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Unit- I
Computer System and Operating System Overview
Overview of computer operating systems:
An operating system is a set of software programs that manages the hardware of a
computer. It also exploits the hardware resources of one or more processors to provide a
set of services to system hardware. An amazing aspect of OS is how varied they are in
accomplishing these tasks.
Mainframe operating systems are designed primarily to optimize the utilization of
hardware resources. Personal Computer (PC) OS supports a wide range between complex
games and business applications.
What an OS do?
A computer system can be divided roughly into four components:
1) The Hardware: The CPU, the memory, and I/O devices provide the basic
computing resources for the system.
2) The Application Programs: The application programs such as word
processors, spreadsheets, compilers, and web browsers define the ways in which
these resources are used to solve user’s computing problems.
3) The Operating System: It controls and coordinates the use of the use of
hardware among the various application programs for the various users.
4) The Users: The users are the ultimate end users of the hardware, operating
system, and the application programs.
User 1
Compilers
User 2
Assembler
User 3
User n
……..
Text Editors ………..
Database System
System and Application Programs
Operating System
Computer Hardware
Fig 1: Abstract View of the components of a computer system
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For example: An OS is similar to a government
Like a government, it performs no useful by itself. It simply provides an
environment within which other programs can do useful work.
User
Application
Operating System
Hardware
Fig 2: Hierarchy of computer system
What does a modern operating system do?
1) Provides Abstractions: Hardware has low-level physical resources with
complicated, idiosyncratic (individual, peculiar) interfaces.
OS provides abstracts that present clean interfaces.
Goal: make computer easier to use.
E.g.: Processes, unbounded memory, files, synchronization and communication
mechanisms.
2) Provides Standard Interface:
Goal: portability.
E.g.: UNIX runs on many very different computer systems. To a first
approximation can port many programs across systems with little effort.
3) Mediates Resource Usage:
Goal: allow multiple users to share resources fairly, efficiently, safely, and
securely.
E.g.:
 Multiple processes share one processor (preemptable resources).
 Multiple programs share one physical memory (preemptable resources).
 Multiple users and file share one disk (non- preemptable resources).
 Multiple programs share a given amount of disk and network bandwidth
(preemptable resources).
4) Consumes Resources:
Goal: Solaris takes up about 8M bytes physical memory (or about $400).
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The major problem facing computer science today is how to build large, reliable
software systems. Operating systems are one of very few examples of existing large
software systems, and by studying OS we may learn lessons applicable to the
construction of larger systems.
Operating System Overview
An operating system or OS is a software program that enables the computer
hardware to communicate and operate with the computer software. Without a computer
operating system a computer would be useless.
OS Types:
As computers have progressed and developed so have the types of OS.
1) GUI (Graphical User Interface): A GUI operating system contains graphics and
icons and is commonly navigated by using a mouse.
E.g.: Widows XP, Windows CE, Solaris etc.,
2) Multi-user: A multi-user operating system allows for multiple users to use the
same computer at the same time and/or different times.
E.g.: Linux, UNIX etc.,
3) Multiprocessing: An OS capable of supporting and utilizing more than one
computer processor.
E.g.: Linux, UNIX, Windows 2000 etc.,
4) Multitasking: An OS that is capable of allowing multiple software processes to
run at the same time.
E.g.: UNIX, Windows 2000 etc.,
5) Multithreading: Operating systems that allow different parts of a software
program to run concurrently.
E.g.: Linux, UNIX, Windows 2000 etc.,
Computer System Overview
Basic Elements:
A computer consists of processor, memory and I/O components, with one or more
modules of each type. These components are interconnected in some fashion to achieve
the main function of a computer, which is to execute programs. There are 4 main
structural elements.
1) Processor: It controls the operation of the computer and performs its data
processing functions. When there is only one processor, it is often referred to as
the CPU.
2) Main Memory: It is used store the data and programs. This memory is typically
volatile; it is referred to as primary memory or real memory.
3) I/O Modules: These are used to move data between the computer and it external
environment. The external environment consists of a variety of devices, including
secondary memory devices, communication equipment, and terminals.
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4) System Bus: It is used to provide the communication among processors, main
memory, and I/O modules.
The following figure depicts these top-level components. One of the processor’s
functions is to exchange data with memory. For this purpose, it typically makes use of
two internal (to the processor) registers:
A. Memory Address Register (MAR): which specifies the address in
memory for the next read or write.
B. Memory Buffer Register (MBR): This contains the data to be written
into memory or receives the data read from memory.
Similarly I/O Address register (I/OAR) specifies a particular I/O device.
I/O Buffer Register (I/OBR) is used for the exchange of data between an I/O
module and the processor.
CPU
PC
MAR
IR
MBR
.
.
.
Instruction
I/OAR
0
1
2
.
.
Instruction
Instruction
Execution
unit
I/OBR
.
.
.
Data
Data
.
.
.
I/O Module
.
.
.
Buffers
n-2
n-1
PC=Program counter
IR=Instruction register
MAR=Memory Address Register
MBR=Memory Buffer Register
I/OAR=Input/Output Address Register
I/O BR=Input/Output Buffer Register
Fig 3: Computer Components: Top-Level View
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A memory module consists of a set of locations, defined by sequentially
numbered addresses. Each location contains a bit pattern that can be interpreted as either
an instruction or data. An I/O module transfers data from external devices to main
memory and processor and vice versa. It contains internal buffers for temporarily storing
data until they can be sent on.
