Transcript cs320ch1powerpointx

CS 320, Operating Systems
Chapter 1
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The god Janus, beardless, Roman coin; in the Bibliothèque Nationale,
Paris (credit: Larousse)
Found at answers.com
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Found at www.pantheon.org
• Janus is the Roman god of gates and doors (ianua),
beginnings and endings, and hence represented with a
double-faced head, each looking in opposite directions.
• He was worshipped at the beginning of the harvest
time, planting, marriage, birth, and other types of
beginnings, especially the beginnings of important
events in a person's life.
• Janus also represents the transition between primitive
life and civilization, between the countryside and the
city, peace and war, and the growing-up of young
people.
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• …
• Janus was represented with two faces, originally
one face was bearded while the other was not
(probably a symbol of the sun and the moon).
• Later both faces were bearded.
• In his right hand he holds a key.
• The double-faced head appears on many Roman
coins, and around the 2nd century BCE even with
four faces.
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1.1 What Operating Systems Do
• System components
– Users
– Application programs
– O/S
– Hardware
• The O/S provides an environment where other
programs can do useful work
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• Purposes of the O/S from the user point of
view
• Provide for ease of use (PC)
• Provide for resource utilization (mainframe)
• Some combination of the two (networked
workstations, handheld devices, etc.)
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• Purpose of the O/S from the system point of
view
• Resource allocator
• Control program
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• The “Janus” summary:
• The O/S can be regarded as a control
program—a means by which the user can
control the machine.
• The O/S can also be regarded as a service
program—it makes available to the user all of
the capabilities of the machine.
• (A perfect antithesis would be, “a means by
which the machine can control the user…”)
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• Defining an O/S
• There is no single definition
• The book’s preference: The program that’s
running on a system at all times
• Commercial aspect: What vendors choose to
include
• Legal aspect: What the DOJ said MS could
include without violating anti-trust laws (no
browser, for example)
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1.2 Computer-System Organization
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• Inside the machine:
– The CPU and controllers run concurrently
– They share and compete for the bus and main
memory
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• Start-up sequence—where does the O/S come
from and how is it loaded onto the machine?
– Hardware switched on
– Bootstrap program loaded (from ROM)
– Bootstrap program initializes hardware (registers,
etc.)
– Bootstrap program loads O/S from disk
– Bootstrap program starts O/S running
– O/S begins in the idle state
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• Interrupts: How the O/S is put into action—
how different capabilities of the O/S are
triggered:
– Hardware interrupts generated by attached
devices
– Software interrupts generated by application
programs
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• When an interrupt is received the machine
instruction pointer is set to the initial address
of the (O/S) handler code corresponding to
the interrupt
• When the interrupt handling code finishes,
the instruction pointer is reset to the address
of the instruction following the last one
executed before interrupt handling began
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• Some systems use a stack-oriented
architecture
• More commonly, systems use a registeroriented architecture
• For the purposes of understanding interrupt
handling and other processing, consider the
registers on a chip such as the Intel 8088
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• AX, BX, CX, DX: General purpose
• SI, DI: Source and destination for string ops
• SP, BP: Stack pointer (top), base pointer
(bottom of current stack frame)
• IP: Instruction pointer or program counter
• Flag register: Miscellaneous information
• CS, DS, SS, ES: Segment registers for forming
addresses
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Background on Memory
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Memory and Interrupt Handling
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• A more complete sequence of events for
interrupt handling
– O/S receives interrupt
– Saves state for current process, including the
instruction pointer (IP)
– The state for a process is saved in a process
control block (PCB)
– The PCB is a data structure, or object, which
contains fields for every important piece of
information about a running program
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– The PCB added to waiting queue
– The waiting queue is a linked structure (software)
maintained by the O/S
– In other words, the waiting queue is a part of the
O/S internals
– The instruction pointer may be specifically saved
in a fixed memory location or on the stack for ease
of access
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• After saving the state of the previous process,
the O/S:
– Looks up interrupt address in the interrupt vector
table (IVT)
– Loads that address into the instruction pointer
register
– Starts execution (of interrupt handling code)
– When that code is finished, the O/S reloads the
previous IP value
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• The Von Neumann execution cycle—how programs
loaded in memory are executed in hardware
– Fetch = retrieve the instruction at the address in the IP
register
– Decode = put the instruction into the instruction register
and determine the operation and operands contained in it
– Execute, possibly leading to various values in general
purpose and other registers
– (Repeat)—the machine automatically increments the IP
unless there is a jump instruction, an interrupt occurs, the
program comes to an end, etc.
