7-1

Communicating with the Computer

• How do programs get in to memory in the first place?

• Input/Output (I/O): – Refers to the process of getting information into and out of the

computer.

computercpu

memory

input device(s)

output device(s)

peripheral devices

7-2

I / O

• Peripheral device: a piece of hardware that is outside the main computer.

– “Clearly” peripheral devices are necessary to use our computer in any meaningful way . . .

• Input units: – Computer hardware devices that receive input and bring that

input into the computer– What are some input devices?

• Output units: – Computer hardware that caries information out of the

computer (i.e., produce output)– What are some output devices?

7-3

Input Devices

• Input devices:– Keyboard

– Mouse

– Scanner

– Digital camera

– Video camera/capture device

– Temperature sensor

– Pressure sensor

– How does a keyboard really work?• You press a key (which is really a switch)

• The ASCII code of the key you’ve pressed is sent into the computer

• (Voltage is set on the wires according to the key you’ve pressed)

7-4

Input Devices

• How does a mouse work (physically) ?• (the buttons are just switches)

• A little ball sits inside the mouse

• The ball rolls against three rollers, two of the rollers act as “rotary switches” so they know when they’ve been moved

• When the mouse moves, the ball rolls and the rotary switches are moved as well

– One rotor is aligned to measure left and right movement

– The other is aligned to measure up and down movement

• The changes in X and Y coordinate updates are sent to the computer as binary numbers

– A program needs to be running on the computer that knows what to do with mouse updates

7-5

Output Devices

– Output devices:• Monitor / Video card

• Printer

• Sound card

• Network card

• Robotic arm

7-6

Display Technologies

• Hi-resolution displays come in two types:– Each takes an input signal and creates a visible image

• Cathode ray tube (CRT) - Streams of electrons make phosphors glow on a large vacuum tube.

– Monitor

• Liquid crystal display (LCD) - A flat panel display that uses crystals to let varying amounts of different colored light to pass through it.

– Developed primarily for portable computers.

– Panel

• How does a monitor really work –i.e., how does the image get there?

7-7

Video Display

– How does a monitor really work –how does the image get there?• The video card or graphics card sends video signals to the monitor

• The video card has it’s own memory and CPU

– The Graphics Processing Unit (GPU) receives instructions (from the CPU) to draw an image (bitmap) to the display

– The image data (bitmap) will be copied from primary memory to video memory (VRAM)

– The video signal is sent periodically ensuring that the monitor is displaying the video image described in the VRAM contents

• this is complicated, but the video card takes care of everything for us

• Let’s consider a much simpler output device --

– Daisy wheel printers

» “letter quality” printer

7-8

“Other” Devices• There are other devices that we have yet to mention that don’t fit

nicely into either “input devices” or “output devices”– Floppy drives,

– Hard disks

– CD-ROM drives

– DVD-ROM drives

– CD-RW/DVD-RW drives

• They are input devices to our computer, but can also be output devices. – There’s something else weird about them, consider the type of the input they process

– We consider “traditional” input devices those that deal with original data.

– And define a classification for devices that handle previously stored data.

7-9

Data Sources

• Computer data can be:– Original data: Data being introduced to a computing system for the

first time.

• The input device directly samples physical things in the world (printed text, pictures, sound, and other common types of information) and converts them to binary for the computer

• Think: non-binary inputs that need to be converted

• Examples: ?

– Digitizing: The process of taking an image, audio recording or any other analog data and converting it to a binary format for the computer.

• Allows our computer (and programs) to represent and manipulate “analog data”

7-10

Data Sources

• Computer data can be :– Previously stored data or information: Data that has already been

processed by a computer and is being stored for later use.

• The input devices directly reads binary data (the data has already been converted to binary).

• The binary form of the data is useful only to a computer

– Using a CDR as a drink coaster doesn’t count as using binary data

• Think: Already converted to binary

• Examples:

– music CDs?

– It would be useful to have “previously stored data”-devices that can both read and write binary data

7-11

Storage Devices

• Storage Devices – “Secondary Memory”– used by a computer to

• Write binary information (store)• Read binary information as needed (retrieve)

– compared to primary memory, it’s slower but less expensive• slower means it takes more time to get a bit from the device

– “Secondary” with respect to the Von Neumann model • Technically a kind of memory

– Can be used to manipulate bits• But calling it memory is really incorrect

– By Von Neumann’s definition, it is secondary – not required– Take bits from memory and output them to a device

» Move papers from desk to cabinet– Take bits from a device and put them in memory

» Or move from filing cabinet to desk

7-12

Storage Devices

• Literally, storage devices are input and output devices– Remember, the CPU can only take instructions from primary memory

• If we have a program to run on a storage device compared to directly in memory

– These devices are slower to access, so it may take longer to “find” or “get” our bits

– Then, we’ll need to copy the instructions into memory before we can run them

• If we want to put results on a storage device we’ll need to copy them from memory

– So, if primary memory were cheap enough, would we still need storage devices?

