An operating system is a program that acts as an intermediary between the user and the computer
hardware. The purpose of an OS is to provide a convenient environment in which user can execute
programs in a convenient and efficient manner.

1. Batched operating systems

2. Multi-programmed operating systems

3. timesharing operating systems

4. Distributed operating systems

5. Real-time operating systems

Operating system controls and coordinates the use of the hardware among the various applications programs
for various uses. Operating system acts as resource allocator and manager. Also operating system is control
program which controls the user programs to prevent errors and improper use of the computer. It is especially
concerned with the operation and control of I/O devices.

Kernel is the core and essential part of computer operating system that provides basic services for all parts of
OS.

Micro kernel is a kernel which run services those are minimal for operating system performance. In this
kernel all other operations are performed by processor.

Macro Kernel is a combination of micro and monolithic kernel. In monolithic kernel all operating system
code is in single executable image.

Deadlock is a situation or condition where the two processes are waiting for each other to complete so that
they can start. This result both the processes to hang.

A program in execution is called a process.

Processes are of two types:
1. Operating system processes
2. User processes

1. New
2. Running
3. Waiting
4. Ready
5. Terminated

Starvation is Resource management problem where a process does not get the resources it needs for a long
time because the resources are being allocated to other processes.

Aging is a technique to avoid starvation in a scheduling system.

Semaphore is a variable, whose status reports common resource, Semaphore is of two types one is Binary
semaphore and other is Counting semaphore.

Transferring the control from one process to other process requires saving the state of the old process and
loading the saved state for new process. This task is known as context switching.

A thread is a program line under execution. Thread sometimes called a light-weight process, is a basic unit
of CPU utilization; it comprises a thread id, a program counter, a register set, and a stack

A situation, where several processes access and manipulate the same data concurrently and the outcome of
the execution depends on the particular order in which the access takes place, is called race condition. To
guard against the race condition we need to ensure that only one process at a time can be manipulating the
same data. The technique we use for this is called process synchronization.

Virtual memory is hardware technique where the system appears to have more memory that it actually
does. This is done by time-sharing, the physical memory and storage parts of the memory one disk when
they are not actively being used

It is a phenomenon in virtual memory schemes when the processor spends most of its time swapping
pages, rather than executing instructions. This is due to an inordinate number of page faults.

When many of free blocks are too small to satisfy any request then fragmentation occurs. External
fragmentation and internal fragmentation are two types of fragmentation. External Fragmentation happens
when a dynamic memory allocation algorithm allocates some memory and a small piece is left over that
cannot be effectively used. Internal fragmentation is the space wasted inside of allocated memory blocks
because of restriction on the allowed sizes of allocated blocks.

1. Mutual exclusion (where at least one resource is non-sharable)

2. Hold and wait (where a process holds one resource and waits for other resource)

3. No preemption (where the resources can?t be preempted)

4. Circular wait (where p[i] is waiting for p[j] to release a resource. i= 1,2, n

j=if (i!=n) then i+1

else 1 )

Cache memory is random access memory (RAM) that a computer microprocessor can access more quickly
than it can access regular RAM. As the microprocessor processes data, it looks first in the cache memory
and if it finds the data there (from a previous reading of data), it does not have to do the more
time-consuming reading of data from larger memory.

Logical address space is generated from CPU; it bound to a separate physical address space is central to
proper memory management. Physical address space is seen by the memory unit. Logical address space is
virtual address space. Both these address space will be same at compile time but differ at execution time.

An interpreter reads one instruction at a time and carries out the actions implied by that instruction. It does
not perform any translation. But a compiler translates the entire instructions.

Throughput ? number of processes that complete their execution per time unit

Turnaround time ? amount of time to execute a particular process

Waiting time ? amount of time a process has been waiting in the ready queue

Response time ? amount of time it takes from when a request was submitted until the first response is
produced, not output (for time-sharing environment).

Hardware device that maps virtual to physical address. In MMU scheme, the value in the relocation
register is added to every address generated by a user process at the time it is sent to memory.

->The user program deals with logical addresses; it never sees the real physical addresses

A real time process is a process that must respond to the events within a certain time period. A real time
operating system is an operating system that can run real time processes successfully

Trapdoor is a secret undocumented entry point into a program used to grant access without normal
methods of access authentication. A trap is a software interrupt, usually the result of an error condition.

The set of dispatch able processes is in a safe state if there exists at least one temporal order in which all
processes can be run to completion without resulting in a deadlock.

Distributed systems work in a network. They can share the network resources, communicate with each
other.

