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Operating Systems:
Components and Duties
NOTE: this lecture and the next two lectures are intended to
provide an overview of OS concepts important to the study of
embedded systems. For more comprehensive information,
please visit the website listed at the bottom of the slides and the
material accessible there.
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
What is an Operating System?
• A program that acts as an intermediary between a
user of a computer and the computer hardware.
• Operating system goals:
– Execute user programs and make solving user problems
easier.
– Make the computer system convenient to use.
• Use the computer hardware in an efficient manner.
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
System Components
Hides the complexity of machine
language from programmer
Instruction Set Architecture
Physical devices grouped
together to form functional units
Integrated circuit chips,
power supply, CRT
The operating system runs in kernel or supervisor mode - protected
from user tampering
Compilers, editors and application programs run in user mode
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Functions of an OS
• User Environment - OS layer transforms bare
hardware machine into higher level abstractions
– Execution environment - process management, file
manipulation, interrupt handling, I/O operations,
language.
– Error detection and handling
– Protection and security
– Fault tolerance and failure recovery
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Functions of an OS (continued)
• Resource Management
– Time management
• CPU and disk transfer scheduling
– Space management
• main and secondary storage allocation
– Synchronization and deadlock handling
• IPC, critical section, coordination
– Accounting and status information
• resource usage tracking
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Typical memory layout
Several jobs are kept in main memory at the same time, and the CPU
is multiplexed among them.
OS tasks:
• Memory management – the
system must allocate the
memory to several jobs.
• CPU scheduling – the system
must choose among several
jobs ready to run.
• Allocation of devices.
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Real-Time Systems
• Designed for a dedicated application such as scientific experiments or industrial
processes, medical imaging and devices, consumer products, avionics, and
automobiles
• Responsibilities have well-defined, fixed time constraints
• Hard real-time:
– MISSED DEADLINE IS A SYSTEM FAILURE
– Secondary storage limited or absent, data stored in short term memory, or
read-only memory (ROM)
– Not supported by general-purpose operating systems.
• Firm real-time
– OCCASIONAL MISSED DEADLINES ARE TOLERABLE but quality of service may
degrade
– a result is useless if it is provided after the deadline
• Soft real-time
– The usefulness of a result and the quality of service degrade if the result is
provided after the deadline
– System may attempt to maximize the number of deadlines met or the number
of high-priority tasks meeting their schedules or to minimize the lateness of
tasks, for example
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Computer-System Operation
• Starting up a computer
– Initial program, or bootstrap program, is run
– Stored in ROM or EPROM within the computer hardware
– Initializes all aspects of the computer (registers, controllers,
memory etc.)
– Loads the operating system and executes it
• Locates and loads the OS kernel
– Once loaded, the OS waits for an event to occur
• Events usually signaled by an interrupt from either the hardware
or software
– Hardware sends a signal to the CPU via the system bus
– Software triggers an interrupt by executing a system call
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Device Controller
• A computer system typically consists of a CPU and
multiple device controllers connected through a common
bus
• Each device controller is in charge of a specific device or
multiple devices (e.g. SCSI controller)
• Device controller maintains local buffer storage and a set
of special purpose registers
• The device controller moves data between the device it
controls and its local buffer storage
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Interrupts
• Interrupts enable software to respond to signals
from hardware
– May be initiated by a running process
• Interrupt is called a trap – software generated caused by
error or user request for an OS service
• Dividing by zero or referencing protected memory
– May be initiated by some event that may or may not be related
to the running process
• Key is pressed on a keyboard or a mouse is moved
– Low overhead
• Polling is an alternative approach
– Processor repeatedly requests the status of each device
– Increases in overhead as the complexity of the system increases
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Handling Interrupts
• After receiving an interrupt, the processor completes execution of the
current instruction, then pauses the current process
• The processor will then transfer to a fixed location and executes the
service routine for the interrupt
• The interrupt handler determines how the system should respond
• Interrupt handlers are stored in an array of pointers called the interrupt
vector
– To handle the interrupt quickly, a table of pointers is
generally stored in low memory which hold the
addresses of the ISR for the various devices
– This array, or interrupt vector, of addresses is then
indexed by a unique device number to provide the
address of the ISR for the interrupting device
• After the interrupt handler completes, the interrupted process is
restored and execution continues from the address of the interrupted
instruction (stored on stack) or the next process is executed
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Interrupt Classes
EXAMPLE: Intel IA-32 exception classes
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Interrupt Classes
EXAMPLE: Intel IA-32 exception classes (continued)
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
I/O Interrupts
• To start an I/O operation, the CPU loads the appropriate
registers within the device controller
• The device controller examines the values and determines
what action to take (e.g. read, write)
• When the transfer is complete, the device controller
informs the CPU via an interrupt
• The device driver returns control to the OS
– Returns the data or pointer to the data if a read was done
– For other operations it returns status information
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
• Synchronous:
I/O Interrupts
– After I/O starts, control returns to user program only upon I/O completion.