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How the O/S Accesses Main Memory
• Storage structure
– Main memory = RAM
– The only large storage area the processor can
address directly
– Structured as an array of words
– Access is on the basis of a linear sequence of word
addresses
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• Memory management unit
• This is specialized hardware on the CPU
• Fast memory access needs hardware support
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• The MMU works this way
– One of the parameters needed for a memory load
instruction is the address of the desired word
– This address is put into the memory address
register.
– The other parameter needed is the register where
the retrieved memory will be put
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– With these parameter set, execution of a LOAD
instruction retrieves the value at the given
memory address and puts it into the specified
general purpose register
– Execution of a STORE instruction is analogous,
taking the value from a general purpose register
and storing it at the desired address
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• More on memory management
– Note that the function of the MMU doesn’t
depend on “what kind” of memory is being
accessed
– The MMU simply sees addresses and loads and
stores values
– A user program may issue instructions that use
the (data) memory allocated to it
– The Von Neumann cycle also automatically
accesses (code) memory allocated to a program
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• In a perfect world, multiple programs and all
of their code and data would reside in main
memory.
• This is not practical for these reasons:
– Main memory is too small
– Main memory is volatile
• Other kinds of storage are needed in a general
purpose machine.
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A system storage hierarchy
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Registers
Cache
Main memory
Electronic disk (dividing line between
semiconductor and non-semiconductor tech—
may or may not be volatile)
• Magnetic disk
• Magnetic tapes, optical disks, etc.
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• Parameters for comparison: Speed, cost, size,
volatility
• From top to bottom: Fastest to slowest
• From top to bottom: Most expensive to least
expensive
• From top to bottom: Smallest to largest
• From top to bottom: Volatile to persistent
• Notice that the top is “best” only in speed.
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I/O structure
• An example of simple I/O is communication
with one-character at a time peripherals, like
keyboards
• One interrupt per character
• Device driver issues instructions to controller
by loading controller registers
• Data is exchanged through O/S buffers in main
memory and device buffers on the controller
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DMA, a more complex form of I/O
• Direct memory access = DMA
• A more efficient model is needed for peripherals which
transfer large blocks of data, like disks
• One interrupt per character would impose too much
overhead
• In main memory a block is reserved
• Registers on the controller are loaded with the block
addresses
• One interrupt triggers the transfer of a whole block in
secondary storage to the reserved block in memory
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1.3 Computer System Architecture
• Single processor systems
– One general purpose CPU
– Running a general purpose instruction set
– Special purpose processors may exist in device
controllers, etc.
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• Multi-processor systems
– >1 processor (general purpose CPU)
– Gives increased throughput
– Economy of scale through sharing of resources
– Increased reliability
• Fault tolerance
• Graceful degradation
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Multi-processor models
Asymmetric multi-processing—master-slave
Symmetric multi-processing (SMP)—peer-to-peer
Dual and multiple core systems (Intel and
analogous architectures) are examples of multiprocessing
• Blade servers also
• The O/S has to be written to run on/handle
multiple processors
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• Clustered systems (Beowulf, for example)
• Multiple, independent, general purpose
systems connected by LAN
• In addition to the local O/S, this requires a
layer of system software to manage the
collaboration between the individual systems
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1.4 Operating-System Structure
• A large part of the purpose of the next few
overheads is to distinguish between these
terms:
• Multi-processing
• Multi-programming
• Multi-tasking
• On tests, it will be necessary for you to use the
terminology the same way the book does.
