CS 641
Name : Muhammad Hamad Albiladi
ID : 1302219
Subject: Main Memory Research
A computer system usually is not constructed using single type of memory . In fact,
several types are used ; they comprice the hierarchical memory system of the
computer. A typical memory hierarchy is shown in figure 9.1. The most well known
element of the memory subsystem is the physical memory , which is constructed
using random access memory (DRAM) chips. When a company advertices a
personal computer for sale with 128 MB (or some other amount) of RAM, they are
reffering to physical memory. [1]
CPU with L1
cache
Figure 1
L2 cache
Physica l memory
Virtual memory
storage
Generic memory hierarchy
The memory unit that communacates directly with CPU is called main memory .
The main memory is central storage unit in computer system. It is a relatively lage
and fast memory used to store programs and data during the computer
operation.[3]
The memory unit is used to store programs and data. Usually, two types of memory
devices are used to form a memory unit : primay storage memory decice and
secondary storage memory device. The primary memory, commonly called main
memory is a fast memory used for the storage of programs and active data (the data
currently in process). The main memory is a semiconductor memory. It consists of a
large number of semiconductor storage cells, each capable of storing one bit of
information . These cells are read or written by the central processing unit in a group
of fixed size called word. The main memory is organized such that the content of one
word, containing n bits, can be stored or retrieved in one write or read operation,
respectively.[3]
To access data from a particular word from main memory each word in the main
memory has distict address. This allows to access any word from the main memory
by specifying corresponding adderss. The number of bits in each word is reffered as
the word length of the computer. Typically, the word length varies from 8 to 64 bits.
The number of such words in the main memory decides the size of memory or
capacity of the memory. This is one of thespecification of the computer main
memory varies from few million words to tens of millions words.[3]
An important characteristics of a memory is access time (the time required to access
one word). This access time for main memory should be as small as possible.
Typically, it is of the order of 10 to 100 nanoseconds. This access time depend on the
type of memory[3].
 Essential concepts:
The maximum size of the memory that can be used in any computer is determined
from the number of address lines provided by the processor used in the computer.
For example, if procesor has 20 address lines , it is capable of addressing upto 2^20 =
1 M (mega) memory locations. Similarly, proccessors whose instructions generate
32-bit address can access a memory taht contains up to 2^32 = 4G (Giga) memory
locations. The number of locations represents the size of the address space of the
computer.[3]
Computer memory consists of a collection of consecutively number (addressed)
registers, each one of which normally holds on byte. A byte is a collection of eight
bits (sometimes reffered to by those in the computer communication community as
an octor). Each register has an an address , reffered to as memory location. A nibble
or nybble, as it is sometimes spelled, refer to a collection of four adjecent bits. The
meaning of the terms “bit” , “byte” and “nibble” are generally agreed upon
regardless of the specifics of an architecture, but the meaning of word depends upon
the particular processor. Typical word sizes are 16,32,64 and 128 bits, with the 32-bit
word size being the common form for ordinary computers these days, and the 64-bit
word growing in popularity. In this text, words will be assumed to be 32 bits wide
unless otherwise specified. A comparison of these data types is shown in figure 2.[4]
Bit
0
Nibble
0110
Nibble
Byte
10110001
16-bit word (half word) 11001001 01000110
32-bit
10110100 00110101 10011001 01011000
64-bit word (double)
01011000 01010101 10110000 11110011
11001110 11101110 01111000 00110101
128-bit word (quad)
01011000 01010101 10110000 11110011
11001110 11101110 01111000 00110101
00001011 10100110 11110010 11100110
10100100 01000100 10100101 01010001
Figure 2 Common sizes for data types.
In byte-addressable machine, the smallest object that an be referenced in memory is
the byte. However, there are usually instructions that read and write multi byte
words. Multi byte words are stored as a sequence of byte , addressed by the byte of
the word that has the lowest address.Most machine today have instructions that
can access bytes, half-word and double-words.[4]
When multi byte words are used, there are two choices about the order in whichthe
byte are stored in memory: most significant byet at lowest address, refferedto as
big-endian , or least significant byte stored at lowest address, reffered to as
little-endian. The term “endian”comes from the issue of whether eggs should be
broken on the big or little end, which caused a war by bickering politicians in
Jonathan Swift’s Travels. Examples of big- and little-endian formats for a 4-byte, 32bit word illustrated in figure 3.[4]
31
X
MSB Big-Endian byte
x+1
x+2
31
x+3
MSB Little-Endian LSB
X+3 X+2 X+1
x
Figure 3 Big-endian and little-endian formats.

