo {H«$`m{ÝdV H$aZo H$s J{V
access time of RAM is of the order of ns à^m{dV hmoVr h¡ Ÿ& Mwå~H$s` _o_moar H$m EŠgog g_`
(nano second) while the access time of ms (milli second) _| VWm AY©MmbH$ _o_moar H$m
magnetic memory is in ms (milli second). ns (nano second) _| hmoVm h¡Ÿ&
4. Destructive and Non-destructive 4. {dZmer d A{dZmer n‹T>Zm: O~ n‹T>Zo Ho$ níMmV²
Readout: When a read operation is {H$gr gob H$s OmZH$mar ZîQ> hmoVr h¡, Vmo Bgo