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• Multi-processing refers to a hardware
architecture which contains more than one
CPU
• Do not confuse this with multi-programming
or multi-tasking
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• Multi-programming = >1 job loaded in memory at
a time (on a single processor system)
• When one job has to wait (typically for I/O)
another can be scheduled to execute
• This maximizes utilization of system resources
• It describes an efficient scheduling scheme for a
batch system
• Note that this term does not imply interactivity.
• Do not confuse it with multi-tasking
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• Multi-tasking = interactive time-sharing
• This is multi-programming where switching
between jobs is fast enough that user
interactivity can be supported.
• Individual response times <1 second are
reckoned good enough to satisfy most users
• User interactions are usually slow I/O
operations (from keyboard for example)
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• Multi-tasking involves scheduling algorithms
• These algorithms may also support switching
between programs even if they haven’t reached
an I/O wait
• The term “process” is used to describe a program
that has been given a memory footprint (loaded
into memory) that is running or ready to be run
• However, do not confuse multi-tasking with
multi-processing (or multi-programming)
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Running programs, a macro view
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• As noted, the term for a runnable program is a
process
• The business of deciding which program to
run is known as process scheduling
• This is the question of which process in
memory is in possession of the CPU
• You can also refer to this as switching between
processes or jobs
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• The business of moving programs back and
forth between secondary storage and main
memory (loading and storing them) is known
as swapping.
• On tests, it will be necessary for you to use the
terminology switching and swapping correctly.
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• Scheduling is a large topic
• A brief preview of some obvious requirements
for successful switching and swapping on a
single processor system are given below.
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• CPU—1 job at a time on a single CPU
• No conflicts are possible if only one job at a
time is allowed
• Moving between memory and CPU =
switching
• Managing multiple CPU’s is an additional topic
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• Memory— >1 job in memory at a time is
allowed
• However, conflicts between jobs in memory
can’t be allowed—memory allocation is
inviolate
• Moving between secondary storage and
memory = swapping
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• Secondary storage— >1 job in storage at a
time is allowed
• However, no conflicts allowed
• Where things are placed in secondary storage
may vary over time, but at any given time
allocation is inviolate
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1.5 Operating-System Operations
• Terminology: Software interrupt = trap
• Traps can be generated by application system
requests
• They can also be generated when the
application causes an error
• For example, division by zero
• In the error case, the error has to be trapped
and handled so that it doesn’t cause a system
crash or adversely affect other applications
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The first line of hardware-O/S level protection
of a system from non-system software
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Dual mode operation
Modern CPU’s contain a mode bit
1 = user mode
0 = kernel mode = supervisor mode = system
mode = privileged mode
• The bit identifies whether the current process is a
user process or a system process
• Put another way, the mode bit has to be set by
the system to indicate what kind of process is
being scheduled
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• The mode bit is a very basic security and
protection mechanism
• The instruction set for a machine includes
both general purpose and system instructions
• System instructions can only be executed in
system mode
• If a user process attempts to execute a system
instruction a trap is generated
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• Example timeline of interrupts and modes
• System startup, mode = 0
• User request to run application, granted and
scheduled by O/S, mode = 1
• User process makes a system call, granted and
scheduled by O/S, mode = 0
• System returns from call and resumes user
process, mode = 1
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Examples of privileged instructions
The most basic: The instruction to change modes
The most common: I/O, file I/O.
Every time a user process issues an instruction to
access memory or secondary storage, this
generates a system request
• Only the system can actually do this, by means of
privileged instructions (which affect the MMU,
for example)
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• Note that context switching, switching
between processes, is a cooperation between
the O/S and hardware
• The O/S does the scheduling and has an
instruction to set the mode bit appropriately
• When a user process, for example, finishes
executing, the machine default has to be to
return control to the O/S, in system mode
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The second line of hardware-O/S level protection of
a system from non-system software
• The timer
• The mode bit protects the system from rogue
processes in one way—preventing a user
process from executing system instructions
• The timer protects the system in another way
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• Suppose a user application intentionally or
unintentionally contains an infinite loop
• When the O/S grants control to a user
application, it sets a timer in hardware
• If the program doesn’t end on its own, the timer
will generate an interrupt when it expires
• The interrupt returns control to the O/S, which
can kill the offending user job or extend it
• Note that the timer instruction is privileged
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• The mode bit prevents a rogue process from
accessing system resources in general.
