3 computer memory

COMPUTER MEMORY
LECTURE # 03 (COMPUTER APPLICATIONS)

INTRODUCTION
• The computer memory stores different kinds of data like input
data, output data, intermediate results, etc., and the instructions.
• Storage may be required for a limited period of time, instantly, or,
for an extended period of time.
• The cache memory, registers, and RAM are fast memories and
store the data and instructions temporarily during the processing
of data and instructions.
• The secondary memory like magnetic disks and optical disks
have large storage capacities and store the data and instructions
permanently, but are slow memory devices.
• The memories are organized in the computer in a manner to
achieve high levels of performance at the minimum cost.

MEMORY REPRESENTATION
• Binary digit or bit is the basic unit of memory.
• A bit is a single binary digit, i.e., 0 or 1 and is the smallest
unit of representation of data in a computer.
• A group of 8 bits form a byte. One byte is the smallest unit
of data that is handled by the computer.
• One byte can store 28, i.e., 256 different combinations of
bits, and thus can be used to represent 256 different symbols.
• In a byte, the different combinations of bits fall in the range
00000000 to 11111111.
• A group of bytes can be further combined to form a word. A
word can be a group of 2, 4 or 8 bytes.

• 1 bit = 0 or 1
• 1 Byte (B) = 8 bits
• 1 Kilobyte (KB) = 210 Bytes = 1024 bytes
• 1 Megabyte (MB) = 220 Bytes = 1024KB
• 1 Gigabyte (GB) = 230 Bytes = 1024 MB = 1024 *1024 KB
• 1 Terabyte (TB) = 240 Bytes = 1024 GB = 1024 * 1024 *1024 KB
• Memory is logically organized as a linear array of locations.
• For a processor, the range of the memory addresses is 0 to the
maximum size of memory. Figure 3.1 shows the organization of a 16
MB block of memory for a processor with a 32-bit word length.

Figure 3.1 Organization of memory

MEMORY HIERARCHY
• The memory is characterized on the basis of two key factors:
1. capacity and
2. access time.
• Capacity is the amount of information (in bits) that a
memory can store.
• Access time is the time interval between the read/ write
request and the availability of data. The lesser the access
time, the faster is the speed of memory.
• The hierarchy of the different memory types is shown in
Figure 3.2.

MEMORY HIERARCHY
• The internal memory and external memory are the two broad
categories of memory used in the computer.
Internal Memory
• The key features of internal memory are:
(1) limited storage capacity,
(2) temporary storage,
(3) fast access, and
(4) high cost.
• Registers, cache memory, and primary memory constitute the
internal memory.

MEMORY HIERARCHY
• Registers are the fastest and the most expensive among all
the memory types. The registers are located inside the CPU,
and are directly accessible by the CPU. The speed of
registers is between 1—2 ns (nanosecond). The sum of the
size of registers is about 200B.
• Cache memory is next in the hierarchy and is placed between
the CPU and the main memory. The speed of cache is
between 2—10 ns. The cache size varies between 32 KB to
4MB.
• Any program or data that has to be executed must be brought
into RAM from the secondary memory. Primary memory is
relatively slower than the cache memory. The speed of RAM
is around 60ns. The RAM size varies from 512KB to 12GB.

MEMORY HIERARCHY
Secondary Memory
• The key features of secondary memory storage devices are:
(1) very high storage capacity,
(2) permanent storage (non-volatile), unless erased by user,
(3) relatively slower access,
(4) stores data and instructions that are not currently being used by
CPU but may be required later for processing, and
(5) cheapest among all memory.
• The storage devices consist of two parts—drive and device.
For example magnetic tape drive and magnetic tape,
magnetic disk drive and disk, and, optical disk drive and
disk.

