2 memory-and-io-devices
TRANSCRIPT
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MEMORY
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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registers
on-chip L1cache (SRAM)
main memory(DRAM)
local secondary storage(local disks)
Larger, slower,
and cheaper (per byte)storagedevices
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 L2cache (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 HierarchyCPU 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 MemoryMost 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 memoryStatic 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 voltageRetains 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 nanosecondsUsed for cache memory
Dynamic RAM (DRAM)Each cell stores bit with a capacitor and transistor.Bit is stored in the form of chargeValue must be refreshed every 10-100 ms.Sensitive to disturbances.Slower and cheaper than SRAM.Almost all computer has DRAMAccess 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 RAM05/02/239
Static RAM Cell
ROMROM 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 small window
Types of ROM3. 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 nanoseconds5. 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
surfacetracks
track k
sectors
gaps
Disk Geometry(Muliple-Platter View)
Aligned tracks form a cylinder
surface 0surface 1surface 2surface 3surface 4surface 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 fixedrotational rate
spindle
By moving radially, arm can position read/write head over any track
Read/write headis attached to endof the arm and flies overdisk surface onthin cushion of air
spindle
Hard Disk Drive
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I/O Devices
• 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 keys2. Numeric keypad3. Function keys4.Modifier keys5.Cursor-movement keys
ACADEMIC RECORDSPassword
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.
Trackpad
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 HandPen based input
Tablet PCs, PDAPen used to
write dataPen used as
a pointerHandwriting recognition
Devices for the hand
3B-30
Touch screensSensors determine
where finger pointsSensors create an
X,Y coordinateUsually presents a
menu to usersFound in cramped
or dirty environments
Devices for the handGame controllers
Enhances gaming experienceProvide custom input to the gameJoystickGame pad
Optical Input DevicesAllows the computer to see inputBar code readers
Converts bar codes to numbersComputer find number in a databaseWorks by reflecting light
Amount of reflected light indicates number
Optical Input DevicesImage scanners
Converts printed media into electronicReflects light off of the imageSensors read the intensityFilters determine color depths
Optical input devicesOptical character recognition (OCR)
Converts scanned text into editable textEach letter is scannedLetters are compared to known lettersBest match is entered into documentRarely 100% accurate
Audiovisual Input DevicesMicrophones
Used to record speechSpeech recognition
“Understands” human speechAllows dictation or control of computerMatches spoken sound to known phonemesEnters best match into document
Audiovisual Input DevicesDigital cameras
Captures images electronicallyNo film is neededImage is stored as a JPG fileMemory cards store the imagesUsed 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 color on a black background
• Grayscale – Shades of gray on a white or off-white background
• Color – From 16 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 struck by electrons.
• In color CRTs, each pixel contains a red, green, and blue dot. These glow at varying intensities to produce color images.
Monitors - Flat-Panel Monitors
• Most flat-panel monitors use liquid crystal display (LCD) technology.
• TFT(Thin-film transistor )displays use multiple transistors for each pixel.
• Flat-panel monitors take up less desk space.
Monitors - Comparing Monitors
When comparing monitors, consider four 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.
• Larger monitors are available, but can be expensive.
The diagonalsize (often 15”)
Comparing Monitors - Resolution
• Resolution is the number of pixels on the screen, expressed as a matrix (such as 600x800).
• A 17" monitor offers resolutions from 640x480 up to 1280x1024.
• The Video Graphics Array (VGA) standard is 640x480. Super VGA (SVGA) monitors provide resolutions of 800x600, 1024x768 or higher.
Comparing Monitors - Refresh Rate
• Refresh rate is the number of times each second that the electron guns scan the screen's pixels.
• Refresh rate is measured in Hertz (Hz), or cycles per second.
• Look for a refresh rate of 72 Hz or higher. A slower rate may cause eyestrain.
Fast scanning = Quick refresh (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 + GR + B + G
Phosphor Dot Color Mixing
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•Ink jet•Laser
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