Operating System19

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Operating System(19)

• Minyi Guo
• Dept. of Computer Science and Engineering,
• Shanghai Jiaotong University

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Input/Output

• Principles of I/O hardware
• Principles of I/O Software
• I/O software layers
• Disks/Clocks/Char-Oriented Terminals
• GUI/Network Terminals
• Power management

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Optical Disks

• Optical disks are becoming popular
• 1980, Philips developed CD format
• CD are said to last for 100 years

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The Making of CD

• Burn a 0.8 micron diameter holes in a coated
  glass master disk (with laser)
• A mold is made from the master disk
• With bumps where the laser holes are
• A polycarbonate disk is made from mold

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The Making of CD

• A very thin layer of reflective aluminum is
  deposited on the polycarbonate
• Place a protective lacquer on the disk
• The depression in the polycarbonate
• substrate are called pits
• The unburned areas between pits called lands

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The Making of CD

• The pits and lands are written in a single
  continuous spiral starting near the hole
• working out a distance of 32mm toward edge
• The spiral makes 22,188 revolutions
• around the disk
• If unwound, it would be 5.6km long

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Structure of a CD or CD-ROM
Rotation rate of the CD?
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Logical data layout on CD-ROM
Single-speed CD-ROM 75 sectors/sec
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CD-ROM

• 1984, Philips Sony developed the CDROM standard
  for storing computer data
• 1986, graphics and multimedia is added
• High Sierra file standard developed
• It becomes IS 9660

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CD-Recordable

• Nick name CD-R
• Can be used to recording data once

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Making of CD-R

• Starts with polycarbonate blanks
• But they contains a 0.6mm groove
• To guide the laser for writing
• The groove has a sinusoidal excursion of 0.3mm at
  a frequency of exactly 22.05kHz
• To provide continuous feedback so that rotation
  speed can be accurately monitored

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Making of CD-R

• CD-Rs look like regular CD-ROMs
• But they are gold coated instead of
• aluminum for the reflective layer
• Two kinds of dyes are used
• Cyanine, which is green
• Pthalocyanine, which is a yellowish orange

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Making of CD-R

• Initially, the dye are transparent and lets laser
  lights pass through and reflect off the gold
  layers
• To write, laser is turned into high power
• (8-16mW), and hits a spot of dye
• The dye heats up and break chemical bond

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Making of CD-R

• This creates a dark spot
• When read back (at 0.5mW), the
• photodetector sees a difference between the
  dark spots and the transparent areas
• This is interpreted as pits and lands

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CD-R
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CD-Rewritable

• Same as CD-R, except it uses
• Alloy of silver, indium, antimony, and
• tellurium for the recording layer, instead of
  cyanine or pthalocyanine

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CD-Rewritable

• CD-RW drive use laser of 3 differing power
• High power for recording
• Lower power for read back
• Middle power for reforming its state
• To become a land again

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DVD

• Digital Video (Versatile) Disk
• Developed by cooperating industries
• Movie, consumer electronics, computer

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DVD

• DVD uses same general design as CDs
• But with the following differences
• Smaller pits (0.4 micron vs. 0.8 micron)
• Tighter spiral (0.74 micron between tracks vs.
  1.6 microns for CDs)
• Red laser (at 0.65 micron vs. 0.78 microns)

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DVD

• Together, the changes raise the capacity
  sevenfold to 4.7GB
• But the switch to red laser means that
• DVD player will need a second laser or
• fancy conversion to read existing CDs

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DVD

• There are four DVD formats
• Single-sided, single-layer (4.7GB)
• Single-sided, double-layer (8.5GB)
• Double-sided, single-layer (9.4GB)
• Double-sided, dual-layer (17GB)

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DVD
A double sided, dual layer DVD disk
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Clocks

• Also called Timers
• Essential for OS to work properly
• Synchronizing various circuits in computer
• Handle the alarms
• Prevents one process from monopolizing CPU
• Keeps time of day (real-time)

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Clock Hardware

• Two types of clocks are commonly used
• Power-line clocks
• Cause interrupts on every voltage cycle
• At either 50Hz (220V) or 60Hz (110V)
• Used to dominate, now very rare
• Crystal Oscillator Clocks
• Now commonly used in computers

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Crystal Oscillator Clocks

• Consists of a crystal oscillator, a counter,
• and a holding register
• Using electronics, output signal frequency can
  reach 1000MHz or even more
• The signal is fed to counter to count down to
  zero, and causes a CPU interrupt

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Crystal Oscillator Clocks
A programmable clock (clock tick)
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Clock Software

• Clock hardware only generates interrupts at known
  intervals
• Everything else involved is done by software
• Also called the clock driver

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Tasks of Clock Drivers

• Maintaining time of day
• Preventing process from running longer than they
  are allowed to
• by recording clock ticks a process has run
• Call scheduler when quantum exhausts
• Accounting for CPU usage

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Tasks of Clock Drivers

• Handling the ALARM system call by users
• Providing watchdog timers
• OS also need to set timer
• Do something after a timer expires, i.e. floppy
  disk needs to get to speed before read
• Profiling, monitoring, and data gathering
• Providing statistical information about system

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Time of Day

• Just requires to incrementing a counter at each
  clock ticks
• Need to watch the length of the counter
• A 32 bit counter will overflow in 2 years
• with clock rate of 60Hz
• 3 approaches proposed to solve the problem

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Time of Day
Three ways to maintain the time of day
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Handling Alarm

• A process can request OS to give warning when
  certain time interval
• Each such process requires a clock
• Thus multiple clocks may be needed
• The of physical clocks are not enough, so we
  must simulate multiple clocks in software

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Handling Alarm

• If many signals are expected, it is more
• efficient to chain requests
• Sorted on time in a linked list
• Each entry tells how many clock ticks
• following the previous one to wait before
  causing a signal

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Handling Alarm
Simulating multiple timers with a single clock
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Soft Timers

• A 2nd programmable clock available for timer
  interrupts
• specified by applications
• no problems if interrupt frequency is low
• Avoid interrupts by kernel checking for soft
  timer before it exits to user mode

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Soft Timers

• How well does soft timers work depends on rate of
  kernel entries
• If kernel entry is rare, application deadline
  could miss
• Example
• Average entry rate is 2 to 18 micro seconds
• Thus, a soft timer goes off every 12 is doable

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Character Oriented Terminals

• A rather frequent used device in old times
• Can only pass ASCII characters, not pixels
• RS-232 Terminal Hardware

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RS-232 Terminal Hardware
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RS-232 Terminal Hardware

• Communicate with computer 1 bit at a time, Called
  a serial line
• Windows uses COM1 and COM2 ports,
• Unix uses /dev/tty1, /dev/tty2
• Computer and terminal are completely
• independent