Hardware abstraction layer
Kernel
Subsystems
System Services.

Increased speed and memory capacity of microprocessors 
together with the support fir virtual memory and
Growth  of client server computing

Page swapping or page replacements are called page 
cannibalizing.

Yes.

With demand paging, a page is brought into memory only when 
a location on that page is actually referenced during 
execution. With pre-paging, pages other than the one 
demanded by a page fault are brought in.

 The selection of such pages is done based on common access 
patterns, especially for secondary memory devices.

For load-time dynamic linking: Load module to be loaded is 
read into memory. Any reference to a target external module 
causes that module to be loaded and the references are 
updated to a relative address from the start base address 
of the application module. 
With run-time dynamic loading: Some of the linking is 
postponed until actual reference during execution. Then the 
correct module is loaded and linked.

Placement algorithms determine where in available real-
memory to load a program. Common methods are first-fit, 
next-fit, best-fit. Replacement algorithms are used when 
memory is full, and one process (or part of a process) 
needs to be swapped out to accommodate a new program. The 
replacement algorithm determines which are the partitions 
to be swapped out.

If a process in a monitor signal and no task is waiting on 
the condition variable, the signal is lost. So this allows 
easier program design. Whereas in semaphores, every 
operation affects the value of the semaphore, so the wait 
and signal operations should be perfectly balanced in the 
program.

It is a technique proposed by Lamport, used to order events 
in a distributed system without the use of clocks. This 
scheme is intended to order events consisting of the 
transmission of messages.

Each system 'i' in the network maintains a counter Ci. 
Every time a system transmits a message, it increments its 
counter by 1 and attaches the time-stamp Ti to the message. 
When a message is received, the receiving system 'j' sets 
its counter Cj to 1 more than the maximum of its current 
value and the incoming time-stamp Ti.

 At each site, the ordering of messages is determined by 
the following rules: For messages x from site i and y from 
site j, x precedes y if one of the following conditions 
holds....(a) if Ti<Tj or  (b) if Ti=Tj and i.

Free memory is maintained in linked lists, each of equal 
sized blocks. Any such block is of size 2k. When some 
memory is required by a process, the block size of next 
higher order is chosen, and broken into two. The two such 
pieces differ in address only in their kth bit. Such pieces 
are called buddies. When any used block is freed, the OS 
checks to see if its buddy is also free. If so, it is 
rejoined, and put into the original free-block linked-list.

Seek time is the time required to move the disk arm to the 
required track. Rotational delay or latency is the time it 
takes for the beginning of the required sector to reach the 
head. 

Sum of seek time (if any) and latency is the access time. 
Time taken to actually transfer a span of data is transfer 
time

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.

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

a trap is a device which is used to detect , capture and 
harm to an intruder.

a trap door is a secret entry point into a program that 
allows someone who is aware trap door to gain access 
without going through the usual security access procedures.

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

In N-Buffer system when sender is sending the message to 
reciever and the buffer is full then at that situation 
sender and receiver both r blocked untill receiver deliver 
the message and any buffer slot is free.

Load Sharing: Processes are not assigned to a particular 
processor. A global queue of threads is maintained. Each 
processor, when idle, selects a thread from this queue.  

Gang Scheduling: A set of related threads is scheduled to 
run on a set of processors at the same time, on a 1-to-1 
basis. Closely related threads or processes may be 
scheduled this way to reduce synchronization blocking, and 
minimize process switching. Group scheduling predated this 
strategy.

Dedicated processor assignment: Provides implicit 
scheduling defined by assignment of threads to processors. 
For the duration of program execution, each program is 
allocated a set of processors equal in number to the number 
of threads in the program. Processors are chosen from the 
available pool.

Dynamic scheduling: The number of thread in a program can 
be altered during the course of execution.