What is the difference between Hard and Soft
- A hard real-time system guarantees that critical tasks
complete on time. This goal requires that all delays in
the system be bounded from the retrieval of the stored
data to the time that it takes the operating system to
finish any request made of it. A soft real time system
where a critical real-time task gets priority over other
tasks and retains that priority until it completes. As
in hard real time systems kernel delays need to be
What is the cause of thrashing? How does the system
Once it detects thrashing, what can the system do to
eliminate this problem? - Thrashing is caused by under
allocation of the minimum number of pages required by a
process, forcing it to continuously page fault. The
system can detect thrashing by evaluating the level of
CPU utilization as compared to the level of
multiprogramming. It can be eliminated by reducing the
level of multiprogramming.
What is multi tasking, multi programming, multi
Multi programming: Multiprogramming is the technique
of running several programs at a time using timesharing.
It allows a computer to do several things at the same
time. Multiprogramming creates logical parallelism. The
concept of multiprogramming is that the operating system
keeps several jobs in memory simultaneously. The
operating system selects a job from the job pool and
starts executing a job, when that job needs to wait for
any i/o operations the CPU is switched to another job.
So the main idea here is that the CPU is never idle.
Multi tasking: Multitasking is the logical extension of
multiprogramming .The concept of multitasking is quite
similar to multiprogramming but difference is that the
switching between jobs occurs so frequently that the
users can interact with each program while it is
running. This concept is also known as time-sharing
systems. A time-shared operating system uses CPU
scheduling and multiprogramming to provide each user
with a small portion of time-shared system. Multi
threading: An application typically is implemented as a
separate process with several threads of control. In
some situations a single application may be required to
perform several similar tasks for example a web server
accepts client requests for web pages, images, sound,
and so forth. A busy web server may have several of
clients concurrently accessing it. If the web server ran
as a traditional single-threaded process, it would be
able to service only one client at a time. The amount of
time that a client might have to wait for its request to
be serviced could be enormous. So it is efficient to
have one process that contains multiple threads to serve
the same purpose. This approach would multithread the
web-server process, the server would create a separate
thread that would listen for client requests when a
request was made rather than creating another process it
would create another thread to service the request. To
get the advantages like responsiveness, Resource sharing
economy and utilization of multiprocessor architectures
multithreading concept can be used.
What is hard disk and what is its purpose?
Hard disk is the secondary storage device, which holds
the data in bulk, and it holds the data on the magnetic
medium of the disk. Hard disks have a hard platter that
holds the magnetic medium, the magnetic medium can be
easily erased and rewritten, and a typical desktop
machine will have a hard disk with a capacity of between
10 and 40 gigabytes. Data is stored onto the disk in the
form of files.
What is fragmentation? Different types of fragmentation?
- Fragmentation occurs in a dynamic memory allocation
system when many of the free blocks are too small to
satisfy any request. External 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. If too much
external fragmentation occurs, the amount of usable
memory is drastically reduced. Total memory space exists
to satisfy a request, but it is not contiguous. Internal
Fragmentation: Internal fragmentation is the space
wasted inside of allocated memory blocks because of
restriction on the allowed sizes of allocated blocks.
Allocated memory may be slightly larger than requested
memory; this size difference is memory internal to a
partition, but not being used
What is DRAM? In which form does it store data?
- DRAM is not the best, but itís cheap, does the job,
and is available almost everywhere you look. DRAM data
resides in a cell made of a capacitor and a transistor.
The capacitor tends to lose data unless itís recharged
every couple of milliseconds, and this recharging tends
to slow down the performance of DRAM compared to
speedier RAM types.
What is Dispatcher?
- 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 Ė time it takes
for the dispatcher to stop one process and start another
What is CPU Scheduler?
- Selects from among the processes in memory that are
ready to execute, and allocates the CPU to one of them.
CPU scheduling decisions may take place when a process:
1.Switches from running to waiting state. 2.Switches
from running to ready state. 3.Switches from waiting to
ready. 4.Terminates. Scheduling under 1 and 4 is
non-preemptive. All other scheduling is preemptive.
What is Context Switch?
- Switching the CPU to another process requires saving
the state of the old process and loading the saved state
for the new process. This task is known as a context
switch. Context-switch time is pure overhead, because
the system does no useful work while switching. Its
speed varies from machine to machine, depending on the
memory speed, the number of registers which must be
copied, the existed of special instructions(such as a
single instruction to load or store all registers).
What is cache memory?
- 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.
What is a Safe State and what is its use in deadlock
- When a process requests an available resource, system
must decide if immediate allocation leaves the system in
a safe state. System is in safe state if there exists a
safe sequence of all processes. Deadlock Avoidance:
ensure that a system will never enter an unsafe state.
What is a Real-Time System?
- 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
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