Here we are giving chapter wise notes of operating system based on operating system concepts by galvin





these are listed under the fallowing headings::-






Introduction

Computer System Stuctures

O S Structures

CPU Scheduling

Memory

Process

Treads

Virtual Memory

Galvin Operating system

6:33 AM

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Here we are giving chapter wise notes of operating system based on operating system concepts by galvin

Virtual Memory

q) how virtual memory is different from other memory management techniques?
a) it allows the execution of the process that need not be comletely in memory.

q) what are its advantages?
a) the process may be larger than the physical memory.allows processes to easily share files and address space.

note:- entire program is not needed always, onlysometimes:-
1) code that handles exceptions.
2) arrays ,lists,tables are often allocated more memory, than actually needed.

benefits of keeping in memory only the required instructins and data:-
1) more programs can be there in memory, more cpu uilization and throughput.
2) less I/O required to load or swap the processes.

note:- systems that support virtual memory, donot bother programmer to design overlays.

q) what is key concept behind virtual memory?
a) demand paging. demand segmentation can also be used, but segment replacement algorithms are very complex as they have variable sizes.

note:- here we can use pager instead of swapper.now a process needs to be in memory ,before its execution starts.but now instead of bringing all pages into memory, pager does not bring a page unless it is required.in begining pager makes a guess,which pages will be used and loads only those pages into memory, thus decreasing the swap time, and memory needed.

q) how we modify page table to know that which pages are in memory and which are not?
a) help of valid-invalid bit is taken.if a bit is valid, it means it is in logical address space and also is present in memory, if its invalid , it means either the page in not a valid logical address or right now its on disk and not in memory.

note:- page table entry for page on disk , contains invalid bit, or address of page on disk.

q) what is page - fault trap?
a) access to page that is currently not in memory is called page-fault.mapping hardware then sends trap to the o.s.

q) what is pure demand paging?
a) this concept says that never bring a page into memory until it is required.a process could start with no page in the memory.

q) what is swap space?
a) the portion of disk that contains the pages not in memory.

note:- if a page fault occurs when fetching a instruction , it needs to be fetched again. if page fault occurs when fetching an operand, then fetch and decode of instruction need to be done again.

note:-trying to increase degree of multiprogramming can increase the page fault rate.

q) what is modify/dirty bit?
a) when a page fault occurs and no free frame is there, we need to swap out a victim page and bring in the desired page.this takes double time to service page fault ie swapin+swapout. to reduce it we use modify bit.it is attached to each frame in physical memory. if frame is modified it is set. in this case it needs to be written back to memory. but if not changed only we swap in desired page overwritting it.read only pages help a lot in reducing the page fault service time, if they are the victims.

q) what do frame allocation algorithms do?
a) decide how many frames are to be allocated to a process.it also has a lot of affect on the page fault rate.

q) what is refernce string?
a) the string of memory references is called reference string.

q) how to implement FIFO page replacement algorithm?
a) way1) with each page in memory associate the time, when it came in, then remove the oldest page.
way2) make a FIFO queue of pages and remove 1 at the head.

q) what is belady's anomaly?
a) FIFO(some others are also there) suffers from belady's anomaly. here the number of page faults increase even with allocation of more frames to the process.

note:- of all the page replacement algorithms OPR has least page fault, though not practically possible.

q) what is logic of OPR?
a) remove the page that will not be used for the longest period of time.

q) what is logic of LRU?
a) here the page that has not been used for a long period of time is replaced. it is approximation to OPR.with each page its time of last use is attached.

q) implementation of LRU?
a) 1) counters:- with each page we have time-of-use field and we add to cpu a clock or counter, which is incremented for each memory reference.contents are copied to the referenced page.here we need to search page with least count. here we write to memory, and also counter is to be maintained to prevent overflow.