• Yes! Remember, memory contents are lost when power is removed from the computer. This is not the case with storage devices.

• How is a storage device able to “keep” it’s bits where as RAM can not?

7-13

How do we really store data?

• Storage Technologies for Binary Information:– Electronic

– Magnetic

– Optical

7-14

Device Storage

• Electronic Circuits– Most expensive of the three forms for storing binary information.

– A flip-flop circuit has either one electronic status or the other. It is said to flip-flop from one to the other.

– Electronic circuits come in two forms:

• Permanent

• Non-permanent

7-15

Device Storage

• Magnetic Technology– Very inexpensive form of storage

– A special surface can hold magnetic information (+/-)

• Tiny spots on the surface are magnetized to represent 1s and 0s

• It has a magnetic sensing “head” moves over the surface

• Can perform non-destructive reading

• Can destructively write to a surface (over-writing existing 1s and 0s)

– Examples:

» Floppy Drives

» Hard Drives

» Zip Drive

» Tape Drives

7-16

How the disk drive really works …

0111001011100( not to scale :-P )

• The disk is covered in magnetic 1s and 0s

– The disk spins

– The head moves back and forth, reading or writing 1s and 0s from/to the different areas on the disk.

• Like a record player, the head only needs to go in a straight line across the disk, and the disc will spin under it

• Seek time: time it takes to find the desired bit on the physical device.

– complicated, but the hard disk takes care of all of this for us

• Floppy drives, Hard drives . . .– How would a tape drive work?

7-17

Device Storage

• Optical– “Slower” than magnetic media, price is on par– Uses lasers to “read” the binary information from a surface.– Millions of tiny holes are “burned” into the surface of the disc.

• The surface of re-writable optical media is a chemical that changes and reforms when hit with the “low-power” laser head

• The laser “burns” the chemical hence “burning” a CD• The holes are interpreted as 1s. The absence of holes are

interpreted as 0s.

» So, this looks just like before, but instead of a magnetic head, we have a laser head to read the gapping as 0s & 1s

» This is the mechanism for CDs, DVDs, CDRs, Laserdiscs, etc

7-18

Optical Storage Devices

• Optical Disks: CD-ROM, DVD, etc– CD-ROM (Compact Disc - Read Only Memory)

• By its definition, CD-ROM is Read Only.– types of CDs:

» CD-R (Compact Disc - Recordable)» CD-RW (Compact Disc - Rewritable)

– DVD, DVD-R, DVD+R, DVD-RW, DVD+RW, DVD-R9/DL• DVD really didn't stand for anything when it was introduced, yet

there are two often used names– Digital Video Disc and Digital Versatile Disc– Neither is “official”

• Writing to an Optical device must be done “in one sitting” as opposed to magnetic media which is easy to change “on the fly”

7-19

Characterizing Capacity

• Capacity - The amount of information that can be stored on the medium.

Unit Description Approximate Size

1 bit 1 binary digit1 nibble 4 bits1 byte 8 bits 1 character (ASCII)1 kilobyte 1,024 bytes 1/2 page, double spaced1 megabyte 1,048,576 bytes 500,000 pages

1 million bytes1 gigabyte 1,073,741,824 bytes 5 million pages

1 billion bytes1 terabyte 1 trillion bytes 5 billion pages

7-20

Capacity continued

• How much storage capacity is needed for…– One keystroke on a keyboard. 1 Byte (8 bits)– One page single-spaced document. 4.0 KB– One second of uncompressed (WAV) sound.176 KB– One hour of uncompressed WAV sound 619MB– One hour of MPEG2 video ~2.3GB

• How much data can be stored on…– One inch of 1/2 in. wide magnetic tape. 4 K– One 3 1/2” floppy disk, high density. 1.4 MB– One Compact Disc. 700 MB– One DVD-R. ~4.3 GB– One Dual-Layer DVD. ~9 GB

7-21

Characterizing Devices

• Type of Access• Sequential - Obtained by proceeding through the storage medium

from the beginning until the designated area is reached (as in magnetic tape).

• Random Access – “Direct” access (as in floppy and hard disks).

• Type of Access directly influences speed– Seek times are longer for sequential access devices – Sequential devices are slower than random access devices

7-22

Characterizing Devices

• Speed (Access time) - How fast information (bits) can be taken from or stored to the device.

– Electronic circuits: Fastest to access.

• 40 billionths of a second.

– Floppy disks: Very slow in comparison.

• Takes up to 1/2 second to reach full speed before access is even possible.

• Why? Have to spin the disk and get the head into the right place. Then the reading of magnetic charge is converted into a 1 or 0

• Why do we tolerate slow devices?– Cost

– Portability?