– Wait instruction or tight loop (Loop: jmp Loop) idles the
CPU until the next interrupt
– At most one I/O request is outstanding at a time, no
simultaneous I/O processing.
• Asynchronous:
– After I/O starts, control returns to user program without
waiting for I/O completion.
– Increased system efficiency by increasing CPU utilization
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Device-Status Table
Device-status table contains entry for each I/O device indicating its type,
address, and state.
Operating system indexes into I/O device table to determine device status
and to modify table entry to include interrupt.
If the device is busy with a request, the type of request and other parameters
are stored in the table entry for that device
A queue will contain a list of all those requests waiting for a device
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Hardware Protection
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•
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•
Dual-Mode Operation
I/O Protection
Memory Protection
CPU Protection
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
•
•
Dual-Mode Operation
Sharing system resources requires operating system to
ensure that an incorrect program cannot cause other
programs to execute incorrectly as well as to protect the
OS from errant user programs
Provide hardware support to differentiate between at
least two modes of operations.
– User mode – execution done on behalf of a user.
– Monitor mode (also kernel mode or system mode) –
execution done on behalf of operating system.
– MS-DOS for the Intel 8088 had no mode bit
•
•
User program can wipe out the OS
Multiple programs can write to a device simultaneously
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Dual-Mode Operation (Cont.)
•
•
Mode bit added to computer hardware to indicate the current mode: monitor (0) or
user (1).
When an interrupt or trap (program error) occurs hardware switches to monitor mode.
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
I/O Protection
• All I/O instructions are privileged instructions.
• Must ensure that a user program could never gain
control of the computer in monitor mode (i.e., a
user program that, as part of its execution, stores a
new address in the interrupt vector).
• System call - A privileged instruction provides a
means for the user to interact with the OS to
perform tasks that only the OS can do
– Treated by the hardware as a software interrupt
– Switch to monitor mode jumping to the address
determined by the interrupt vector
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Memory Protection
• Must provide memory protection at least for the
interrupt vector and the interrupt service routines.
• In order to have memory protection, add two
registers that determine the range of legal addresses
a program may access:
– Base register – holds the smallest legal physical memory
address.
– Limit register – contains the size of the range
• Memory outside the defined range is protected.
• Base and limit registers can be loaded only by the OS
which uses a special privileged instruction (which
can be executed only in monitor mode)
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Use of A Base and Limit Register
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Hardware Protection
• CPU hardware compares every address generated in
user mode with the registers
• Any violation results in a trap to the monitor which
treats it as a fatal error
• When executing in monitor mode, the operating system
has unrestricted access to both monitor and user’s
memory.
• The load instructions for the base and limit registers
are privileged instructions.
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
CPU Protection
• Timer – interrupts computer after specified period
to ensure operating system maintains control.
– Timer is decremented every clock tick.
– When timer reaches the value 0, an interrupt occurs.
• Timer commonly used to implement time sharing
– interrupt every N milliseconds (a time slice) at which
time a context switch occurs
• Performs housekeeping on the program just stopped, saves
registers, internal variables, buffers etc. and prepares for the
next program to run
• Load-timer is a privileged instruction.
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Protection and Security
• Protection controls the access of processes or
users to the resources of the computer system
– Distinguish between authorized and
unauthorized use
• Security defends a system from external and
internal attacks
– Viruses, worms, denial of service attacks,
identify theft
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Operating-System Structures
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•
•
•
•
System Components
Operating System Services
System Calls
System Programs
System Structure
Virtual Machines
System Design and Implementation
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Common System Components
A system as large and complex as an OS can be
created only by partitioning it into smaller pieces*
•Process Management
•Main Memory Management
•File Management
•I/O System Management
•Secondary Management
•Networking
•Protection System
•Command-Interpreter System
*italicized topics will not be discussed here
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Process Management
• A process is a program in execution. A process needs
certain resources, including CPU time, memory, files,
and I/O devices, to accomplish its task.
– A process is the unit of work in a system
• The operating system is responsible for the following
activities in connection with process management.
– Process creation and deletion.
– Process suspension and resumption.
– Provision of mechanisms for:
• process synchronization
• process communication
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
I/O System Management
• Hide the peculiarities of specific hardware
devices from the user
• The I/O system consists of:
– A memory management component that includes
buffering, caching and spooling
– A general device-driver interface
– Drivers for specific hardware devices
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Protection System
• Protection refers to a mechanism for
controlling access by programs, processes, or
users to both system and user resources.