In a multiprocessor system there exist several caches each may containing a copy of same variable A.
Then a change in one cache should immediately be reflected in all other caches this process of maintaining
the same value of a data in all the caches s called cache-coherency.

Long term schedulers are the job schedulers that select processes from the job queue and load them into
memory for execution. The short term schedulers are the CPU schedulers that select a process from the
ready queue and allocate the CPU to one of them.

Mutex is the short form for ?Mutual Exclusion object?. A mutex allows multiple threads for sharing the
same resource. The resource can be file. A mutex with a unique name is created at the time of starting a
program. A mutex must be locked from other threads, when any thread that needs the resource. When the
data is no longer used / needed, the mutex is set to unlock.

We encounter cycle stealing in the context of Direct Memory Access (DMA). Either the DMA controller
can use the data bus when the CPU does not need it, or it may force the CPU to temporarily suspend
operation. The latter technique is called cycle stealing. Note that cycle stealing can be done only at specific
break points in an instruction cycle.

The process of packaging and sending interface method parameters across thread or process boundaries.

Daemon is a program that runs in the background without user's interaction. A daemon runs in a
multitasking operating system like UNIX. A daemon is initiated and controlled by special programs known
as ?processes?.

Preemptive scheduling: The preemptive scheduling is prioritized. The highest priority process should
always be the process that is currently utilized. Non-Preemptive scheduling: When a process enters the
state of running, the state of that process is not deleted from the scheduler until it finishes its service time.

The repeated execution of a loop of code while waiting for an event to occur is called busy-waiting. The
CPU is not engaged in any real productive activity during this period, and the process does not progress
toward completion.

Page swapping or page replacements are called page cannibalizing.

To achieve maximum efficiency and reliability a mode of operation known as symmetric multiprocessing
is used. In essence, with SMP any process or threads can be assigned to any processor.

It is the transfer of sufficient amount of the state of process from one machine to the target machine.

Primary memory is the main memory (Hard disk, RAM) where the operating system resides.

Secondary memory can be external devices like CD, floppy magnetic discs etc. secondary storage cannot
be directly accessed by the CPU and is also external memory storage.

Compaction is a process in which the free space is collected in a large memory chunk to make some space
available for processes.

Early operating systems were called residence monitors.

In order to protect the operating systems and the system programs from the malfunctioning programs the
two mode operations were evolved
System mode
User mode.

A list of processes waiting for a particular I/O device is called device queue.

Hard real-time systems required to complete a critical task within a guaranteed amount of time.
Soft real-time computing requires that critical processes receive priority over less fortunate ones.

Absolute path-- Exact path from root directory.
Relative path-- Relative to the current path.

Time taken for switching from one process to other is pure over head. Because the system does no useful
work while switching. So one of the solutions is to go for threading when ever possible.

Data registers - can be assigned to a variety of functions by the programmer. They can be used with any
machine instruction that performs operations on data.
Address 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 addresses.

Dynamic Ram stores the data in the form of Capacitance, and Static RAM stores the data in Voltages.

Local replacement means that an incoming page is brought in only to the relevant process' address space.
Global replacement policy allows any page frame from any process to be replaced. The latter is applicable
to variable partitions model only.

In batched operating system the users gives their jobs to the operator who sorts the programs according to
their requirements and executes them. This is time consuming but makes the CPU busy all the time.

SCSI - Small computer systems interface is a type of interface used for computer components such as hard
drives, optical drives, scanners and tape drives. It is a competing technology to standard IDE (Integrated
Drive Electronics).

The set of dispatch able processes is in a safe state if there exists at least one temporal order in which all
processes can be run to completion without resulting in a deadlock.

We encounter cycle stealing in the context of Direct Memory Access (DMA). Either the DMA controller
can use the data bus when the CPU does not need it, or it may force the CPU to temporarily suspend
operation. The latter technique is called cycle stealing. Note that cycle stealing can be done only at specific
break points in an instruction cycle.

The special thread a dispatcher will execute when no ready thread is found.

It is a fault tolerance disk driver for Windows NT.

Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this
involves: Switching context, Switching to user mode, Jumping to the proper location in the user program
to restart that program, dispatch latency ? time it takes for the dispatcher to stop one process and start
another running.

In message passing, it is the condition in which, both, the sender and receiver are blocked until the
message is delivered.

When the OS at the explicit request of another process creates a process, this action is called process
spawning.

1) swapping
2) interactive user request
3) timing
4) parent process request

1) Network redirector/ Server
2) Cache manager.
3) File systems
4) Network driver
5) Device driver

1) ability to support multiple processors
2) virtual storage
3) source level debugging

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