2) stack:- stack of page numbers. a page when referenced is moved to the top of stack.tail pointer points to LRU page easy to implement using doubly linked list.

note:- OPT and LRU donot suffer from belady's anomaly.

q) what are stack algorithms?
a) these never exhibit belady's anomaly.these are algorithms for which the set of pages in memory for n frames is always the subset of set of pages for n+1 frames.

note:- these algorithms slow down the process execution as for each memory reference , there is a need to update the timer or stack structures.

q) what are referenece bits?
a) its a technique used to assist LRU. with each page we associate a reference bit.initially all bits are cleared to 0 by O.s when it is referenced the hardware sets it to 1. so we can know which pages have been used and which have not been.

q) what additional feature added to reference bit?
a) now with each page a byte is attached ie 8 bits.it stores history.every interval(set by hardware), the refernce bit is set to high order bit, and all other bits are moved 1 postion right. smallest number is LRU.if same value FIFO is used.

q) what is 2nd chance alghorithm?
a) here only 1 refrence bit is used. if page is to be replaced it's reference bit is examined.if 0 it is replaced.if not, then it gets a 2nd chance ie bit is cleared and arrival time is set to current time, and moves further. it can also be implemented using the circular queue,where page with 0 reference bit is searched. if all are set, it acts as FIFO.

note:- we can also enhance the 2nd chance algorithm by using modify bit also along with. now we get 3 classes here:-
(0,0),(0,1),(1,0),(1,1).
case 1 : neither recently used nor modified- best page to replaced.
case 2 : not recently used but modified - not quite as good, page need to be written out on the disk before replacement
case 3 : recently used but not modified - probably will be used again soon
case 4 : recently used and modified - probably will be used again soon.

q) what are counting based page replacement algorithms?
a) here for each page , count the number of refernces made to it.
2 types are there:-
1) LFU:- page with low count is removed.
disadvantage:- a page that was used heavily in begining, but not needed now remains in memory.

2) MFU:-it argues that page with small count has just been brought in , and yet to be used.

note:- the minimum number of frames to be allocated to a process is also determined by the instruction -set architecture.ie enough pages must be there that the single instruction can reference.

note:- limit has been put on the levels of indirection to support virtual memory in context of minimum number of frames to be allocated to a process.

q) what is meant by equal allocation?
a) if we have m free frames and n processese, give each process m/n frames.

q) what is proportional allocation?
a) here we allocate number of frames to each process as s(i)/S *m; where S is total sum of virtual memory for each process. s(i) virtual memory for individual.

note:- we also need to consider the priority of the process while allocating frames rather than size.

q) what is global replacement and local replacement?
a) global replacement allows to select replacement frame from all frames even allocated to other processes.local replacement requires each process select replacement frame from its own set of allocated frames.

note:- higher priority process can select replacement frame from its own set or from lower proirity processes.

q) what is disadvantage of global page replacement?
a) a process cannot control its page faults.

note:- if number of frames allocated to a process fall below the minimum required by computer architecture, then process need to be swapped out completely.

q) what is thrashing?
a) its high paging activity, where process spend more time paging than execution.

note:- thrashing occurs because of over multiprogramming and global replacment.all processes are in pager device queue than ready queue.

q) how to prevent thrashing?
a) by allocating the process the number of frames it need. for this we use the locality model of process execution.

q) what is locality?
a)it is set of pages that are used together.program structure defines locality. eg:- a procedure its instructions,variables global variables form a locality.

note:- if fewer frames than its locality are assigned then the process will thrash.

