OS- LINKER

  A linker or link editor is a program that takes one or more objects generated by a compiler and combines them into a single executable program.

Many operating system environments allow dynamic linking, that is the postponing of the resolving of some undefined symbols until a program is run. That means that the executable code still contains undefined symbols, plus a list of objects or libraries that will provide definitions for these.

Linkers can take objects from a collection called a library. Some linkers do not include the whole library in the output; they only include its symbols that are referenced from other object files or libraries. Libraries exist for diverse purposes, and one or more system libraries are usually linked in by default.

The linker also takes care of arranging the objects in a program's address space. This may involve relocating code that assumes a specific base address to another base. Since a compiler seldom knows where an object will reside, it often assumes a fixed base location (for example, zero). 

Linking is the process of combining various pieces of code and data together to form a single executable that can be loaded in memory.
Linking can be done at compile time, at load time (by loaders) and also at run time (by application programs). The process of linking dates back to late 1940s, when it was done manually. Now, we have linkers that support complex features, such as dynamically linked shared libraries.

Linkers and loaders perform various related but conceptually different tasks:

• Program Loading.:  This refers to copying a program image from hard disk to the main memory in order to put the program in a ready-to-run state. In some cases, program loading also might involve allocating storage space or mapping virtual addresses to disk pages.
• Relocation.:  Compilers and assemblers generate the object code for each input module with a starting address of zero. Relocation is the process of assigning load addresses to different parts of the program by merging all sections of the same type into one section. The code and data section also are adjusted so they point to the correct runtime addresses.
• Symbol Resolution.: A program is made up of multiple subprograms; reference of one subprogram to another is made through symbols. A linker's job is to resolve the reference by noting the symbol's location and patching the caller's object code.
  
  So a considerable overlap exists between the functions of linkers and loaders.
  One way to think of them is: the loader does the program loading; the linker does the symbol resolution; and either of them can do the relocation.

OS- CONTEXT SWITCHING

A context switch is the computing process of storing and restoring state (context) of a CPU so that execution can be resumed from the same point at a later time. This enables multiple processes to share a single CPU. The context switch is an essential feature of a multitasking operating system.

A context switch can mean a register context switch, a task context switch, a thread context switch, or a process context switch.


The scheduler is the part of the operating systems that manage context switching, it perform context switching in one of the following conditions:

1. Multitasking: Within a preemptive multitasking operating system, the scheduler allows every task (according to its priority level) to run for some certain amount of time, called its time slice where a timer interrupt triggers the operating system to schedule another process for execution instead.
   If a process will wait for one of the computer resources or will perform an I/O operation, the operating system schedules another process for execution instead.
2. Interrupt handling: Some CPU architectures (like the Intel x86 architecture) are interrupt driven. When an interrupt occurs, the scheduler calls its interrupt handler in order to serve the interrupt after switching contexts; the scheduler suspended the currently running process till executing the interrupt handler.
3. User and kernel mode switching: Context switching can be described in slightly more detail as the kernel suspending execution of one process on the CPU and resuming execution of some other process that had previously been suspended. When a transition between user mode and kernel mode is required in an operating system, a context switch is not necessary; a mode transition is not by itself a context switch. However, depending on the operating system, a context switch may also take place at this time.

for more details visit:   http://www.linfo.org/context_switch.html
                                  http://simple.wikipedia.org/wiki/Context_switch
                                  http://en.wikipedia.org/wiki/Context_switch

OS- Spooling and Buffering

SPOOLING:


Acronym for simultaneous peripheral operations on-line, spooling refers to putting jobs in a buffer, a special area in memory or on a disk where a device can access them when it is ready. Spooling is useful because devices access data at different rates. The buffer provides a waiting station where data can rest while the slower device catches up.

pls do refer :        http://www.webopedia.com/TERM/S/spooling.html
                           http://www.techterms.com/definition/spooling

Spooling refers to a process of transferring data by placing it in a temporary working area where another program may access it for processing at a later point in time.

Spooling refers to copying files in parallel with other work. The most common use is in reading files used by a job into or writing them from a buffer on a magnetic tape or a disk. Spooling is useful because devices access data at different rates.

The most common spooling application is print spooling .



BUFFERING:

buffer- A temporary storage area, usually in RAM. The purpose of most buffers is to act as a holding area, enabling the CPU to manipulate data before transferring it to a device.

Buffers are commonly used when burning data onto a compact disc, where the data is transferred to the buffer before being written to the disc.
Another common use of buffers is for printing documents. When you enter a PRINT command, the operating system copies your document to a print buffer (a free area in memory or on a disk) from which the printer can draw characters at its own pace. This frees the computer to perform other tasks while the printer is running in the background. Print buffering is called spooling.


