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Operating Systems
1.1
Introduction
An operating system (OS) is the system software responsible
for direct control and management of basic operations of a
computing system; it provides a platform upon which
application programs are run. Examples of Operating System
are Windows Vista, UNIX and Linux
1.2 – Hardware Basics and Protection Issues
Hardware Basics. Advances in hardware technology (fueled
by Moore’s Law) make the Operating Systems of today
feasible. In the past, Operating Systems were constructed
with little or no hardware assists, and they were slow!
They worked, but slowly. The intelligent controllers
mentioned earlier, make multi-programming possible.
Storage hierarchies (especially cache) allow faster and
faster processors to execute instructions in a single
processor, von Neumann architecture. RISC architectures
allow multiple instruction and/or data streams at the
machine cycle level. Hardware memory managers allow
address translation and fetch of contents from memory to be
done quickly. These, and other hardware advances, allow
multi-user, multi-process systems to function with a
minimal amount of software overhead. (A first definition
of OS overhead – i.e. the time spent by the OS in
management tasks. CPU time is spent by the OS on behalf of
processes, but the processes themselves do no work.)
Protection. In the multi-user, multi-process system
protection and security become critical. Processes need to
be protected from each other. The OS needs to be protected
from all other processes. (Almost all programmers from the
MS-DOS era, remember writing a C language program that made
use of pointers. We remember writing over instructions of
our debugger or MS-DOS itself and “trashing” the system.
The solution was simple – simply reboot. That’s not a
feasible option in today’s multi-user systems.)
The first line of defense in protection of OS resources and
the OS itself from errant processes is the “dual mode of
operation”. Using this model, certain instructions in the
machine architecture are declared to be “privileged”. These
always include basic I/O instructions (to controllers or
memory mapped devices), the Halt instruction, timer
management instructions, and interrupt processing
instructions. A special hardware bit is added to the
system whose binary value indicates the mode of execution.
This bit is changed by the hardware itself in response to a
specific software interrupt. Privileged instructions can
only be executed by kernel mode instructions. User mode
instructions (making up the user process itself) are
executed in user mode. When the user process desires to
use OS resources or to have the OS execute instructions or
manage resources on its behalf, it issues what is known as
a System Call.
A System Call typically involves a function call from
within the program, a software interrupt, a mode change,
execution of kernel instructions, and a return to user
mode. Many times, a number is passed to the OS along with
parameter values (in registers, on the stack, or in
accessible memory) to indicate which OS module to execute.
Topic 1_3 will discuss the System Call interface in more
detail.
Security. In addition to this low level mode or
protection, higher level security features provided by the
OS provide defense of a system from external and internal
attacks. The older, basic UNIX mechanism revolved around
the assignment of id’s to a user. In UNIX/Linux systems
this consisted of two numbers, a user ID (UID) and a group
ID (GID). This combination of ID numbers defined the
access rights of the user to his/her and other files.
The Windows XP security system is more sophisticated (and
harder to manage). It also is based on the idea of users
and user ID’s, with the addition that every resource access
managed by the OS has an associated access control list.
When a process requests access to or use of such a
resource, a security monitor checks the process’ security
token against the access control list. In essence, each
object knows which processes can access it.