COS 109 Wednesday October 21
• Housekeeping
– Midterm next week
90 minutes to take it
No class next Wednesday (can do midterm here then)
Will be available from Wednesday 1201 AM to Friday 1159PM on blackboard
Midterm review Next Monday (10/26) at 7PM in room to be announced
–
• Today’s class
– Software we depend on
The operating system (continued)
The file system
The cloud
Fetching as part of Fetch/Execute cycle
• Fetch is Event driven
– The window manager listens for an event
From keyboard
Key down/key up
From mouse
Mouse motion, mouse button up or down
From other device
Game controller
Speech interface
• Identify where the command occurred
– In which window on the screen
• Identify the action that must be taken
– Examples
Type text into Word window
Click command in window of executing program
Click on resize corner of window
Click on icon from window
Click on icon from task bar
Parsing commands
– Type text into Word window
Text is passed to Word program
– Click command in window of executing program
Command is passed to executing program
– Click on resize corner of window
Window manager
Listens to mouse for stretching
changes the geometry of the screen
The application might tell the window manager how
– Click on icon from window
Action to be taken is identified
– Click on icon from task bar
Sleepy program comes back to life
Parsing commands
• Possibilities
– Pass information to an existing command
Type text into Word window
Click command in window of executing program
– Start new command
Click on icon from window
– Wake up existing command
Click on icon from task bar
– Modify the geometry of the display
Click on resize corner of window
Executing existing commands
• Type text into Word window
– Text is passed to Word program
• Word program has previously been put into RAM. It is storing
the text in space it has in RAM, so the new text is added to that
RAM.
– Von Neumann model
– How is RAM managed?
What if you need more
What if you overflow your memory?
What happens when you quit?
Starting new command (cont.)
• Have to identify the command
– How to start Word
Enter from a command line
Select Word from the Start menu
Click on icon for Word
Click on a shortcut to the icon for Word
Click on a file called Something.doc
Click on an unidentified file and tell it to use Word to open
Starting new command
• Have to identify the command
– How to start Word
Enter from a command line
Start>Run>
C:\Program Files\Microsoft Office\Office15\WinWord.exe
– Icons are just an improved User Interface
In UNIX, use keyboard to create the command line
In Windows, use mouse to click on icons.
Starting new command (cont.)
• Click on a file called Something.doc or Something.docx
• Windows registry keeps track of such things
– Basic file associations
.doc or .docx Word
.jpg Default Image viewer
.vbs Visual Basic
.mp3 .wav etc. various music formats
– A newly installed program may change file associations
Starting new command (cont.)
• Once the command is found
•
•
•
•
– C:\Program Files\Microsoft Office\Office15\WinWord.exe
Locate the command on the disk
Move the command to RAM
Add it to the list of commands being executed
Let it run
How do you find it on disk?
How do you know where to load it in RAM?
How do you keep track of all running programs?
The Window Manager
• Maintains the geometry
• Identifies events with windows
• Confusion
– Windows 7,8,10, … is the operating system
– Underneath runs the window manager
• How does the window manager work?
Unresolved questions
•
•
•
•
•
How
How
How
How
How
is RAM managed?
do you find it on disk?
do you know where to load it in RAM?
do you keep track of all running programs?
does the window manager work?
• Answer
– This is what the operating system does
What an operating system does
• manages CPUs, schedules and coordinates running programs
– switches CPU among programs that are actually computing
– suspends programs that are waiting for something (e.g., disk, network)
– keeps individual programs from hogging resources
• manages memory (RAM)
–
–
–
–
loads programs in memory so they can run
swaps them to disk and back if there isn’t enough RAM (virtual memory)
keeps separate programs from interfering with each other
and with the operating system itself (protection)
• manages and coordinates input/output to devices
– disks, display, keyboard, mouse, network, ...
– keeps separate uses of shared devices from interfering with each other
– provides uniform interface to disparate devices
• manages files on disk (file system)
– provides hierarchy of directories and files for storing information
To run a program, the operating system must
• fetch program to be run (usually from disk)
• load it into RAM
– maybe only part, with more loaded as it runs (dynamic libraries)
• transfer control to it
• provide services to it while it runs
– reading and writing info on disk
– communications with other devices
• regain control and recover resources when program is finished
• protect itself from errant program behavior
• share memory & other resources among multiple programs
running "at the same time"
– manage memory, disks, network, ...
– protect programs from each other
– manage allocation of CPUs among multiple activities
Managing memory
• Initially
– Some space reserved for the operating system
– Rest of space for user programs
• When a new program starts
– Gets space for execution
– Gets space for data
• As a program executes
– Can ask for more space
– Can free up space
• When a program stops
– It’s memory is released
Memory management
Initially
Op. Sys
Memory management
Initially
Start Word
Op. Sys
Op. Sys
Word.exe
Word
data
Memory management
Initially
Op. Sys
Op. Sys
Start Word
Word.exe
Word needs
More memory
Word
data
Word
data2
Memory management
Initially
Op. Sys
Op. Sys
Start Word
Word.exe
Word needs
More memory
Word
data
Word
data2
Start Browser
Broswer
Memory management
Initially
Op. Sys
Op. Sys
Start Word
Word.exe
Word needs
more memory
Word
data
Word
data2
Start Browser
Word needs
more memory
Broswer
Word
data3
memory manager
makes sure that
boundaries
between
programs are
maintained
Memory management
Op. Sys
Op. Sys
Start Word
Word.exe
Word.exe
Word needs
more memory
Word
data
Word
data
Word
data2
Word
data2
Initially
Start Browser
Word needs
more memory
Op. Sys
Browser
Free Space
Word
Word
data3
data3
Memory management
Initially
Start Word
Op. Sys
Word needs
more memory
Start Browser
Op. Sys
Op. Sys
Word.exe
Word.exe
Word
data
Free space
Word
data2
Word
data2
Word needs
more memory
Free Space
Free Space
Exit Browser
Word
Word
Word frees space
data3
data3
What if you run out of memory?