MEMORY HIERARCHY
• The speed of magnetic disk is around 60ms. The capacity of a
hard disk ranges from 500 GB to 3000 GB (3 Tera Bytes).
• The speed of memories is dependent on the kind of technology
used for the memory.
• The registers, cache memory and primary memory are
semiconductor memories. They do not have any moving parts and
are fast memories.
• The secondary memory is magnetic or optical memory, has
moving parts and has slow speed.
• To get the fastest speed of memory with largest capacity and least
cost, the fast memory is located close to the processor. The
secondary memory, which is not as fast, is used to store
information permanently, and is placed farthest from the
processor.

MEMORY HIERARCHY
• With respect to CPU, the memory is organized as follows:
Registers are placed inside the CPU (small capacity, high cost, very
high speed)
Cache memory is placed next in the hierarchy (inside and outside
the CPU)
Primary memory is placed next in the hierarchy
Secondary memory is the farthest from CPU (large capacity, low
cost, low speed)

MEMORY HIERARCHY
Figure 3.3 CPU and the memory

CPU REGISTERS
• Registers are very high-speed storage areas located inside the
CPU.
• After CPU gets the data and instructions from the cache or RAM,
the data and instructions are moved to the registers for
processing.
• Registers are manipulated directly by the control unit of CPU
during instruction execution. That is why registers are often
referred to as the CPU’s working memory.
• Since CPU uses registers for the processing of data, the number
of registers in a CPU and the size of each register affect the
power and speed of a CPU.
• The more the number of registers (ten to hundreds) and bigger the
size of each register (8 bits to 64 bits), the better it is.

CACHE MEMORY
• Cache memory is placed in between the CPU and the RAM.
• Cache memory is a fast memory, faster than the RAM. When the CPU
needs an instruction or data during processing, it first looks in the
cache.
• If the information is present in the cache, it is called a cache hit, and
the data or instruction is retrieved from the cache.
• If the information is not present in cache, then it is called a cache miss
and the information is then retrieved from RAM.
• The content of cache is decided by the cache controller (a circuit on the
motherboard). The most recently accessed information or instructions
help the controller to guess the RAM locations that may be accessed
next.
• To get good system performance, the number of hits must be greater
than the misses. The two main factors that affect the performance of
cache are its size and level (L1, L2 and L3).

PRIMARY MEMORY
• Primary memory is the main memory of computer. It is a chip mounted
on the motherboard of computer.
• Primary memory is categorized into two main types:
1. Random Access Memory (RAM), and
2. Read Only Memory (ROM)
• RAM is used for the temporary storage of input data, output data and
intermediate results. The input data entered into the computer using the
input device, is stored in RAM for processing.
• After processing, the output data is stored in RAM before being sent to
the output device. Any intermediate results generated during the
processing of program are also stored in RAM.
• Unlike RAM, the data once stored in ROM either cannot be changed or
can only be changed using some special operations. Therefore, ROM is
used to store the data that does not require a change.

RANDOM ACCESS MEMORY
• RAM is used to store data and instructions during the operation
of computer.
• The data and instructions that need to be operated upon by CPU are first
brought to RAM from the secondary storage devices like the hard disk.
• CPU interacts with RAM to get the data and instructions for processing.
• RAM loses information when the computer is powered off. It is a
volatile memory.
• RAM provides random access to the stored bytes, words, or
larger data units. This means that it requires same amount of time
to access information from RAM, irrespective of where it is
located in it.
• RAM can be read from and written to with the same speed.
• The size of RAM is limited due to its high cost. The size of RAM
is measured in MB or GB.

• The performance of RAM is affected by:
• Access speed (how quickly information can be retrieved). The speed of
RAM is expressed in nanoseconds.
• Data transfer unit size (how much information can be retrieved in one
request).
• RAM affects the speed and power of a computer. More the RAM,
the better it is.
• Nowadays, computers generally have 1 GB to 12 GB of RAM.
• RAM is a microchip implemented using semiconductors.
• There are two categories of RAM, depending on the technology
used to construct a RAM:
(1) Dynamic RAM (DRAM), and
(2) Static RAM (SRAM).

