2 memory-and-io-devices

Memory
 The memory unit is an essential component in any digital computer
since it is needed for storing programs and data
 Not all accumulated information is needed by the CPU at the same time
 Therefore, it is more economical to use low-cost storage devices to
serve as a backup for storing the information that is not currently used
by CPU
 The memory unit that directly communicate with CPU is called the
main memory or Primary Memory
 Devices that provide backup storage are called auxiliary memory or
Secondary Memory
 The memory hierarchy system consists of all storage devices employed
in a computer system from the slow by high-capacity auxiliary memory
to a relatively faster main memory, to an even smaller and faster cache
memory

An Example Memory Hierarchy
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3
registers
on-chip L1
cache (SRAM)
main memory
(DRAM)
local secondary storage
(local disks)
Larger,
slower,
and
cheaper
(per byte)
storage
devices
remote secondary storage
(tapes, distributed file systems, Web servers)
Local disks hold files
retrieved from disks on
remote network
servers.
Main memory holds disk
blocks retrieved from local
disks.
off-chip L2
cache (SRAM)
L1 cache holds cache lines
retrieved from the L2 cache
memory.
CPU registers hold words
retrieved from L1 cache.
L2 cache holds cache lines
retrieved from main memory.
L0:
L1:
L2:
L3:
L4:
L5:
Smaller,
faster,
and
costlier
(per byte)
storage
devices

Memory
 The main memory occupies a central position by being able to communicate
directly with the CPU and with auxiliary memory devices through an I/O
processor
 A special very-high-speed memory called cache is used to increase the speed of
processing by making current programs and data available to the CPU at a rapid
rate

Memory Hierarchy
CPU logic is usually faster than main memory access time, due to
this reason processing speed of CPU is limited primarily by the
speed of main memory
The cache is used for storing segments of programs currently
being executed in the CPU and temporary data frequently
needed in the present calculations
The typical access time ratio between cache and main memory is
about 1 to 7~10
Auxiliary memory access time is usually 1000 times greater than
that of main memory

Main Memory
Most of the main memory in a general purpose
computer is made up of RAM integrated circuits chips,
but a portion of the memory may be constructed with
ROM chips
RAM– Random Access memory
Static and Dynamic
ROM– Read Only memory

Types of RAM
 Static RAM (SRAM)
Each cell stores bit with a six-transistor circuit.
Bit is stored in the form of voltage
Retains value indefinitely, as long as it is kept powered.
Relatively insensitive to disturbances such as electrical noise.
Faster (8-16 times faster) and more expensive (8-16 times more expensice as well)
than DRAM.
Access time around 10 nanoseconds
Used for cache memory
 Dynamic RAM (DRAM)
Each cell stores bit with a capacitor and transistor.
Bit is stored in the form of charge
Value must be refreshed every 10-100 ms.
Sensitive to disturbances.
Slower and cheaper than SRAM.
Almost all computer has DRAM
Access time 60 – 70 nanoseconds
 Note: a nanosecond is one billionth of a second!

Types of RAM
Synchronous Dynamic Random Access Memory (SDRAM)
• Quicker than DRAM
• Access time less than 60 nanoseconds
Direct Rambus Dynamic Random Access Memory (DRDRAM)
• New type of RAM architecture
• Access time 20 times faster than DRAM
• More expensive
Cache memory
• Small amount of memory typically 256 or 512 kilobytes
• Temporary store for often used instructions
• Level 1 cache is built within the CPU (internal)
• Level 2 cache may be on chip or nearby (external)
• Faster for CPU to access than main memory
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Types of RAM
Video Random Access memory
• Holds data to be displayed on computer screen
• Has two data paths allowing READ and WRITE to occur at the same time
• A system’s amount of VRAM relates to the number of colours and resolution
• A graphics card may have its own VRAM chip on board
Virtual memory
• Uses backing storage e.g. hard disk as a temporary location for programs
and data where insufficient RAM available
• Swaps programs and data between the hard-disk and RAM as the CPU
requires them for processing
• A cheap method of running large or many programs on a computer system
• Cost is speed: the CPU can access RAM in nanoseconds but hard-disk in
milliseconds (Note: a millisecond is a thousandth of a second)
• Virtual memory is much slower than RAM
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ROM
ROM is used for storing programs that are PERMANENTLY
resident in the computer and for tables of constants that do not
change in value once the production of the computer is completed
The ROM portion of main memory is needed for storing an initial
program called bootstrap loader, witch is to start the computer
software operating when power is turned off
ROM stores Bit in the form of Diode.
Access time of between 10 and 50 nanoseconds

