CORE–1: DIGITAL LOGIC (Unit– 4)

Memory System: memory refers to a device that is used to store information for
immediate use in a computer. The term "memory", meaning "primary storage" or
"main memory” is often associated with addressable semiconductor memory, i.e.
integrated circuits consisting of silicon-based MOS transistors, and used as primary
storage. The organization of memory is as shown in the following figure.

There are two main kinds of semiconductor memory, volatile and non-volatile.
Volatile memories loose data when power goes off. But non-volatile memories retain
data even after power goes off. Various types of memories are shown in the
following diagram.
Computer Memory

Primary memory Secondary

(Semiconductor memory) memory

RAM ROM HDD CD DVD Flash

(volatile) (non-volatile) (magn (Optical memory
etic) memory) (Semicond
uctor)
SRAM DRAM PROM EPROM EEPROM

Asynchronous Synchronous
DRAM DRAM

Primary memory includes ROM and RAM, and is located close to the CPU on the
computer motherboard, enabling the CPU to read data from primary memory very
quickly. It is used to store data that the CPU needs imminently so that it does not
have to wait for it to be delivered.

Random Access Memory (RAM):It is also called as read write memory or

the main memory or the primary memory. The programs and data that the CPU
requires during execution of a program are stored in this memory. It is a volatile
memory i.e. the data loses when the power is turned off. RAM is further classified
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into two types- SRAM (Static Random Access Memory) and DRAM (Dynamic
Random Access Memory).
SRAM: SRAM stands for Static RAM, and it is a particular type of RAM which is
faster than DRAM, but more expensive and bulker, having six transistors in each
cell. For these reasons SRAM is generally used as a data cache within a CPU itself
or as RAM in very high-end server systems. A small SRAM cache of the most
imminently-needed data can result in significant speed improvements in a system.
DRAM: DRAM stands for Dynamic RAM, and it is the most common type of RAM
used in computers. The oldest type is known as single data rate (SDR) DRAM, but
newer computers use faster dual data rate (DDR) DRAM. DDR comes in several
versions including DDR2 , DDR3, and DDR4, which offer better performance and
are more energy efficient than DDR. However different versions are incompatible, so
it is not possible to mix DDR2 with DDR3 DRAM in a computer system. DRAM
consists of a transistor and a capacitor in each cell.

SRAM vs, DRAM

Feature DRAM SRAM
Cost Cheaper More expensive
Slower: Off-chip memory On-chip memory with minimal
Performance
with longer access time access time;
Use case Main memory Level 1 and Level 2 caches
Less density per cell (1
Denser (6 transistors per chip) but
Density transistor per chip) but can
can fit fewer cells into space
pack more cells into space
Generally higher: Capacitors Generally lower: No charge leakage.
Power leak power requiring regular However, SRAM can consume as
power refreshes. much or more power as DRAM.
Larger: Connects directly to
Storage Smaller: Acts as cache; storage
CPU bus, volatile storage
capacity measured in MBs
measured in GBs
Volatile: Must have active Volatile: Does not require additional
Volatility power supply plus frequent charges while it is receiving power,
charges while active. but eventually loses data without it.
Physical Processors or between processor
Motherboard
placement and main memory