Memory Organization:
 Characteristics of memory systems:
The Table 1 lists the key characteristics of memory systems:
Location
:
CPU
Internal (main)
External (Secondary)
Capacity
:
Word size
Number of words
Unit of Transfer
:
Word
Block
Access Method
:
Random access
Serial access
Performance
:
Access time, Cycle time,
Transfer rate
Physical Type
:
Semiconductor
Magnetic surface
0
Phgysical characteristics
:
Volatile / non volatile
Organisation
:
Erasable/nonerasable
Table 1
Location : The computer memory is placed in three different locations.[3]
CPU : It is in the form of CPU registers and its internal cache memory (16 k bytes in
case of pentium).[3]
Internal : It is the main memory of the system which CPU can access directly.[3]
External : It is in the form of secondary storage devices such as magnetic disk, tapes,
etc. The CPU access this memory with the help of I/O controllers. [3]
Capacity: It is expressed using two terms : Word size and number of words.[3]
Word size: It is expressed in bytes (8-bit). The common word sizes are 8, 16 and 32
bits.[3]
Number of Word: This term specifies the number of words available in the particular
memory device. For exmaple, if memory capacity is 4k x 8 then its word size is 8 and
number of words are 4k = 4096.[3]
Unit of Transfer: It is the maximum number of bits that can be read or written intb
the main memory at a time. In case of main memory, most of the times it is equal to
word size. In case of external memory, unit of transfer is not limited top word size , it
is often larger than a word and it is refferd to as blocks.[3]
Access Method: A fundemental characteristic of a memory is the order or sequence
in which information can be accessed.[3]
Random Access; if storage locations can be accessed in any order and access time is
independent of the location being accessed , the access method is known as random
access. The memory tht provides such as access is known as random access memory
(RAM) IC (semiconductor) memories are generally of this type.[3]
Serial Access: if storage locations can be accessed only in a certain predetermined
sequence, the access method is known as serial access. The memory that provides
such access is known as serial access memory. Magnetic disk and tapes, as well as
optical memories like CD_ROMs , employ serial access methods.[3]
Semirandom Access: Memory devices such as magnetic hard disks and CD-ROMs
contains many rotaing storage tracks. If each trck has its own read-write head, the
tracks can be accessed randomly, but access within each track is serial. In such cases
the access method is semirandom.[3]
Performance: The performance of the memory system is determined using three
parameters.[3]
Access Time: In case of random access memory, it is the time taken by memory to
complete read/write operation from the instant that an address is sent to the
memoty. On the other hand, for nonrandom access memory , access time is the
time it takes to position the read-write mevhanism at the desired location.
Transfer Time : It is defined as the rate at which data can be transferred into or out
of memory unit.[3]
Physical Type: Two most common physical types used today are semiconductor
memory and magnetic memory.[3]
Physical characteristics :
Volatile/nonvolatile if memory can hold data even if power turn off, it is called
nonvolatile memory, otherwise volatile memory.[3]
Memory Access time : This term is used only in cincern with random access memory
and is is defined as access time plus additional time required befor a second access
can commence.[3]
Erasable/Nonrasable: The memories in which data is once programmed cannot be
erased are called as noonerasable memories. On the hand , if data in the memory is
called erasable memory.[3]
The basic elememnt of semiconductor memory is the memory cell. Although a
variety of electronic technologies are used, all semiconductor memory cells share
certain properties:
o They exhibit two stable (or smistable) status , which can be used to
represent binary 1 and 0.
o They are capable of being written into (at least once) , to set the state.
o They are capable of being read to sense thw state.
Figure 4 depicts the operation of memory cell. Most commonly, the cell has three
functional terminals capable of carrying an electronic signal. The select terminal,as
the name suggests, select a memory cell for a read or write operation. The control
terminal indicates read or write. For writting, the other terminal provides an
electroic signal the sets the state of the cell 0 or 1. For reading, that terminal is used
for output of nthe cell’s state. The details of the internal organization, functioning,
and timing of the memory cell depend on the specific integrated technology used
and are beyond the scope of this book, except for brief summary. For our purpose,
we will take it as given that individual cells can be selected for reading and writting
operations.[5]
Control
Select
Cell
Control
Data in
(a) Write
Select
Cell
Sense
(b) Read
Figure 4 Memory Cell Operation.
 DRAM and SRAM:
Table 2 Lists are the major types of semiconductor memory. The most common is
reffered to as random access memory (RAM). This is , course, a misuse of the
term , because all of the types listed in the table are random access. One it is
distinguishing charactertic of RAM is that it is possible both to read data from the
memory and to write new data into the memory easily and rapidly.Both the
reading and writting are accomplished through the use of electronic signals. The
othe distinguishing characteristic of RAM is that it is volatile. A RAM must be
provided with constant power supply. If the power supply is off, the data are lost.
Thus, RAM can be used only as temporary storage. The two traditional forms of
RAM used in computer are DRAM and SRAM.[5]
Memory Type
Category
Erasure
Write
Volatiltity
Mechanism
Random-access
memory(RAM)
Read-write
memory
Electrically, bytelevel
Electrically
Read-only
memory(ROM)
Programmable
ROM(PROM)
Erasable PROM
(EPROM)
Electronic
Erasable PROM
(EEPROM)
Flash memory
Volatile
Masks
Read-only
memory
Not possible
UV light,
chip-level
Read-mostly
memory
Electrically,
Byte-level
Electrically
Block-level
Table 2 semiconductor Memory Types
Electrically
Nonvolatile
Dynamic RAM
It is made with cells that store data as charge on capacitors. The presence or
absence of the charge in a capacitor is interpreted as a binary 0 or 1. Because
capacitors have a natural tendency to discharge, dynamic RAMs require periodic
charge refreshing to maintain data. The term dynamic refers to this tendency of
the stored charge to leak away , even with power continuously applied.