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Galvin Operating system

6:30 AM

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Here we are giving chapter wise notes of operating system based on operating system concepts by galvin


q) what is a thread?
a) it is a lightweight process, a basic unit of cpu utilization. it contains a thread id,program counter,register set and
stack.

q) what do a thread share with other threads of same process?
a) code section and data section.

note:- if a process has more than 1 thread it can do more than 1 task at a time.

q) give example of application with multiple threads?
a) web browser:- 1 thread display images ot text while other retrieves data from network.
word processor:- display graphics,read keystrokes and third one perform grammar and spelling check.

q) what are 2 kinds of threads?
a) 1) user threads and 2) kernel threads.

q) how are user thread implemented?
a) thread libraray.library gives support for thread creation, scheduling and management with nO help from kernel.

q) what is many to one model of threads?
a) there are many user-level threads but only 1 kernel thread.eg:- green threads(a thread library) for Solaris2.

q) what is 1-1 model?
a) each user thread is mapped to a kernel thread.here a user thread creation requires a corresponding kernel thread also.
there is a restriction on the number of threads suported by system. windows nt,2000 use this model.

q) what is a many-many model?
a) many user-level threads mapped to smaller or equal number of kernel threads.

q) how many kernel threads we can have in many-many model?
a) depends either on the particular application or a particular machine.

q) in case of multithreaded program, how can a fork behave?
a) 1) IT DUPLICATES all threads of its parent.
2) it is single threaded.

q) what is thread cancellation?
a) it is the task of terminating a thread before its completion.

q) any example of thread cancellation?
a) 1) if multiple threads are searching through database and 1 returns the result,remaining are cancelled.
2) if u click stop button in web browser,it stops web page from loading any further.
the thread that is to be cancelled is called the target thread.

q) what are 2 different kinds of thread cancellation?
a) 1) asynchronuos:- one thread immidiately terminates the target thread.
2) deffered:- target thread checks itself periodically to check if it should terminate.

q) what is difficulty in cancellation?
a) in situations where resources have been allocated to the cancelled thread, or if thread is cancelled in middle of
updating data it is sharing with other threads. in case of asynchronous cancellation it may occur that o.s is not able to
reclaim all resources from the thread.

q) what is signal used for ?
a) used in unix systems to notify a process that a particular event has occured. it may be recieved synchronously or
asynchronously depending on source and reason for event being signalled.

q) tell about synchronous signal?
a) it occurs when illegal memory access is there or division by 0 takes place.synchronous are delived to same process that
performed the operation causing signal.

note:- asynchronous signal is sent to other process.

q) what are various signal handlers?
a) 1) default signal handler.
2) user-defined -do-.

note:- take example of web server. it creates a new thread when a new request comes to it.if more requests are there, then
more threads may exhaust the system resources.to solve this we have thread pool.

q) what is concept of thread pool?
a) at time of process startup, create a number of threads and place them in pool to wait for work.they finish work and then
come back to pool. if request come and no thread is available, then request has to wait.

q) what is thread specific data?
a) each thread share data of process, but sometimes it may need its own copy of data. such data is called thread specific
data.

q) what are Pthreads?
a) it refers to a POSIX standard defining an API for thread creation and synchonization. it is specification for thread
behaviour not its implementation. o.s developer can use these specifications in any way he likes.generally libraries
implementing Pthreads are confined to the unix-based system like Solaris2.

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Galvin Operating system

6:25 AM

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Here we are giving chapter wise notes of operating system based on operating system concepts by galvin

PROCESS


q) what is a process?
a) a program in execution.

q) what is difference b/w the program and process?
a) program is passive entity,such as contents of file on disk. whereas process is a active entity, with program counter
indicating next instruction to be executed, and has set of associated resources.

q) how many states can a process be in?
a) 1)new 2)running 3)waiting 4)ready 5)terminated.

q) what is process control block?
a) each process in o.s is represented by process control block. it contains all information related to process. it contains:-
1) process state
2) program counter
3) cpu registers:- state of process is saved
4) cpu scheduling information:- process priority, pointers to scheduling queues.
5) memory management information:- limit and base registers, page tables.
6) accounting information:- amount of cpu and real time used.
7) i/o status information:- list of i/o devices allocated and list of open files.

q) what is ready queue?
a) processes that are in memory and are ready and are waiting to get executed are kept on list called ready queue, generally
implemented as linked list.

q) what is there in the ready queue?
a) process control blocks.

note:- there is device queue also for each device.

