Operating Systems

Lecture-1
OS Concepts
Tahira Alam
University of Asia Pacific
 What is 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
 Computer System Structure(1)
• What are the components?
Computer System Structure(2)

Application
User Software

Hardware
OS
 Computer System Structure(3)
• Hardware
• Operating System
• Application Programs
• Users
Computer System Structure(4)
– Hardware – provides basic computing resources
• CPU, memory, I/O devices
– Operating system
• Controls and coordinates use of hardware among
various applications and users
Computer System Structure(5)
– Application programs – define the ways in which
the system resources are used to solve the
computing problems of the users
• Word processors, compilers, web browsers, database
systems, video games
– Users
• People, machines, other computers
Computer System Structure(6)
 Some Useful Definitions(1)
Kernal
• kernel is the most important program in the
operating system.
• “The one program running at all times on the
computer”
• . Everything else is either a system program
or an application program.
 Some Useful Definitions(2)
System Program:
• A program that controls some aspect of the
operation of a computer.
• used to program the operating system
software.
• Example: operating system, networking
system, web site server, data backup server
etc
 Some Useful Definitions(3)
System Call:
• A system call is the programmatic way in
which a computer program requests a service
from the kernel of the operating system on
which it is executed.
 Some Useful Definitions(4)
Shell:
• a shell is a user interface for access to
an operating system's services.
• In general, operating system shells use either
a command-line interface (CLI) or graphical
user interface (GUI)
 Some Useful Definitions(4)
Program:
• A computer program is a collection of
instructions that can be executed by a
computer to perform a specific task.
 What Happens When We Run a
Program(1)
• A compiler translates high level programs into an
executable file
• The exe contains instructions that the CPU can
understand, and data of the program (all
numbered with addresses)
• Instructions run on CPU: hardware implements
an instruction set architecture (ISA)
• CPU also consists of a few registers, e.g.,
– Pointer to current instruction (program counter or PC)
– Operands of instructions, memory addresses
 What Happens When We Run a
Program(2) Exe in
• To run an exe, CPU Memory
CPU (RAM)
– fetches instruction pointed at
by PC from memory
– loads data required by the PC
instructions into registers Code
– decodes and executes the
Reg.
instruction
– stores results to memory Reg. Data
• Most recently used
instructions and data are in Reg.
CPU caches for faster access

Result
 So, what does the OS do?
• OS manages program
memory
– Loads program executable Memory
(code, data) from disk to
memory
• OS manages CPU
– Initializes program
counter (PC) and other
registers to begin
execution Disk
• OS manages external devices
– Read/write files from disk.
 OS manages CPU
• OS provides the process abstraction
– Process: a running program P P P
– OS creates and manages processes 1 2 3
• Each process has the illusion of
having the complete CPU, i.e., OS
virtualizes CPU
• Timeshares CPU between processes CPU
• Enables coordination between
processes
 OS manages memory
Memory
• OS manages the memory
of the process: code, data, Of a Process
stack, heap etc 0
• Each process thinks it has a 1 Code
dedicated memory space 2
. Data
for itself, numbers code .
and data starting from 0 RAM .
(virtual addresses) Stack
• OS abstracts out the details
of the actual placement in . Heap
.
memory, translates from n
virtual addresses to actual
physical addresses
 OS manages devices
• OS has code to manage disk, network card,
and other external devices: device drivers
• Device driver talks the language of the
hardware devices
– Issues instructions to devices (fetch data from a
file)
– Responds to interrupt events from devices (user
has pressed a key on keyboard)
• Persistent data organized as a filesystem on
disk
 Interrupt
• An interrupt is a signal sent to the processor
that interrupts the current process.
• It may be generated by a hardware device or
a software program.
• A hardware interrupt is often created by
an input device such as a mouse or keyboard.
 Common Functions of Interrupts
• Interrupt transfers control to the interrupt service
routine generally, through the interrupt vector, which
contains the addresses of all the service routines.
• Interrupt architecture must save the address of the
interrupted instruction.
• Incoming interrupts are disabled while another
interrupt is being processed to prevent a lost interrupt.
• A trap is a software-generated interrupt caused either
by an error or a user request.
• An operating system is interrupt driven.
 Interrupt Handling
• The operating system preserves the state of the
CPU by storing registers and the program counter.
• Determines which type of interrupt has occurred:
– polling
– vectored interrupt system
• Separate segments of code determine what action
should be taken for each type of interrupt.
 Design goals of an operating system
• Convenience, abstraction of hardware
resources for user programs
• Efficiency of usage of CPU, memory, etc.
• Isolation between multiple processes
Thank You