A Buffer is a region of memory used to temporarily hold data while it is being moved from one place to another. Typically, the data is stored in a buffer as it is retrieved from an input device (such as a keyboard) or just before it is sent to an output device (such as a printer). However, a buffer may be used when moving data between processes within a computer. This is comparable to buffers in telecommunication. Buffers can be implemented in either hardware or software, but the vast majority of buffers are implemented in software. Buffers are typically used when there is a difference between the rate at which data is received and the rate at which it can be processed, or in the case that these rates are variable, for example in a printer spooler.

Buffering: is reading and writing data from hdd takes long time.so to improve the speed for data processing the data next required by processor is stored is cache memory or cpu register.for e.g. to cut certain line from text file to copy into another file.cut data get stored in to buffer (cpu register) to get back stored into another file.

Read more: http://wiki.answers.com/Q/What_is_the_meaning_of_buffering_an_operating_system#ixzz1EWjvzuHi

OS- MULTIPROCESSING

Multiprocessing is running a system with more than one processor.
Multiprocessing is the use of two or more central processing units (CPUs) within a single computer system.

Multiprocessing can be said to be either asymmetric or symmetric.
Asymmetric multiprocessing designates some processors to perform system tasks only, and others to run applications only. This is a rigid design that results in lost performance during those times when the computer needs to run many system tasks and no user tasks, or vice versa.
Symmetric multiprocessing, often abbreviated SMP, allows either system or user tasks to run on any processor, which is more flexible and therefore leads to better performance. SMP is what most multiprocessing PC motherboards use.

Multiprocessing operating systems enable several programs to run concurrently.
UNIX is one of the most widely used multiprocessing systems.

A system can be both multiprocessing and multiprogramming, only one of the two, or neither of the two.

 For more details refer : http://en.wikipedia.org/wiki/Multiprocessing

OS- MULTITASKING

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Multitasking, in an operating system, is allowing a user to perform more than one computer task (such as the operation of an application program) at a time. The operating system is able to keep track of where you are in these tasks and go from one to the other without losing information.


Multitasking operating systems allow more than one program to run at a time. They can support either preemptive multitasking, where the OS *doles out time to applications (virtually all modern OSes) or cooperative multitasking, where the OS waits for the program to give back control (Windows 3.x, Mac OS 9 and earlier).

A multitasking operating system is any type of system that is capable of running more than one program at a time. Most modern operating systems are configured to handle multiple programs simultaneously, with the exception of some privately developed systems that are designed for use in specific business settings. As with most types of communications technology, the multitasking operating system has evolved over time, and is likely to continue evolving as communication demands keep growing in many cultures.  for more details visit :      http://en.wikipedia.org/wiki/Computer_multitasking

*doles = distributes something.

OS- MULTIPROGRAMMING

Multiprogramming is one of the more basic types of parallel processing that can be employed in many different environments. Essentially, multiprogramming makes it possible for several programs to be active at the same time, while still running through a single processor. Multiprogramming is very different from the multiprocessing because even though there may be several programs currently active, the uniprocessor is not simultaneously executing commands for all the programs. Instead, the processor addresses each program, executes a single command, then moves on to the next program in the queue. The previous program remains active, but enters into a passive state until the uniprocessor returns to the front of the queue and executes a second command.  Multiprogramming is the rapid switching of the CPU between multiple processes in memory. It is done only when the currently running process requests I/O, or terminates. It was commonly used to keep the CPU busy while one or more processes are doing I/O. It is now mostly  super ceded by multitasking, in which processes also lose the CPU when their time quantum expires. Multiprogramming makes efficient use of the CPU by overlapping the demands for the CPU and its I/O devices from various users. It attempts to increase CPU utilization by always having something for the CPU to execute. The prime reason for multiprogramming is to give the CPU something to do while waiting for I/O to complete. If there is no DMA, the CPU is fully occupied doing I/O, so there is nothing to be gained (at least in terms of CPU utilization) by multiprogramming. No matter how much I/O a program does, the CPU will be 100% busy. This of course assumes the major delay is the wait while data is copied. A CPU could do other work if the I/O were slow for other reasons (arriving on a serial line, for instance).

MCA timetable..

	MON	TUES	WED	THURS	FRI
9:15	CS (Pracs-BOTH BATCHES)	DS	P&S	OS	OS (PRACS-2) DS (PRACS-1)
10:15	CS (Lec- Shirin maam)	DS	CS	DS
11:15	CS (Lec- Rashmi maam)	OS	OS (PRACS-1) CG (PRACS-2)
12:40	FM	CG		CG (PRACS-1) DS (PRACS-2)	DS
1:40	OS	FM		CG	DS
2:40	P&S	FM	FM	CG	CG
3:40	P&S	FM (TUT)	OS		P&S (TUT)