RAMBUS Memory: Rambus DRAM (RDRAM), and its successors Concurrent

Rambus DRAM (CRDRAM) and Direct Rambus DRAM (DRDRAM), are types
of synchronous dynamic random-access memory (SDRAM) developed
by Rambus Inc. from the 1990s through to the early-2000s. Rambus DRAM was
developed for high-bandwidth applications, and was positioned by Rambus as
replacement for various types of contemporary memories, such as SDRAM.
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DRDRAM was initially expected to become the standard in PC memory, especially
after Intel agreed to license the Rambus technology for use with its future chipsets.
Further, DRDRAM was expected to become a standard for graphics memory.
However, RDRAM quickly lost out on grounds of price and performance with an
alternative technology—DDR SDRAM. By around 2003, DRDRAM was no longer
supported by any personal computer.
Synchronous DRAMS vs. Asynchronous DRAMS
 The key difference between synchronous and asynchronous DRAM is that
the synchronous DRAM uses the system clock to coordinate the memory
access while asynchronous DRAM does not use the system clock to
coordinate the memory access.
 Asynchronous DRAM is an older type of DRAM used in the first personal
computers. It is called "asynchronous" because memory access is
not synchronized with the computer system clock. Modern PCs
use SDRAM (synchronized DRAM) that responds to read and write operations
in synchrony with the signal of the system clock.
 Synchronous DRAM provides high performance while Asynchronous DRAM
provides low performance.
Read Only Memory (ROM) –
 Stores crucial information essential to operate the system, like the program
essential to boot the computer.
 It is not volatile i.e. always retains its data.
 Used in embedded systems or where the programming needs no change.
 Used in calculators and peripheral devices.
Types of Read Only Memory (ROM): ROM is further classified into 3 types-
PROM, EPROM and EEPROM.
1. PROM (Programmable read-only memory) – It can be programmed by user.
Once programmed, the data and instructions in it cannot be changed.
2. EPROM (Erasable Programmable read only memory) – It can be
reprogrammed. To erase data from it, expose it to ultra violet light. To
reprogram it, erase all the previous data.
3. EEPROM (Electrically erasable programmable read only memory) – The
data can be erased by applying electric field, no need of ultra violet light. Only
some portions of the chip can be erased.
Flash Memory: Flash memory storage is a form of non-volatile memory that was
born out of a combination of the traditional EPROM and EEPROM. Flash memory
uses the same method of programming as the standard EPROM and the erasure
method of the EEPROM. Data typically erased and written in blocks.
Advantages of Flash Memory
 Non-volatile memory
 Easily portable (e.g. USB memory sticks, camera flash cards, etc)
 Mechanically robust

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Disadvantages of Flash Memory
 Higher cost per bit than hard drives
 Slower than other forms of memory
 Data typically erased and written in blocks
Secondary Storage: Primary memory has limited storage capacity and is
volatile. Secondary memory overcomes this limitation by providing permanent
storage of data and in bulk quantity. Secondary memory is also termed as external
memory and refers to the various storage media on which a computer can store
data and programs. The Secondary storage media can be fixed or removable. Fixed
Storage media is an internal storage medium like hard disk that is fixed inside the
computer. Storage medium that are portable and can be taken outside the computer
are termed as removable storage media.
Difference between Primary Memory and Secondary Memory:
PRIMARY MEMORY SECONDARY MEMORY
Secondary memory is not accessed
directly by the Central Processing Unit
Primary memory is directly accessed (CPU). Instead, data accessed from a
by the Central Processing Unit (CPU). secondary memory is first loaded into
Random Access Memory (RAM) and is
then sent to the Processing Unit.
RAM provides much faster accessing Secondary Memory is slower in data
speed to data than secondary accessing. Typically primary memory is six
memory. times faster than the secondary memory.
Primary memory, i.e. Random Access Secondary memory provides a feature of
Memory (RAM) is volatile and gets being non-volatile, which means it can
completely erased when a computer hold on to its data with or without electrical
is shut down. power supply.

Uses of Secondary Media:

 Permanent Storage: Primary Memory (RAM) is volatile, i.e. it loses all
information when the electricity is turned off, so in order to secure the data
permanently in the device, Secondary storage devices are needed.
 Portability: Storage medium, like the CDs, flash drives can be used to
transfer the data from one device to another.
Magnetic Hard Disks: A magnetic disk is a storage device that uses a
magnetization process to write, rewrite and access data. It is covered with a
magnetic coating and stores data in the form of tracks, spots and sectors. Hard
disks and floppy disks are common examples of magnetic disks.

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A magnetic disk primarily consists
of a rotating magnetic surface
(called platter) and a mechanical
arm that moves over it. Together,
they form a “comb”. The
mechanical arm is used to read
from and write to the disk. The
data on a magnetic disk is read
and written using a magnetization
process. The platter keeps
spinning at high speed while the
head of the arm moves across its
surface.
Data is organized on the disk in the form of tracks and
sectors, where tracks are the circular divisions of the
disk. Tracks are further divided into sectors that
contain blocks of data. All read and write operations
on the magnetic disk are performed on the sectors.
The floating heads require very precise control to
read/write data due to the proximity of the tracks.
With the invention of RAID (redundant array of inexpensive disks) technology that
combines multiple disk drives, the storage capacity of devices increased year after
year. Magnetic disks have traditionally been used as secondary storage devices in
computers, and represented the mainstream technology for decades. With the
advent of solid-state drives (SSDs), magnetic disks are no longer considered the
only option, but are still commonly used.
Optical Disks: An optical disk is any computer disk that uses optical storage
techniques and technology to read and write data. It is a computer storage disk that
stores data digitally and uses laser beams (transmitted from a laser head mounted
on an optical disk drive) to read and write data. An optical disk is primarily used as a
portable and secondary storage device. Compact disks (CD), digital versatile/video
disks (DVD) and Blu-ray disks are currently the most commonly used forms of
optical disks. These disks are generally used to:
 Distribute software to customers
 Store large amounts of data such as music, images and videos
 Transfer data to different computers or devices
 Back up data from a local machine
Magnetic Tape Systems: In magnetic tape only one side of the ribbon is used
for storing data. It is sequential memory which contains thin plastic ribbon to store
data and coated by magnetic oxide. Data read/write speed is slower because of
sequential access. It is highly reliable which requires magnetic tape drive writing and
reading data. The width of the ribbon varies from 4mm to 1 Inch and it has storage
capacity 100 MB to 200 GB.