Types of ROM
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1. PROM – Programmable ROM – a type of memory whose contents can be
programmed by the user
• OTP – One Time Programmable, a PROM is OTP if contents can be
programmed only once.
• Has polysilicon wires, arranged in matrix, works as diode or fuses.
• Burning the fuses to program the ROM called “Burning the PROM”
2. EPROM –Erasable PROM-Can be programmed, erased and
reprogrammed
• The EPROM chip has a small window on top allowing it to be erased by shining
ultra-violet light on it
• After reprogramming the window is covered to prevent new contents being
erased
• Access time is around 45 – 90 nanoseconds
Fig: EPROM with smallwindow

Types of ROM
3. Electrically Erasable Programmable Read Only Memory (EEPROM)
• Reprogrammed electrically without using ultraviolet light
• Must be removed from the computer and placed in a special machine to do this
• Access times between 45 and 200 nanoseconds
4. Flash ROM
• Similar to EEPROM
• However, can be reprogrammed while still in the computer
• Easier to upgrade programs stored in Flash ROM
• Used to store programs in devices e.g. modems
• Access time is around 45 – 90 nanoseconds
5. ROM cartridges
• Commonly used in games machines
• Prevents software from being easily copied
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Fig: Flash Memory

Secondary memory-Hard Disk
Geometry
 Disks consist of platters, each with two surfaces
 Each surface consists of concentric rings called tracks
 Each track consists of sectors separated by gaps
spindle
surface
tracks
track k
sectors
gaps

Disk Geometry
(Muliple-Platter View)
Aligned tracks form a cylinder
surface 0
surface 1
surface 2
surface 3
surface 4
surface 5
cylinder k
spindle
platter 0
platter 1
platter 2
Each Platter has two read write Head

Disk Operation (Single-Platter View)
The disk surface
spins at a fixed
rotational rate
spindle
By moving radially, arm can
position read/write head over
any track
Read/write head
is attached to end
of the arm and flies over
disk surface on
thin cushion of air
spindle

• A standard computer keyboard has about 100 keys.
• Most keyboards use the QWERTY layout, named for the first six
keys in the top row of letters.
The Keyboard - Standard Keyboard Layout
Most keyboards have keys arranged in five
groups:
1.Alphanumeric keys
2. Numeric keypad
3. Function keys
4.Modifier keys
5.Cursor-movement keys

ACADEMIC RECORDS
Password
ENTER Invalid Password
* * * * *

• The keyboard controller detects the keystroke.
• The controller places a scan code in the keyboard
buffer, indicating which key was pressed.
• The keyboard sends the computer an interrupt
request, telling the CPU to accept the keystroke.
When you press a key:
The Keyboard - How a Keyboard Works

The Keyboard - How a Keyboard Works

The Mouse - What is a Mouse?
• The mouse is a pointing device. You use it to move a graphical
pointer on the screen.
• The mouse can be used to issue commands, draw, and
perform other types of input tasks.
1. Pointing; Move the mouse to move the on-screen pointer.
2. Clicking; Press and release the left mouse button
once.
3. Double-clicking; Press and release the left mouse
button twice.
4. Dragging; Hold down the left mouse button as you
move the pointer.
5. Right-clicking; Press and release the right mouse button.
Mouse Techniques:Using the mouse
involves five techniques:

Variants of the Mouse
• Trackballs
• Trackpads
• Integrated Pointed Devices

Variants of the Mouse - Trackballs
• A trackball is like a mouse turned upside-down.
• Use your thumb to move the exposed ball and
your fingers to press the buttons.
Many styles of
trackball are
available.

Variants of the Mouse - Trackpads
• A trackpad is a touch-sensitive pad that provides
the same functionality as a mouse.
• To use a trackpad, you glide your finger across its
surface.
• Trackpads provide a set of buttons that function
like mouse buttons.