• DRAM is the most common type of memory chip. DRAM is
mostly used as main memory since it is small and cheap.
• It uses transistors and capacitors. The transistor and capacitor
are paired to make a memory cell.
• DRAM must be refreshed continually to store information.
For this, a memory controller is used.
• DRAM gets its name from the refresh operation that it
requires to store the information; otherwise it will lose what
it is holding. The refresh operation occurs automatically
thousands of times per second. DRAM is slow because the
refreshing takes time.
• Access speed of DRAM ranges from 50 to 150 ns.

• SRAM chip is usually used in cache memory due to its high
speed.
• SRAM uses multiple transistors (four to six), for each memory
cell. It does not have a capacitor in each cell.
• A SRAM memory cell has more parts so it takes more space on a
chip than DRAM cell.
• It does not need constant refreshing and therefore is faster than
DRAM.
• SRAM is more expensive than DRAM, and it takes up more
space.
• It stores information as long as it is supplied with power.
• SRAM are easier to use and very fast.
• The access speed of SRAM ranges from 2–10 nanosecond.

READ ONLY MEMORY
ROM is a non-volatile primary memory. It does not lose its content
when the power is switched off. The features of ROM are described
as follows:
• ROM has only read capability and no write capability. After the
information is stored in ROM, it is permanent and cannot be
corrected.
• ROM comes programmed by the manufacturer. It stores standard
processing programs that permanently reside in the computer.
ROM stores the data needed for the start up of the computer. The
instructions that are required for initializing the devices attached
to a computer are stored in ROM.
• The ROM memory chip (Figure 3.5) stores the Basic Input
Output System (BIOS). BIOS provides the processor with the
information required to boot the system. It does not load from
disk but instead is stored in a ROM memory chip.

READ ONLY MEMORY
• When the computer is turned on, the BIOS does the following
things:
1. Power On Self Test (POST) It checks that the major hardware
components are working properly.
2. BIOS setup program, lets the user set the many functions that control
how the computer works.
3. Bootstrap Loader is a program whose purpose is to start the computer
software for operation when the power is turned on. It loads the
operating system into RAM and launches it
• ROMs are of different kinds. They vary in the number of re-
writes and the method used for the re-writing.
• PROM (Programmable ROM) can be programmed with a special tool,
but after it has been programmed the contents cannot be changed.

READ ONLY MEMORY
• EPROM (Erasable Programmable ROM) can be programmed in a
similar way as PROM, but it can be erased by exposing it to ultra
violet light and re-programmed
• EEPROM (Electrically Erasable Programmable ROM) memories
can be erased by electric charge and re-programmed.
• Flash Memory is a kind of semiconductor-based non-
volatile, rewritable computer memory that can be electrically
erased and reprogrammed. It is a specific type of EEPROM.
• Examples of flash memories are memory used in digital
camera, mobile phone, printer, laptop computer, and MP3
players.

SECONDARY MEMORY
• In addition to primary memory, an auxiliary or secondary
memory is required by a computer. The secondary memory
is also called the storage device of computer.
• In comparison to the primary memory, the secondary
memory stores much larger amounts of data and information
(for example, an entire software program) for extended
periods of time.
• The data and instructions stored in secondary memory must
be fetched into RAM before processing is done by CPU.
• Magnetic tape drives, magnetic disk drives, optical disk
drives and magneto-optical disk drives are the different types
of storage devices.

ACCESS TYPES OF SECONDARY
MEMORY / STORAGE DEVICES
• The information stored in storage devices can be accessed in two
ways:
1. Sequential access
2. Direct access
• Sequential Access Devices: Sequential access means that
computer must run through the data in sequence, starting from the
beginning, in order to locate a particular piece of data. They are
slow.
• Direct Access Devices: Direct access devices are the ones in
which any piece of data can be retrieved in a non-sequential
manner by locating it using the data’s address.
• Based on access, magnetic tapes are sequential access devices,
and, magnetic disks, optical disk and magneto-optical disks are
direct access devices.