• The protection mechanism must:
– distinguish between authorized and unauthorized
usage.
– specify the controls to be imposed.
– provide a means of enforcement.
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Command-Interpreter System
• Serves as the interface between the user and the OS
– User friendly, mouse based windows environment in the Macintosh and in
Microsoft Windows
– In MS-DOS and UNIX, commands are typed on a keyboard and displayed on
a screen or printing terminal with the Enter or Return key indicating that a
command is complete and ready to be executed
• Many commands are given to the operating system by control
statements which deal with:
–
–
–
–
–
–
–
process creation and management
I/O handling
secondary-storage management
main-memory management
file-system access
protection
networking
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Command-Interpreter System (Cont.)
• The program that reads and interprets control
statements is called variously:
– command-line interpreter
– shell (in UNIX)
Its function is to get and execute the next command
statement.
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
System Calls
• System calls provide the interface between a running program and the
operating system.
– For example – open input file, create output file, print message to console,
terminate with error or normally
– Generally available as routines written in C and C++
– Certain low-level tasks (direct hardware access) may be written in assemblylanguage
• Mostly accessed by programs via a high-level Application Program Interface
(API) rather than direct system call use
– Provides portability (underlying hardware handled by OS)
– Hides the detail from the programmer
• Three most common APIs are Win32 API for Windows, POSIX API for POSIXbased systems (including virtually all versions of UNIX, Linux, and Mac OS X),
and Java API for the Java virtual machine (JVM)
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Example of Standard API
• Consider the ReadFile() function in the
• Win32 API—a function for reading from a file
• A description of the parameters passed to ReadFile()
–
–
–
–
–
HANDLE file—the file to be read
LPVOID buffer—a buffer where the data will be read into and written from
DWORD bytesToRead—the number of bytes to be read into the buffer
LPDWORD bytesRead—the number of bytes read during the last read
LPOVERLAPPED ovl—indicates if overlapped I/O is being used
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
System Call Implementation
• Typically, a number associated with each system call
– System-call interface maintains a table indexed according to
these numbers
• The system call interface invokes intended system call in
OS kernel and returns status of the system call and any
return values
• The caller need know nothing about how the system call is
implemented
– Just needs to obey API and understand what OS will do as a
result call
– Most details of OS interface hidden from programmer by API
• Managed by run-time support library (set of functions built into
libraries included with compiler)
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
API – System Call – OS Relationship
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Standard C Library Example
• C program invoking printf() library call, which
calls write() system call
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
System Call Parameter Passing
• Often, more information is required than simply identity
of desired system call
– Exact type and amount of information vary according to OS
and call
• Three general methods used to pass parameters to the OS
– Simplest: pass the parameters in registers
• In some cases, may be more parameters than registers
– Parameters stored in a block, or table, in memory, and address
of block passed as a parameter in a register
• This approach taken by Linux and Solaris
– Parameters placed, or pushed, onto the stack by the program
and popped off the stack by the operating system
– Block and stack methods do not limit the number or length of
parameters being passed
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Passing Parameters as a Table
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Types of System Calls
• Process control
– end, abort, load, execute, allocate/free memory
• File management
– Create/delete file, open, close, read, write
• Device management
– Request/release device, read, write
• Information maintenance
– Get/set date or time, get process, get/set system data
• Communications
– Send/receive message, create/delete comm link
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Example: UNIX System Calls
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
UNIX Running Multiple Programs
FreeBSD is a multitasking system


Shell accepts a command to run a program but, unlike MSDOS, continues to run while the other program is executing
To start a new process, the shell executes a fork system call
which enables another program to be loaded into memory
and executed



A process can run in the “background” but then it can not
accept input from the keyboard (as it is dedicated to the shell)
but can read and writes files
The shell again waits for further commands – run another
program, monitor the progress of the running processes,
change priorities, etc.
When a process terminates it issues an exit system call
returning a status code
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
UNIX Running Multiple Programs
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
System Programs
• System programs provide a convenient environment for program
development and execution. The can be divided into:
–
–
–
–
–
–
File manipulation
Status information (date,time, # of users, free memory etc.)
File modification (text editors)
Programming language support (compilers, assemblers, etc)
Program loading and execution (loaders, linkage editors)
Communications (create virtual connections among processes, users and
different computer systems)
– System utilities or application programs (web browsers, word processing,
spreadsheets etc)
• Most users’ view of the operating system is defined by system programs,
not the actual system calls.