Variants of the Mouse-
Integrated Pointing Devices
• An integrated pointing device is a small joystick built into the
keyboard.
• To use an integrated pointing device, you move the joystick.
• These devices provide a set of buttons that function like mouse
buttons

Devices for the Hand
Pen based input
Tablet PCs, PDA
Pen used to
write data
Pen used as
a pointer
Handwriting recognition

Devices for the hand
3B-30
Touch screens
Sensors determine where
finger points
Sensors create an X,Y
coordinate
Usually presents a menu to
users
Found in cramped or dirty
environments

Devices for the hand
Game controllers
Enhances gaming experience
Provide custom input to the game
Joystick
Game pad

Optical Input Devices
Allows the computer to see input
Bar code readers
Converts bar codes to numbers
Computer find number in a database
Works by reflecting light
Amount of reflected light indicates number

Optical Input Devices
Image scanners
Converts printed media into electronic
Reflects light off of the image
Sensors read the intensity
Filters determine color depths

Optical input devices
Optical character recognition (OCR)
Converts scanned text into editable text
Each letter is scanned
Letters are compared to known letters
Best match is entered into document
Rarely 100% accurate

Audiovisual Input Devices
Microphones
Used to record speech
Speech recognition
“Understands” human speech
Allows dictation or control of computer
Matches spoken sound to known phonemes
Enters best match into document

Audiovisual Input Devices
Digital cameras
Captures images electronically
No film is needed
Image is stored as a JPG file
Memory cards store the images
Used in a variety of professions

• Categories of Monitors
• CRT Monitors
• Flat-Panel Monitors
• Comparing Monitors
• Video Controllers

Monitors - Categories of Monitors
Monitors are categorized by the technology they use:
• Cathode ray tube (CRT) monitors
• Flat-panel displays
• LCD
• LED
• Plasma
And by the way they display colors:
• Monochrome – One coloron a blackbackground
• Grayscale – Shades of gray on a white oroff-white
background
• Color– From16 to 16 million unique colors

Monitors - CRT Monitors
• In CRT monitors, electrons are fired at
phosphor dots on the screen.
• The dots are grouped into pixels, which
glow when struckby electrons.
• In colorCRTs, each pixel contains a red,
green, and blue dot. These glow at varying
intensities to produce colorimages.

Monitors - Flat-Panel Monitors
• Most flat-panel monitors use liquid crystal display
(LCD) technology.
• TFT(Thin-film transistor)displays use multiple
transistors foreach pixel.
• Flat-panel monitors take up less desk space.

Monitors - Comparing Monitors
When comparing monitors, considerfour
features:
• Size
• Resolution
• Refresh rate
• Dot pitch

Comparing Monitors - Size
• A monitor's size is the diagonal
measurement of its face, in inches.
• Early days, 15"monitors (13"viewing area)
were standard.
• Today, 17"monitors (15"viewing area) are
common.
• Largermonitors are available, but can be
expensive.

The diagonal
size (often 15”)

Comparing Monitors - Resolution
• Resolution is the numberof pixels on the
screen, expressed as a matrix (such as
600x800).
• A 17"monitoroffers resolutions from
640x480 up to 1280x1024.
• The Video Graphics Array (VGA) standard is
640x480. SuperVGA (SVGA) monitors
provide resolutions of 800x600, 1024x768
orhigher.

Comparing Monitors - Refresh Rate
• Refresh rate is the numberof times each
second that the electron guns scan the
screen's pixels.
• Refresh rate is measured in Hertz (Hz), or
cycles persecond.
• Lookfora refresh rate of 72 Hz orhigher.
A slower rate may cause eyestrain.

Fast scanning = Quickrefresh (less flicker)

Comparing Monitors - Dot Pitch
• Dot pitch is the distance between the phosphor dots that make
up a single pixel.
• In color monitors, three dots (red, green, and blue) comprise
each pixel.
• Look for a dot pitch no greater than 0.28 millimeter.
Fine dot pitch = Crisp displays

R + BB + G
R + G
R + B + G
PhosphorDot ColorMixing

PC Projectors
• A PC projector connects to a PC and is used to project images
on a large screen.
• Many PC projectors provide the same resolutions and color
levels as high-quality monitors.
Printer
•Dot Matrix
•Inkjet
•Laser

2 memory-and-io-devices

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