q) what is queueing daigram?
a) it is used to represent process scheduling,ie rectangles for queues,circles for resources etc.

q) how many types of schedulers are there?
a) in batch system, more processes are submitted than can be executed. they are spooled to disk. now we have long-term-
scheduler that selects process from the pool and load into memory. cpu or short-term scheduler selects process from memory
to be given cpu.cpu scheduler needs to take decision fast.

q) when is long term scheduler invoked?
a) when a process leaves the system.

note:- long-term scheduler must send proper mix of i/o bound and cpu bound processes to the memory.

q) which systems donot have long term scheduler?
a) time sharing systems like unix. new process is put in memory for cpu scheduler.

q) what is medium level scheduler?
a) it brings in the concept of swapping.

q) what is context switch?
a) to switch cpu to another process, needs saving state of old process and loading saved state of new process.

q) what is system call to create a new process?
a) create-process.

note:- child processes may get different resouces from its parent process or they may share resources.

q) in unix , how to create new process?
a) each process has a pid, and new process are created by fork system call.

q) what are types of concurrent processes in the system?
a) they may be independent(do not share data) or may be cooperating(share data).

q) why process cooperation is needed?
a) 1) information sharing:- shared file.
2) computation speed up:- task broken into subtasks.here we need multiple processing elements.
3) modularity

q) give examples of the producer consumer problem?
a) print program produce characters to be used by printer driver.compiler produces assembly code for assembler.

note:- a synchonization is required between producer and consumer. a buffer is required in between shared by both.

q) where do we get buffer from?
a) may programmer need to code it , or provided by o.s through use of IPC facility.

q) what is IPC?
a) it is inter process communication facility. it allows processes to communicate without sharing same address space.it is
mostly used in distributed systems. it is provided by message passing system. ex: chat program on WWW.

q) where is message passing used in computer?
a) in b/w micro kernels.

q) what operations do IPC provide?
a) at least 2 ie send(message) and recieve(message).

q) what is direct communication?
a) here we have:-
1) send(P,message):- send a message to process P.
2) recieve(Q,message):- receive a message from process Q.

q) what is indirect communication?
a) here messages are sent to and recieved from mailboxes,or ports.2 processes need to share a mailbox. it works as:-
1) send(A,message):- send a message to mailbox A.
2) recieve(A,message):- receive a message from mailbox A.

note:- mailbox can be owned by a process or by operating system.in second case o.s allows processes to create,send and
recieve and then delete a mailbox.

q) how many ways of message passing?
a) it can be blocking and non-blocking, also called as synchronous and asynchronous. it can be:-
1) blocking send
2) non blocking send
3) blocking recieve
4) non blocking recieve


q) what is rendezvous?
a) when both send and recieve are blocking ,then there is rendezvous b/w sender and reciever.

q) what is buffering here?
a) messages exchanged b/w processes reside in temporary queue. this queue can be implemented in 3 ways:-
1) 0 capacity:- no message should wait,so sender need to get blocked till recipient recieves.
2) bounded capacity:- sender need to block if queue is full.
3) unbounded capacity:- sender never blocks.

q) what is socket?
a) it is defined as end point for communication. processes communicating over network employ it.it is made up of IP address
concatenated with port number.
in genaral sockets use client-server architecture.

q) what are the ports that server listens to provide certain services?
a) telnet server listens to port 23
ftp server listens to port 21
http server listens to port 80.

note:- all ports below 1024 are used for standard services.

q) when a client process request for connection, what happens?
a) it is assigned port number >1024 by host computer. then connection will cosist of pair of sockets. then packets travel .
b/w client and server.

note:- processes on same host, when try to get connected to same server ,it would be given unique port number. all
connections must be unique.

q) how many types of sockets , java provide?
a) 3 types:-
1) connection oriented(TCP) :- implemented with Socket class.
2) connectionless(UDP) sockets use DatagramSocket class.
3) MulticastSocket class allows data to be sent to multiple recipients.

q) what is a thread?
a) it is also called light weight process. it consist of thread id,program counter,register set. it shares with other
threads belonging to same process code,data section and other o.s resources. traditional heavy weight process has single
thread of control. if a process has multiple threads,it can do more than 1 task at a time.

q) give an example of thread?
a) a word processor may have a thread for displaying graphics, another for reading keystrokes, third for performing
spelling and grammar check.they also play an important role in remote procedure calls(RPC).

note:- instead of process creation on web server to handle a client a thread is created to service request.