Figure 5.1 is typical DRAM structurefor an individual cell that stores one bit.[5]
For the Write operation , a voltage signal is applied to the bit line ; a high voltage
represents 1 , qnd a low voltage represents 0. A signal is then applied to the
address line , allowing a charge to be transferredto the capacitor.
For Read operation, when the address line is selected , the transistor turns on and
the charge on the capacitor is fed out onto a bit line and to a sense amplifier.
Although the DRAM cell is used to store a signle bit (0 or 1), it is essentially analog
device.[5]
Address line
Transistor
Storage Capacitor
Bit line B
Ground
(a) Dynamic RAM (DRAM) cell
Figure 5.1 Typical Memory Cell Structure
STATIC RAM
It is a digital device, using the same logic elements used in the processor. In a SRAM,
binary values are stored using traditional flip-flop configration . A static RAM will
hold its data as long as power supplied to it. Figure 5.2b is a typical SRAM Structure
for an individual cell.[5]
T3
T4
C2 T6
T1
T2
Ground
Bit line A
Address Line
Bit Line B
(b) Static RAM (SRAM) cell
Figure 5.1 Typical Memory Cell Structure
SRAM versus DRAM:
Both static and dynamic RAMs are volatile;that is, power musst be continously
supplied to the memory to preserve the bit values. A dynamic memory cell is smaller
and simpler than static memory cell. Thus, a DRAM is more dence (smaller
cells=more cellsm per unit area) and less expensive than corresponding SRAM.
DRAM requires the supporting refresh circuitry. For the larger memories , the fixed
cost of the refresh circuitry is more than compensated for by the smaller variable
cost of DRAM cells.Thus, DRAMs tend to be favored for large memory requirement.
A final point is that SRAM is used are generally somewhat faster than DRAms.
Because of these relative characteristics , SRAM is used for cache memory (both on
and off chip), and DRAM is used for main memory.[5]
Types of ROM:
As the name suggests. A read-only memory (ROM) contains a permanent pattern of
data taht cannot be changed .A ROM is nonvaltile ; that is no power source is
required to maintain the bit values in memory. While it is possible to read a ROM , it
is not possible to write new data into it. An important aplications of ROMs include
the multiprgramming.[5] The other potential applications include :
 Library subroutines for frequently wanted functions.
 System programs.
 Function table.
For modest-size requirement, the advantage of ROM is that the data or programs is
permanently in main memory and need never be loaded from a secondray storage
device. A ROM is created like any other integrated circuit chip , with data actually
wired into the chip as part of the fabrication process.[5] The presence two problems:
 The data insertion step includes large fixed cost whether one or thousands of
copies of particular ROM are fabricated.
 There is no room for error. If one bit is wrong , the whole batch of ROMs must
be thrown out.
When a small number of ROMs with a particular memory content is needed, a less
expensive alternative is the programming ROM (PROM). The PROM is nonvolatile
and may be written into only once. For the PROM, the writing process is performed
electrically and may be performed by supplier or customer at a time than the original
chip fabrication. The PROM remains attractive for high-volume production runs.[5]
The optically erasable programmable read-only memory (EPROM) is read and written
electrically, as with PROM. However, before a write operation, all the storage cells
must be erased to the same initial state by exposure of the packaged chip to
utraviolet rediation. The EPROM can be altered multiple times and it can hold its
data virtually indefinitely. For comparable amount of storage, the EPROM is more
expensive than PROM , but it has the advantage of the multiple update capability.[5]
A more attractive form of read-mostly memory is electrically erasable programmable
read-only memory (EEPROM). This is a read-mostly memory that can be written into
at any time without erasing contents, only the byte or bytes addressed updated. The
EEPROM combines the advantage of nonvolatility with the flexbility of being
updatable in place, using ordinary bus control, address and data lines. EEPROM is
more expensive than EPROM and also is less dense, supporting fewer bits per
chip.[5]
 References:
1. John D. Carpinelli. (2001). Computer System Organisation &
Architecture. USA: Addison Wesley.
2. M. Morris Mano. (2002). Computer System Architecture. India:
Prentice-Hall of india private limited
3. A. P. Godse and D. A. Godse.
(2008). Computer Organisation. India:
technical publication pune.
4. Miles J. Murdocca and Vincent P. Heuring. (2000). Principles of
Computer Architecture. USA: Prentice-Hall.
5. William Stallings. (2006). Computer Organisation & Architecture.
USA: Prentice-Hall.