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					Operating Systems and Basics


OS is system software, which may be viewed as collection of software
consisting of procedures for operating the computer & providing an
environment for execution of programs. It’s an interface between user &
computer
Types of Processing:
1. Serial Processing
2. Batch Processing
3. Multiprogramming.

Types of OSs:
1. Batch OS
2. Multiprogramming OS

         ?   Multitasking/Multiprocessing
         ?   Multiuser OS
         ?   Time Sharing OS
         ?   Real Time OS

3. Network OS
4. Distributed OS

OS Structure:

1. Layered Structure
2. Kernel Structure
        ? Create & Delete process
        ? Processor scheduling, mem mgmt & I/O mgmt.
        ? Process synchronization.
        ? IPC help

3. Virtual Machine
4. Client Server model

Process Managment

Process Status: New, ready to run, running, suspended, sleep, wait,
terminate.

Types of Scheduler:
1. Long term/Job Scheduler
2. Medium term/
3. Short term/ CPU Scheduler
Processes can be either CPU bound or I/O bound. Scheduling performance
criteria:
CPU utilisation

        Throughput
        Turnaround time
        Waiting time
        Response Time
Scheduling Algorithms:
Preemptive
        First-come-first served

Non-Preemptive
        Shortest-job-first
        Round Robin.
        Priority based scheduling
        Multi-level Queue
Processing the interrupt to switch the CPU to another process requires
saving all the registers for the old process & then loading the registers
for new process is known as Context Switching.
Synchronization & IPC

The shared storage may be in main memory or it may be a shared file. Each
process has a segment of code, critical Section, which accessed shared
memory or files. Some way of making sure that if one process is executing
in its critical section, other process will be excluded from doing the
same thing is known as Mutual Exclusion. Hardware support is available
for mutual exclusion called “Test & set instruction”, it is designed to
allow only one process among several concurrent processes to enter in its
critical section. Semaphore: It’s a synchronization tool, it’s a variable
which accepts non-negative integer values and except for initialization
may be accessed and manipulated through two primitive functions wait() &
signal().
Disadvantages :

1. Semaphores are unstructured.
2. Semaphores do not support data abstraction.

Alternative to Semaphores:
1. Critical region
2. Conditional critical region
3. Monitors
4. Message Passing

Reason for Deadlock:

1. Mutual exclusion
2. Hold & wait
3. No preemption
4. Circular condition
Memory Management

In a single process, system memory is protected through hardware
mechanism such as dedicated register called Fence register. In a multi
programming, memory can be allocated either Statically or Dynamically.
Partition information is stored in partition Description Table. Two
strategies are used to allocate memory to ready process are First Fit &
Best Fit. Loading program into memory by relocating load or linker in a
static allocation is known as Static relocation. In Dynamic method, run
time mapping of virtual address into physical address with support of
some hardware mechanism such as base register & limit registers.
Protection is served by using Limit registers to restrict the program to
access memory location, sharing is achieved by using dedicated common
partition. Static allocation does not support data structures like stack
& queues. It limits degree of multi programming. Compaction is a process
of collecting free space in to a single large memory chunk to fit the
available process. It is not done, bcoz it occupies lot of CPU time. It
is only supported in Mianframe & SuperComputers Paging is a memory
management technique that permits a program’s memory to be non-contiguous
into physical memory, thus allowing a program to be allocated physical
memory whenever is required. This is done by Virtual Address, later these
address are converted to physical address. Memory is divided into number
of fixed size blocks called Frames. The virtual address space or logical
memory of a process is also broken into blocks of the same size called
pages. When a program is to be run, its pages are loaded into any frame
from the disk. Mapping is done thru Page Map Table which contains the
base address of each page in physical memory. Hardware support is given
to paging using Page Map Table Register (PMRT) which will be pointing to
beginning of the PMT. Look side memory or Content addressable memory is
used to overcome the problem of PMT. Address translation is done by
Associative Memory which will convert virtual to physical address by page
& offset values by looking into PMT. Segmentation is Memory management
scheme its sophisticated form Address translation. It is done by Segment
table, which is a important component in segmented system. Segment
accessing supported by Segment Table Base register (STBR). Protection is
enforced by Segment table Limit register (STLR). Virtual memory is memory
management technique which splits the process into small chunks called
Overlay. First overlay will can next overlay before quitting the CPU, the
remaining overlays will be on Hard disk, the swapping is done by OS.
Advantages:

1.         What ever the size program, memory can be allocated easily.
2.          Since the swapping is done between main & secondary memory,
the CPU utilization & throughput will be increased.
3.         It reduces the external fragmentation & Increases the program
execution speed. In Demand paging, pages are loaded only on demand, not
in advance, its same as paging with swapping feature. Page fault occurs
due to missing of page in the main memory, it means that the program is
referring the address of the page which is not brought into memory
File Management

Some systems support a single uniform set of file manipulation features
for both file & I/O device management, this feature is known as Device
Independent I/O or Device Independence. Printer is a one of a such
example.

File organization may be

1.         Byte Sequenced in which OS does not impose any structure on
the file organization.
2.         Record Sequenced, it’s a sequence of fixed sized records,
arbitrary records read or written, but records can’t be inserted or
deleted in middle of the file.
3.        ISAM files are inserted in disc blocks which will have keys to
inserted, its look like a tree of blocks.,
Responsibility of File Management.

1. Mapping of logical file address to physical disk address
2. Management of disk space & allocation - deallocation.
3. Keeping track of all files in system
4. Support for protection & sharing of files. Method to access the file
organized in hierarchical form. Absolute pathname & relative pathname.
File & directory searching is done using

1. Linear List organization which takes O(n) comparisons to locate a
file.
2. Hashing Technique
3. Balanced binary tree which takes O(log n) comparisons to locate file,
it always provides sorted list files which will increase the efficiency.
Collection of tracks on all surfaces that are at the same distance is
called a Cylinder.

Disk Space Management Methods:

1. Linked list
2. Bit Map
Disk allocation Methods

1. Contiguous: It supports both Sequential & Direct Accessing. Allocation
is done using First Fit & Best Fit methods.
2. Linked List

Advantages:
        Simple
       No disk Compaction.
Disadvantages

It doesn’t support direct accessing since blocks are scattered over the
disk. Slow direct accessing of any disk block Space requirement for
pointers Reliability
3. Indexed Uses Index Block to support direct accessing. Problem with
same can be solved by Multiple Level indexing, indirect blocks, double
indirect block.

Advantages
        No external fragmentation
        Efficient random access
        Indexing of free space is done with Bitmap
        Can keep the index of bad blocks.
Disk Scheduling

       1. First come first served (FCFS)
       2. Shortest Seek Time First
       3. Scan Scheduling also called Elevator Algorithm

				
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