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Galvin Operating system

6:23 AM

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Here we are giving chapter wise notes of operating system based on operating system concepts by galvin

MEMORY

q) on what do scheme for memory management depends?
a) hardware.

note:- memory can be viewed as the large array of bytes , each having an address.

q) what is the input queue?
a) collection of processes on the disk , waiting to be brought into the memory for execution , forms the input queue.

note:- compiler maps the symbolic address of the program into the relocatable address.loader convert it into physical address

q) different types of binding of instructions and data to memory address?
a) 1) compile time:- if we know at compile time where the program will reside in memory.if location changes, we need to
recompile it. eg:- MS-DOS .COM -format programs use this technique.
2) load time:-
3) execution time:- if process is to be moved in memory during execution time,run time binding is used.

q) what is logical address space and the physical address space?
a) address generated by cpu is logical address and address seen by memory is physical address. set of addresses is called
address space.

note:- in case of compile time and load time binding , the logical address and physical address are the same, whereas in
run time they are not.

q) what is memory management unit?
a) run time logical to physical address mapping is done by MMU.

q) what is concept of dynamic loading?
a) here the routinue is not loaded into memory until it is called.only main program is loaded.after call, routinue is loaded
and program's address table is also updated.to take advantage of this concept, programmer has to take proper care,not the
responsibilty of the O.S.

q) what is there in linking?
a) here the other modules and system libraries are included into the program image.

q) what is dynamic linking?
a) here all the system libraries or atleast the routines referenced in them are linked only when call is made at run time.

q) what is a stub?
a) it is used to support the dynamic linking.it is included for each library routine reference.it is a small piece of code,
that tells how to locate the library-routine in memory or how to load if routine not present.
** its advantage is that all the processes use only 1 copy of the routine.

q) what is concept of overlays?
a) it enables the process to be larger than the memory allocated to it. only required instructions and data are to be kept in the
memory. when other instructions are needed , they are brought in place of previous data, no longer needed.
eg:- 2 pass assembler.

note:- an overlay driver is used that reads code into memory.it also is responsibility of programmer to take advantage of
this concept.programmer only designs the overlay, o.s only do I/O here.

q) what is swapping?
a) it is process of shifting a process to backing store(swap out), and bringing a process to memory(swap in).

q) what is rollin and roll out?
a) it is variant of swapin and swap out. it is done when a higher priority process wants to come into the memory.

q) do a process swapped out , needs to be swapped in , in the same location?
a) in case of compiletime and loadtime binding---yes, but not in case of run time address binding.

q) when a process can't be swapped out?
a) if it's not idle.if a process is waiting for the I/O ,and I/O is asynchronously accessing the user memory for I/O buffers,
then it can't be swapped out.

**** in case of contigous memory allocation:-

note:- memory divided into 2 parts:-
1) o.s 2) user programs.

q) how to provide memory protection?
a) using a relocation and limit register.

note:- logical address is first compared with the limit register.

q) what is transient operating -system code?
a) o.s contains code and buffer space for devicedrivers. if device driver not commonly used,no need to keep the code and
data in memeory. such code that comes and goes as needed is called transient o.s.

q) what is MFT?
a) it is multiprogramming with fixed number of tasks. here the memory is divided into fixed sized of partitions. in 1
partition 1 process can reside at a time.it was used by IBM OS/360, now not in use.