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Advantages:
1. These are inexpensive, i.e., low cost memories.
2. It provides backup or archival storage.
3. It can be used for large files.
4. It can be used for copying from disk files.
5. It is a reusable memory.
6. It is compact and easy to store on racks.
Disadvantages:
1. Sequential access is the disadvantage, means it does not allow access
randomly or directly.
2. It requires caring to store, i.e., vulnerable humidity, dust free, and suitable
environment.
3. It stored data cannot be easily updated or modified, i.e., difficult to make
updates on data.
Difference between Magnetic Tape and Magnetic Disk
MAGNETIC TAPE MAGNETIC DISK
The cost of magnetic tape is less. The cost of magnetic disk is high.
Reliability of magnetic tape is less. Reliability of magnetic disk is more.
Access time for magnetic tape is more. Access time for magnetic disk is less.
Data transfer rate for magnetic tape is Data transfer rate for magnetic disk is
comparatively less. more.
Magnetic disk is used as a secondary
Magnetic tape is used for backups.
storage.
In magnetic tape data accessing rate is In magnetic disk data accessing rate is
slow. high or fast.
Magnetic tape is more portable. Magnetic disk is less portable.
Magnetic tape contains reels of tape Magnetic disk contains round platters
which is in form of strip of plastic. which are made up of plastic or metal.
In magnetic tape for data recording, While in magnetic disk for data
magnetic material is coated on only one recording, magnetic material is coated on
side of the tape. only both sides of the platters.

Difference Between Magnetic Disk and Optical Disk

Key Magnetic Disk Optical Disk
Media Type Multiple Fixed Type Single Removable Disk
Data Access Random Access. Sequential Access.
Examples Hard Disk, Floppy Disk, CD, DVD, Blu-ray.
Magnetic Tape.
Reuse Highly reusable and used for Most of the optical disks are
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Difference Between Magnetic Disk and Optical Disk
Key Magnetic Disk Optical Disk
random read/write operations. read-only once written.
Cost Costly per MB Cheaper per MB

Speed, Size and Cost of Memory:

Increasing Processor Increasing Increasing
size speed cost per bit
Registers

Primary Cache L1

Secondary Cache L2

Main memory

Magnetic disk
Secondary memory

 Fastest access is to the data held in processor registers. Registers are at the
top of the memory hierarchy.
 Relatively small amount of memory that can be implemented on the processor
chip. This is processor cache.
 Two levels of cache. Level 1 (L1) cache is on the processor chip. Level 2 (L2)
cache is in between main memory and processor.
 Next level is main memory. It is much larger, but much slower than cache
memory.
 Next level is magnetic disk. It is huge amount of inexpensive storage.

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Internal Organization of Memory Chips:

 The above figure shows a memory chip consisting of 16 words of 8 bits each,
which is usually referred to as a 16 x 8 organization.
 The data input and the data output of each Sense/Write circuit are connected
to a single bi-directional data line in order to reduce the number of pins
required.
 One control line, the R/W (Read/Write) input is used to specify the required
operation and another control line, the CS (Chip Select) input is used to select
a given chip in a multichip memory system.
 It can store 16 x 8 = 128 bits.

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Structure of Large Memories:

(Organization of a 2M x 32 memory module using 512K x 8 memory chips)

 The above figure shows implementation of 2M x 32 (2M words of 32 bits each)

memory module using 512K x 8 memory chips.
 A19 and A20 address lines are given as input to a 2 x 4 decoder which select a
particular line of 4 memory chips of 512K x 8
 A0 to A18 (19 address lines) select 8-bit words in the 512K x 8 memory chips in
that line which is activated by the decoder.
 D31-24, D23-16, D15-8 and D7-0 are data input / output lines.

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