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| OPERATING
SYSTEM _ INTRODUCTION |
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| Operating-System
Operations: |
Modern
operating systems are interrupt driven. Events are
almost always signaled by the occurrence of an interrupt
or trap. A trap is a software-generated interrupt
caused either by an error or by a specific request
from a user program that an operating-system service
be performed. For each type of interrupt, separate
segments of code in the operating system determine
what action should be taken. An interrupt service
routine is provided that is responsible for dealing
with the interrupt.
Since the operating system and the users share the
hardware and software resources of the computer system,
we need to make sure that an error in a user program
could cause problems only for the one program that
was running. A properly designed operating system
must ensure that an incorrect (or malicious) program
cannot cause other programs to execute incorrectly.
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| Dual-Mode
Operation: |
In
order to ensure the proper execution of the operating
system, we must be able to distinguish between the
execution of operating-system code and user-defined
code. The approach taken by most computer systems
is to provide hardware support that allows us to differentiate
among various modes of execution.
There are two separate modes of operations: user mode
and kernel mode(also called supervisor mode, system
mode, or privileged mode). A bit, called mode bit,
is added to the hardware of the computer to indicate
the current mode: kernel(0) or user(1). With the mode
bit, we are able to distinguish between a task that
is executed on behalf of the operating system and
one that is executed on behalf of the user.
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Transition
from user to kernel mode |
When the computer system is executing on behalf of
a user application, the system is in user mode. However,
when a user application requests a service from the
operating system via system call, it must transition
from user to kernel mode.
At system boot time, the hardware starts in kernel
mode. The operating is then loaded and starts user
applications in user mode. Whenever a trap or interrupt
occurs, the hardware switches from user mode to kernel
mode (that is, changes the state of the mode bit to
0). Thus, whenever the operating system gains control
of the computer, it is in kernel mode. The system
always switches to user mode (by setting the mode
bit to 1) before passing control to a user program.
The dual mode of operation provides us with the means
for protecting the operating system from erratic users.
We accomplish this protection by designating some
of the machine instructions that may cause harm as
privileged instructions.
The lack of a hardware-supported dual mode can cause
serious shortcomings in an operating system. A user
program running awry (crooked) can wipe out the operating
system by writing over it with data and multiple programs
are able to write to a device at the same time, with
possibly disastrous results.
Once hardware protection is in place, errors violating
modes are detected by the hardware. These errors are
normally handled by the Operating System. If a user
program attempt either to execute an illegal instruction
or to access memory that is not in the user’s
address space, then the hardware will trap to the
operating system. The trap transfers control through
the interrupt vector to the operating system and the
program terminate abnormally.
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| Timer: |
We
must prevent a user program from getting stuck in
an infinite loop or not calling system services and
never returning to the operating system. A timer can
be used to accomplish this goal. A timer can be set
to interrupt the computer after a specified period.
The period may be fixed or variable. A variable timer
is generally implemented by a fixed-rate clock and
a counter. The operating system sets the counter.
Every time the clock ticks, the counter is decremented.
When the counter reaches zero, an interrupt occurs.
Thus, we can use the timer to prevent a user program
from running too long. A simple technique is to initialize
a counter with the amount of time that a program is
allowed to run. For example, a program with a 7 minute
time limit would have its counter initialized to 420.
Every second, the timer interrupts and the counter
is decremented by 1. When the counter becomes negative,
the operating system terminates the program for exceeding
the assigned time limit.
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