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Operating Systems
CS208
What is Operating System?
• It is a program.
• It is the first piece of software to run after
the system boots.
• It coordinates the execution of all other
software.
• It provides various common services
needed by users and applications.
Operating System Definition
• Short definition:
– a program that manages a computer’s
resources and acts as an intermediary between
a user and those resources
computer
user
application
operating system
application
user
hardware
application
Operating System (OS) Functions
• For the User: Provides a user interface
to let the user access hardware and
software resources.
• For the System: Manages all the
system tasks and resources to provide
security, and fair, efficient use.
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.
Parts of an Operating System
• User Interface
– Menu
– Command Language
– Graphical User Interface (GUI)
• System Security
– Login control
• Process Management
– CPU Scheduler
Parts of an Operating System
• Memory Management
– Swapper/Pager
• Temporarily unused pages are stored on disk
(swapped out)
• When they are needed again, they are brought
back into the memory (swapped in)
– Garbage Collection
• Resource Management
– File Management
– Device Management
Operating System Terminology
• Kernel
– Software component that controls the
hardware directly, and implements the core
privileged OS functions.
• Process
– An executing program.
Time Sharing
• Allows multiple processes to run on the same
computer, seemingly at the same time.
• CPU is multiplexed among several processes that
are kept in memory (the CPU is allocated to a
process only if the process is in memory).
– A process is swapped in and out of memory to
the disk as needed.
• OS must manage process scheduling and provide
memory protection to keep one program from
crashing the system or corrupting other
programs.
CPU Time Slicing
for Time Sharing
• Only ONE process can be running on the CPU at
a time.
– Each process is allocated a “slice” of time in
the CPU.
– When the time runs out, the process is
interrupted, and another process is loaded into
the CPU.
• The act of giving each process a small slice of
time to run is called time slicing.
CPU Time Slicing
CPU
Main Memory
Allocate CPU
to Process 1
Current
Process
Allocate CPU
to Process 2
Allocate CPU
to Process 3
Repeat until all processes have completed.
Process 1
Process 2
Process 3
CPU execution modes
• CPUs supports (at least) 2 execution modes:
– User mode
• The code of the user programs
– Kernel (supervisor, privileged, monitor,
system) mode
• The code of OS
• The execution mode is indicated by a bit in the
processor status word (PSW) (a register in the
CPU)
Protecting Kernel mode
• OS code executes in the Kernel mode
– Called via interrupts and system calls
• Only the OS code is allowed to be executed in
the Kernel mode
• The user code must never be executed in the
Kernel mode
– The program counter (PC) is only set to point
to the OS code when the CPU goes to the
Kernel mode
Interrupts
• An interrupt is the way by which the
hardware informs the OS of special
conditions that require OS attention
• Interrupts cause the CPU not to execute the
next instruction
• Instead, the control is passed to OS
System Calls
• A System Call is used by a process to request
a service from the OS
• Typical system calls
– Open/read/write/close the file
– Get the current time
– Create a new process
– Request more memory
Parallel Systems
• Multi-processor systems with more than one
CPU in close communication.
• Tightly coupled system – processors share
memory and a clock; communication usually
takes place through the shared memory.
• Advantages of parallel system:
– Economical
– Increased throughput
– Increased reliability
Parallel Systems
• Symmetric multiprocessing (SMP)
– Each processor runs an identical copy of the operating
system.
– Many processes can run at once without performance
deterioration.
– Most modern operating systems support SMP
• Asymmetric multiprocessing
– Each processor is assigned a specific task; master
processor schedules and allocates work to slave
processors.
– More common in extremely large systems
Distributed Systems
• Distribute the computation among several
physical computers.
– Each has its own CPU, local memory, stable storage,
I/O paths connecting to the environment
• Interconnections
– Loosely coupled system – each processor has its own
local memory; processors communicate with one
another through various communications lines, such as
high-speed buses or telephone lines.
– Systems cooperate to maintain shared state and
coordinate global information
Distributed Systems
• Advantages of distributed systems.
– Inherent distribution
– Speedup - improved performance due to load
sharing
– Fault tolerance/Reliability
– Resource Sharing
– Scalability
– Flexibility
Distributed Systems
• Network Operating System
– provides file sharing
– provides communication scheme
– runs independently from other computers on the
network
• Distributed Operating System
– less autonomy between computers
– gives the impression there is a single operating
system controlling the network.