Processor Registers:
A processor includes a set of registers that provide a type of memory that is faster
and smaller than main memory. There are two types of registers, they are:
1. User-Visible Registers
2. Control and Status Registers
1) User-Visible Registers:
These are used to enable the machine or assembly language programs to minimize
main memory references by optimizing register use.
Types of registers that are typically available are data, address, and condition code
registers.
Data Registers: These can be assigned to a variety if functions by the programmer.
These registers contain main memory addresses of data and instructions, or they contain a
portion of the address that is used in the calculation of the complete or effective address.
E.g.:
a) Index Registers: Indexed addressing is a common mode of addressing that involves
adding an index to a base value to get the effective address.
b) Segment Pointer: With segmented addressing, memory is divided into segments,
which are variable length blocks of words.
c) Stack Pointer: If there is user-visible stack addressing, then there is a dedicated
registers that points to the top of the stack.
2) Control and Status Registers:
A variety of processor registers are employed to control the operations of the
processor. On most machines, most of these are not visible to the users.
In addition to the MAR, MBR, I/OAR and I/OBR registers, the following are
essential to instruction execution:
a) Program Counter: PC contains the address of the next instruction to be fetched.
b) Instruction Register: IR contains the instructions most recently fetched.
Instruction Execution:
A program to be executed by a processor consists of a set of instructions stored in
memory. In its simplest form the instruction processing consists of two steps:
i.
The processor reads (fetches) instruction from memory one at a time and
executes each instruction.
ii. Program execution consists of repeating the process of instruction fetch
and instruction execution.
The processing required for a single instruction is called an instruction cycle.
Instruction cycle includes two steps: 1) Fetch Stage, 2) Execute Stage.
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Instruction Fetch and Execute:
Start
Fetch Next
Instruction
Execute
Instruction
Fetch Stage
Execute Stage
Halt
Fig 4: Basic Instruction Cycle
Interrupts:
An interrupt is a signal which 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 interrupt instruction. It is a
signal to the processor to suspend the current executing program and run a specified
routine. Whenever the execution of routine is completed, the processor switches to its
original program.
Basically, interrupts are of four types. They are:
1. Program Interrupts: These types of interrupts are generated by illegal
instructions in the program such as division by zero, arithmetic overflow and
reference outside a user’s allowed memory space. Whenever the program
interrupts happened the OS sends signal to the processor to suspend the current
job.
2. Timer Interrupts: These types of interrupts are generated by clock within the
processor. Whenever the time slot of the processor expired, the operating system
sends the interrupt signal to the processor to suspend the current job.
3. I/O Interrupts: These types of interrupts are generated by I/O controller. For
example, if the CPU issues read command to the I/O module, the I/O module is
ready at that time, and then the I/O module sends an interrupt signal to the CPU
about the status.
4. Hardware Interrupts: These types of interrupts are generated by hardware
components, if any hardware component fails due to the technical problem, then
they generates an interrupt. For example, memory failure, power failure to a
particular hardware device.
The interrupt signal is detected by the interrupt handler, if there are no other
interrupts are pending, the interrupt controller processes the interrupt immediately. If
another one is in progress, the interrupt suspends temporarily.
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Interrupts and Instruction Cycle:
Interrupts are provided primarily as a way to improve processor utilization.
Whenever an interrupt occurs, the processor will respond after the execution of an
instruction currently it is executing. And the processor does not fetch the subsequent
instruction rather than it executes the interrupt controller. The CPU responds to interrupts
by saving the current value of the program counter and resetting the program counter to a
new address of the interrupt handler. This allows the CPU to return to executing at the
point the interrupt occurred, after it has executed a procedure for handling the interrupt.
The address the CPU jumps to is determined by the hardware architecture. On
some machines, a unique address is associated with each interrupt. More commonly, the
architecture defines an address in memory to be location of an interrupt vector. Each
interrupt is associated with an index into that vector. The branch address is determined by
the contents of the memory location in the vector pointed to by the interrupt’s index.
Other status information is stored in a special register called Program Status Word
(PSW) register. With interrupts, the processor can be engaged in executing other
instructions while an I/O operation is in progress.
For the user program, an interrupt suspends the normal sequence of execution.
When the interrupt processing is completed, execution resumes. See the following fig 5.
User Program
Interrupt Handler
1
2
i
.
.
.
.
.
.
i+1
.
.
.
.
M
Fig 1.6: Transfer of control via Interrupts
To accommodate interrupts, an interrupt stage is added to the instruction cycle. In
the interrupt stage, the processor checks to see if any interrupt have occurred, indicated
by the presence of an interrupt signal. If no interrupts are pending, the processor suspends
execution of current program and executes an interrupt-handler routine. The interrupthandler routine is generally part of the operating system. Typically, this routine
determines the nature of the interrupt and performs whatever actions are needed. The
following fig 6 shows the instruction cycle with interrupts.
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Interrupts
Disabled
Start
Fetch Next
Instruction
Execute
Instruction
Interrupts
Enabled
Check for
interrupt;
initiate interrupt
handler
Halt
Fig 6: Instruction Cycle with Interrupts
Interrupt processing:
The occurrence of an interrupt triggers a number of events, both is the processor
hardware and in software. The following fig 7 shows a typical sequence. The following is
the sequence of events when an I/O operation is completed by the I/O device.
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