q) what is MVT?
a) it is multiprogramming with variable number of tasks.primarily used in batch environment.

q) how MVT is implemented?
a) OS maintains a table indicating which parts of memory are free and which are allocated.

q) what is a hole?
a) free part of memory is called a hole.

q) how dynamic storage allocation problem solved?
a) it relates to how to satisfy the request of size n from the available holes.many strategies are there:- first fit, best
fit,worst fit.

q) what is the disadvantage of these algorithms above?
a) they suffer from external fragmentation.

q) what is external fragmentation?
a) it exists when we have total free memory to accomodate a process, but it is not contiguous.

q) what is internal fragmentation?
a) when physical memory is broken into fixed sized partitions.here the process may be smaller than the size of the partition.
so rest of space in partition is wasted. it is called internal fragmentation.

q) what is compaction?
a) it is one of the solution to the external fragmentation.here the free memory is moved to one part and all pragrams and
data on other side. this can be done if address binding is done at run time. it involves a lot of cost also.

note:- other solution to external fragmentation is paging and segmantation(here program need not to be in contiguous memory)

q) what is paging?
a) it allows the physical address space of the process to be non-contiguous. earlier when processes used to be swapped out
we need to search for free space on disk(backing store) too. so, external fragmantation on disk also occured.

q) concept of paging?
a) physical memory is broken into fixed sise blocks called frames.logical memory is broken into pages of same size of frames.
backing store is also divided into the blocks of size of frames.
when a process is to be loaded , its pages must be loaded into the frames.

q) now how cpu generates address?
a) now address has 2 parts:- p and d. p is the index into the page table, from where we get the frame number, which combines
with d part to form physical address.

q) how to decide how many bits for p and d?
a) page size is defined by hardware.this size is generally the power of 2.if size of logical address space is pow(2,m) and
the page size is pow(2,n), then higher order m-n bits will tell the page number and the n lower order bits will tell the
page offset.

note:- in paging we dont have external fragmentation, but we do have internal fragmentation.

q) so, can we have small page size to reduce the internal fragmentation?
a) but then the page table will grow in size , and overhead to maintian will increase.also with smaller page size disk I/O
will not be efficient.

note:- when a process arrives in the system , its size in pages is expressed.

q) what is frame table?
a) it is a table maintained by the O.s, which has 1 entry for each frame of physical memory.it tells whether a frame is free
or not, if not, then to which process it is allocated.

note:- a system call is made,suppose for I/O, then buffer address is passed as the parameter.

note:- for each process there is a page table.

q) do paging increase context switch time?
a) yes, because dispatcher has also to define hardware page table.

note:- the page table pointer is also stored in the process control block.

q) how to implement page table?
a) way 1) use of dedicated set of registers:- dispatcher needs to load these registers , in context switch.
this method is not feasible if page table has millions of entries.
way 2) here we store the page table in the main memory and PTBR stores its address.here context switch time is less
but here to make a access in memory , 2 memory accesses are required.

q) what is TLB(translation look-aside buffer)?
a) it is a high speed associative memory, with 2 fields tag and value.upto 60 to 1024 entries it can handle.it will contain
only few of the page table entries.

q) what if a search in TLB makes a miss?
a) then the reference to memory page table is made.the new entry is also added to the TLB. if no entry free, 1 need to be
replaced.

q) what do u mean that some entries in TLB are wired down?
a) it means that they cannot be removed from there.entries for kernel code are wired down.

q) what are protection bits?
a) they are stored into the page table only and they indicate during address mapping, that the particular page is read-write
or read-only.

q) what is valid-invalid bit?
a) it is 1 more bit attached to the page table entry.valid bit indicates that this page is in process's logical address space
and thus legal, and invalid bit indicates that the page is not in the logical address space of the process.they cause trap to
O.S.

note:- advantage of paging is sharing common code.it can be done by setting entry in the page table.

q) what is reentrant code?
a) that never changes during the execution. eg:- code of a text editor.it can be executed by more processes at same time.