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
OS Design Implementation
• Mechanisms determine how to do something,
policies decide what will be done.
• The separation of policy from mechanism is a very
important principle in designing an OS
– It allows maximum flexibility if policy decisions are to be
changed later.
• The mechanism that implements the policy to give priority to
I/O intensive processes over CPU intensive processes should be
written in a general way so that if the policy is changed no or
minimal change to the mechanism would be required
• The timer provides is a mechanism providing CPU protection.
How long it runs for a particular user is a policy decision.
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
System Implementation
• Traditionally written in assembly language, operating systems can now be
written in higher-level languages.
• Code written in a high-level language:
– can be written faster.
– is more compact.
– is easier to understand and debug.
• Opponents to OS written high-level language claim slower performance and
increased storage
• Proponents argue:
– Modern compilers can produce highly optimized code
– Todays OSs run on highly complex hardware which can overwhelm the
programmer with details
– Better data structures and algorithms will truly improve the performance
of OSs
– Only a small amount of code is critical to high performance – bottlenecks
can be replaced with assembler code later on
• An operating system is far easier to port (move to some other hardware) if it
is written in a high-level language.
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Operating System Structure: MS-DOS
• MS-DOS – written to provide the most
functionality in the least space
– Not well divided into modules
– Designed without realizing it was
going to become so popular
– Although MS-DOS has some
structure, its interfaces and levels
of functionality are not well
separated
– Vulnerable to system crashes when
a user program fails
– Written for Intel 8088 which had
no dual mode or hardware
protection
MS-DOS layer structure
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Operating System Structure: UNIX
• UNIX – the original UNIX operating system had
limited structuring due to limited hardware
functionality.
• The UNIX OS consists of two separable parts.
– Systems programs
– The kernel – series of interfaces and device drivers
• Consists of everything below the system-call interface and
above the physical hardware
• Provides the file system, CPU scheduling, memory
management, and other operating-system functions; a large
number of functions for one level.
• All this functionality in one level makes UNIX difficult to
enhance – difficult to determine impact of change in one part
of the kernel on other parts
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
UNIX System Structure
kernel
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Virtual Machines
• A virtual machine treats hardware and the operating system kernel as
though they were all hardware.
• A virtual machine not provide any additional functionality but provides an
interface identical to the underlying bare hardware. Each process is
provided with a (virtual) copy of the underlying computer
• The operating system creates the illusion that a process has its own
processor with its own (virtual) memory.
• The resources of the physical computer are shared to create the virtual
machines.
– CPU scheduling can create the appearance that users
have their own processor.
– Spooling and a file system can provide virtual card
readers and virtual line printers.
– A normal user time-sharing terminal serves as the
virtual machine operator’s console.
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
System Models
Non-virtual Machine
Virtual Machine
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Virtual Machines
• The virtual-machine concept provides complete protection
of system resources since each virtual machine is isolated
from all other virtual machines. This isolation, however,
permits no direct sharing of resources.
• A virtual-machine system is a perfect vehicle for operatingsystems research and development. System development is
done on the virtual machine, instead of on a physical
machine and so does not disrupt normal system operation.
• The virtual machine concept is difficult to implement due to
the effort required to provide an exact duplicate to the
underlying machine
• Virtual machines are a means to solve system compatibility
problems
– Windows applications to run on Linux-based computers
• Intel instructions are translated into the native instruction set
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Java Virtual Machine
• A platform is the hardware or software
environment in which a program runs. Some of the
most popular platforms are Windows 2000, Linux,
Solaris, and MacOS.
• Most platforms can be described as a combination
of the operating system and hardware. The Java
platform differs from most other platforms in that
it's a software-only platform that runs on top of
other hardware-based platforms.
• Compiled Java programs are platform-neutral
bytecodes executed by a Java Virtual Machine
(JVM).
• The JVM is specific for each system and it abstracts
the system in a standard way to the Java program
eliminating code changes when ported from one
platform to another
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Communication Models
Communication between processes may take place using either message passing
or shared memory.
Message passing

Information is exchanged through an interprocess
communication facility provided by the OS



Computers have host names, processes have process names for
identification purposes
Useful when smaller number of data need to be exchanged
Easier to implement than shared memory
Shared memory

Processes use map memory system calls to gain access to
regions of memory owned by other processes


Allows maximum speed and convenience of communication
Problems arise in the area of protection and synchronization
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)
Communication Models
Message Passing
Shared Memory
Ref: http://userhome.brooklyn.cuny.edu/irudowdky/OperatingSystems.htm & Silberschatz, Gagne, & Galvin, Operating Systems Concepts, 7th ed, Wiley (ch 1-3)