• The time specifically prevents a rogue process
from monopolizing the most basic system
resource—CPU cycles.
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1.6 Process Management
• A process is a program that has been loaded
into memory and is ready to be scheduled
• The process concept can be thought of as the
unit of work on a computer system
• Processes need system resources in order to
run: CPU time, memory, files, I/O devices, etc.
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• A complete system typically contains a
collection of processes, some system
processes and some user processes
• Process scheduling on a single processor
system means switching back and forth
between them concurrently.
• This is what is known as multi-programming or
multi-tasking
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• The O/S is responsible for these process
management functions
– Creating and deleting system and user processes
– Suspending and resuming processes
– Inter-process communication
– Process synchronization and deadlock handling
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1.7 Memory Management
• O/S responsibilities with respect to memory
– Allocating and deallocating memory space as needed
– Deciding which (parts of) processes should be moved
into and out of memory from secondary storage
– Keeping track of which memory addresses are used by
what processes
– In the most advanced implementation, virtual
memory, mapping a logical memory space to a
physical space
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1.8 Storage Management
• The O/S manages storage in a way that
extends through three levels
– User application level: Storage is viewed logically
as a set of files in directories. This is transparent
– Memory: Logical pages of data are mapped to the
addresses of physical frames in physical memory
– Secondary storage: Pages/frames of user data are
mapped to the disk, platter, side, track, and sector
or other characteristics of the physical medium
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• O/S responsibilities with respect to storage
– Creating and deleting files
– Creating and deleting directories and organizing
files in them
– Supporting additional commands such as move,
copy, change directory, for manipulating and
navigating files and directories
– Backing up files on stable media
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O/S management of storage
• What the O/S does from the hardware point
of view, vs. the user application point of view
• It allocates disk space
• It manages the free space remaining
• It schedules the disk
• When programs issue file I/O commands, the
O/S handles the requests and coordinates
between multiple requests
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Caching
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• Any storage hierarchy falls on a continuum of
parameter values
• Cache stands between the CPU and main
memory and mediates between them
• It has the ability to increase the performance of
memory access, i.e., speed it up
• This depends on locality of reference and the
ability to load a viable working set
• Performance is improved if the speed of the
cache hits outweighs the cost of the cache misses
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I/O systems
• The O/S hides device specific characteristics
behind an interface
• The Unix I/O subsystem illustrates the idea
• It has drivers for specific hardware devices,
storage, communication, etc.
• It has a general device driver interface—
”Everything is a file”
• It has a memory management component that
includes buffering, caching, and spooling to
support one-character at a time or DMA transfer
of I/O to and from memory
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1.9 Protection and Security
• Already discussed: The mode bit and timer,
interaction between hardware and O/S
• The O/S also supports logins and passwords
• The O/S supports groups and group membership
• The O/S supports the granting and revoking of
privileges
• A question: How much of the security function
(e.g., anti-virus, etc.) should be built into the O/S
and how much should be add-on?
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1.10 Distributed Systems
• Network O/S = loosely coupled autonomous
machines with network software linking them
• Distributed O/S = tightly coupled machines,
not autonomous, presenting a monolithic
interface
• The system software for both of these is
beyond the scope of this course
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1.11 Special-Purpose Systems
• This is just a statement of existence.
• The operating system software for these is
also beyond the scope of this course
– Real-time embedded systems
– Multi-media systems
– Handheld systems—constraints
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Memory size
Processor speed
I/O interface size (screen, keyboard)
Battery life
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1.12 Computing Environments
• This topic is just another way of analyzing the
world into a bulleted list
– Traditional computing (understood to include
aspects of mainframe and PC O/S’s)
– Client-server computing
– Peer-to-peer computing
– Web-based computing
• This course only covers “traditional
computing” in a simple, general sense
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1.13 Open-Source Operating Systems
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Linux, Unix, Solaris (partially open)
The book gives Web addresses
The book has related exercises
This is one of the updated sections in the
latest edition of the book
• You may find it of interest, but it will not be
pursued in this course
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The End
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