• Use virtual memory
– Have some memory in RAM, some in disk
– Hide the details from the user
– Make choices effectively
paging
Virtual Memory
Browser data 2
Op. Sys
Word.exe
Word
Word
data
data2
Browser
Browser data
Word
Memory mapper
data3
Requests for memory
go through the memory
mapper which finds the
needed pages
Virtual memory on disk
Sidebar on speed
• Difference between register, cache, RAM, disk
– Registers are right there – can access right away
– Cache is with the CPU – quickly accessible
– RAM requires
a request onto the bus
Address decoding
Returning data on the bus
– Disk requires
Request onto the bus
Determining where data is on disk
Spinning disk platter to get there
Gathering data, putting it on the bus.
Operating system controls devices
• operating system hides physical properties of devices
– device has specific capabilities, parameters, etc.
– hardware and software in device and OS present these at higher level
– e.g., printer
logical view: put characters out in 66 lines of 80 characters
physical view: paint individual bits of characters in raster across page
– e.g., CD-ROM
logical view: file system just like the one on the hard drive
physical view: long spiral of individual bits read by a laser
• OS uses device drivers to control physical devices
– driver code has detailed knowledge of how to operate a particular device
– implemented as functions that provide interface between specific
capabilities of a device and what the operating system expects
– loaded as part of OS as needed, e.g., when a device is plugged in
("Windows has found new hardware")
• drivers insulate OS and application programs from specific
properties of devices
Virtual machines
• running other OS's on top of an OS
– e.g., VMWare, VirtualBox, Parallels, Xen, HyperV, ...
• system calls from applications to "guest" OS are intercepted
by "host" OS
– e.g., guest == Windows 7 or Linux, host == MacOSX
• passed to guest OS, which handles them by converting into
system calls to host OS
• not the same as "dual boot"
Win app
Win app
Windows 7
Mac app(s)
Virtualbox (Mac app)
Mac OSX
A quantitative moment
"The number of American children killed by guns
has doubled every year since 1950."
Nancy Day, Violence in Schools: Learning in Fear
(1996)
Much too high, much too low, or plausible?
Website of the day
• http://projectalexandria.net/
File systems and databases: managing information
• file: sequence of bytes stored on a computer
– content is arbitrary; any structure is imposed by the creator of the file,
not by the operating system
• file system: software that provides hierarchical storage and
organization of files, usually on a single computer
– part of the operating system
• database: integrated collection of logically related records
– data is organized and structured for efficient systematic access
– may be distributed across lots of machines & geographically dispersed
• database system: software that provides efficient access to
information in a database
– not usually part of the operating system
File systems: managing stored information
• logical structure: users and programs see a
hierarchy of folders (or directories) and files
– a folder contains references to folder and files
– "root" folder ultimately leads to all others
– a file is just a sequence of bytes
contents determined and interpreted by programs, not the operating system
– a folder is a special file that contains names of other folders & files
plus other information like size, time of change, etc.
contents are completely controlled by the operating system
• physical structure: disk drives operate in tracks, sectors, etc.
– other storage devices have other physical properties
• the operating system converts between these two views
– does whatever is necessary to maintain the file/folder illusion
– hides physical details so that programs don't depend on them
– presents a uniform interface to disparate physical media
• the "file system" is the part of the operating system that does
this conversion
[ Physical devices for file systems ]
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hard disk (inside or external, e.g., via USB)
CD-ROM, DVD
flash drives (USB, SD card, ...)
network drive
RAM
[ Disks ]
•
•
•
a place to store information when the power is turned off
usually based on rotating magnetic surfaces or solid-state devices
logical / functional structure: folders (directories) and files
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–
–
–
your information: papers, mail, music, web page, …
programs and their data: Firefox, Word, iTunes, …
operating system(s): Windows, MacOS, Unix/Linux, ...
bookkeeping info: where things are physically
How the file system converts logical to physical
• disk is physically organized into sectors, or blocks of bytes
– each sector is a fixed number of bytes, like 512 or 1024 or …)
– reading and writing always happens in sector-sized blocks
• each file occupies an integral number of blocks
– files never share a block
– some space is wasted: a 1-byte file wastes all but 1 byte of the block
• if a file is bigger than one block, it occupies several blocks
– the blocks are not necessarily adjacent on the disk
• need a way to keep track of the blocks that make up the file
• this is usually done by a separate "file allocation table" that lists
the blocks that make up each file
– this table is stored on disk too so it persists when machine is turned off
– lots of ways to implement this
Converting logical to physical, continued
• every block is part of some file, or reserved by operating system,
or unused
• "file allocation table" keeps track of blocks
– by chaining/linking them together
first block of a file points to second, second points to third, etc.
last block doesn't point to a successor (because it doesn't have one)
– or (much more common) by some kind of table or array
that keeps track of related blocks
• also keeps track of unused blocks
– disk starts out with most blocks unused ("free")
some are reserved for file allocation table, etc.
– as a file grows, blocks are removed from the unused list and attached to
the list for the file:
to grow a file, remove a block from the list of unused blocks
and add it to the blocks for the file
Converting logical to physical: directories
• a directory / folder is a file
– stored in the same file system
– uses the same mechanisms
• but it contains information about other files and directories
• the directory entry for a file tells where to find the blocks
• the directory entry also contains other info about the file
– name (e.g., midterm.doc)
– size in bytes, date/time of changes, access permissions
– whether it's an ordinary file or a directory
• the file system maintains the info in a directory
– very important to keep directory info consistent
– application programs can change it only indirectly / implicitly