MAGNETIC TAPE
• Magnetic tape is a plastic tape with magnetic coating (Figure
3.7). It is a storage medium on a large open reel or in a
smaller cartridge or cassette (like a music cassette).
• Magnetic tapes are cheaper storage media. They are durable,
can be written, erased, and re-written. Magnetic tapes are
sequential access devices.
• Data is recorded on tape in the form of blocks, where a block
consists of a group of data also called as records. Each block
is read continually.

MAGNETIC DISK
• Magnetic disk is a direct access secondary storage device. It
is a thin plastic or metallic circular plate coated with
magnetic oxide and encased in a protective cover.
• Data is stored on magnetic disks as magnetized spots. The
presence of a magnetic spot represents the bit 1 and its
absence represents the bit 0.
• The features of magnetic disk are cheap, portable, fast,
reliable and can store a large amount of data

FLOPPY DISK
• Floppy disk (FD) is a flat, round, single disk made of Mylar
plastic and enclosed in square plastic jacket (Figure 3.11).
• Floppy Disk Drive (FDD) is the disk drive for floppy disk.
• The floppy disk is inserted into the floppy disk drive to read
or write data to it.

HARD DISK
• A hard disk (HD)
consists of one or
more platters divided
into concentric tracks
and sectors.
• It is mounted on a
central spindle, like a
stack. It can be read
by a read/write head
that pivots across the
rotating disks.
• The data is stored on
the platters covered
with magnetic coating
(Figure 3.12).
Figure 3.12 Parts of hard disk

OPTICAL DISK
• Optical disk is a flat
and circular disk
which is coated with
reflective plastic
material that can be
altered by laser light.
• Optical disk does not
use magnetism. The
bits 1 and 0 are stored
as spots that are
relatively bright and
light, respectively.
Figure 3.15 Sectors and track in magnetic disk and
optical disk

CD-ROM
• Originally, Compact Disk (CD) was a popular medium for storing
music. Now, it is used in computers to store data and is called
Compact Disk-Read Only Memory (CD-ROM).
• CD-ROM is an optical disk that can only be read and not written
on.
• CD-ROM is written on by the manufacturer of the CD-ROM
using the laser light.
• A CD-ROM drive reads data from the compact disk.
• The storage density of CD-ROM is very high and cost is low as
compared to floppy disk and hard disk
• It is a commonly used medium for distributing software and large
data.

DVD-ROM
• Digital Video Disk-Read Only Memory (DVD-ROM) is an
optical storage device used to store digital video or computer
data.
• DVDs look like CDs, in shape and physical size.
• It improves on CD technology.
• DVD-ROM uses both sides of the disk and special data
compression technologies. The tracks for storing data are
extremely small.
• Each side of DVD-ROM can store 4.7 GB of data, so a
single DVD can store 9.4 GB of data.

USING THE COMPUTER MEMORY
The computer starts using the memory from the moment the
computer is switched on, till the time it is switched off. The list of
steps that the computer performs from the time it is switched on
are:
• Turn the computer on.
• The computer loads data from ROM. It makes sure that all the
major components of the computer are functioning properly.
• The computer loads the BIOS from ROM. The BIOS provides the
most basic information about storage devices, boot sequence,
security, plug and play capability and other items.
• The computer loads the OS from the hard drive into the system’s
RAM. CPU has immediate access to the OS as the critical parts
of the OS are maintained in RAM as long as the computer is on.
This enhances the performance and functionality of the overall
system.

USING THE COMPUTER MEMORY
• Now the system is ready for use.
• When you load or open an application it is loaded in the RAM.
Since the CPU looks for information in the RAM, any data and
instructions that are required for processing (read, write or
update) is brought into RAM.
• The CPU requests the data it needs from RAM, processes it and
writes new data back to RAM in a continuous cycle. The
shuffling of data between the CPU and RAM happens millions of
times every second.
• When you save a file and close the application, the file is written
to the secondary memory as specified by you. The application
and any accompanying files usually get deleted from RAM to
make space for new data. If the files are not saved to a storage
device before being closed, they are lost.

3 computer memory

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