q) name some programs that can be shared?
a) compilers,database-systems, run-time libraraies,etc. code shared must be reentrant.

q) what is segmentation?
a) here the user view of memory is as the collection of various segments.each segment has a name and length.the address here
consists of the segment number and the offset.
.

note:- when a user program is compiled, the compiler itself constructs various segments.

q) who does mapping here?
a) segmenation table, each entry here has base and limit of segment. here logical address has 2 parts , segment number and
offset, offset is compared against the segment limit.

q) what are advantages of segmentation?
a)1) main, functions,procedures, global variables,stacks,arrays,tables etc are various segments.because of segment containing
entries of same type,protection can be easily provided.
2) sharng of code and data can be done. only need to change process's segment table.

note:- in case of segmentation, the care must be taken while sharing the code segment, as they refer to themselves. all
sharing processes need to have same segment number for that code segment. this problem is not there in case of read only
segments.

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Galvin Operating system

6:21 AM

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Here we are giving chapter wise notes of operating system based on operating system concepts by galvin
CPU Sheduling

q) what is idea behind multiprogramming?
a) a process is executed till it has to wait, generally for I/O request.then cpu given to some other process.

note:- the process execution consists of series of cpu and i/o bursts, starting and ending with cpu burst only.

q) when a process can go from running state to ready state directly?
a) when an interuppt occurs.

q) what is non-preemptive scheduling?
a) here once cpu is given to a process, then the process release it only if it terminates ot need to do i/o. windows3.1 is
eg.

q) what are cases to be considered in case of preemptive scheduling?
a) (we here may need timer)
case1:- a process in middle of updating a data is preempted, other process after it tries to use that inconsistent data.
case2:- when kernel data structures are inconsistent, then preemption should not take place.

q) what is a dispatcher?
a) it gives control of cpu to a process selected by cpu scheduler. its functions are:-
1) context switch.
2) switch to user mode.
3) jump to proper location in program to restart it.

q) what is dispatch latency?
a) time taken by dispatcher to stop 1 process and start another.

q) what is throughput?
a) it is defined as the number of processes completed per unit time.

q) what is response time?
a) it is time between submission of process to first response is produced.

note:- fcfs can be implemented using a queue.when a process is given cpu, then its pcb is removed from queue.

q) what is gantt chart?
a) it is used to find waiting time in case of cpu scheduling algorithms.

q) what is convoy effect?
a) it occurs in case of fcfs scheduling.it occurs as small processes wait in queue for big process to leave cpu.

note:- fcfs is non-preemptive, hence not at all suitable for time-sharing systems.

q) what is key logic of shortest job first scheduling algorithm?
a) here algorithm associates with each each process latter's next cpu burst. cpu is given to process with smallest next cpu
burst.

q) what is difficulty with SJF?
a) knowing the length of next cpu burst. it is generally used in long term scheduling.

q) can we implement it at the cpu level?
a) it is difficult to know next cpu burst length. to implement it cpu burst time is predicted on the basis of the previous
ones.

q) what is nature of SJF?
a) it can be preemptive(SRT) as well as non-preemptive.

q) what is priority scheduling?
a) here with each process a priority is attached, and process with highest priority is selected for cpu.

q) how to define these priorities?
a) can be defined internally or externally:-
*internally:- by factores like memory requirements,number of open files,ratio of average i/o burst and cpu burst.
*externally:- importance of process,funds paid, political factors.

note:- it can be preemptive or non-preemptive.

q) what is indefinite blocking or starvation?
a) a system where stream of high priority processes comes continously, then a low priority process in queue, keeps waiting
indefinitely.

q) what is solution to this?
a) solution is aging.here we keep on increasing the priority of a process waiting for long time after some fixed time, till
it becomes high priority process and gets executed.

q) what is RR scheduling?
a) it is special for time sharing systems.here timer also is set before process execution starts.

note:- the time unit above is called time quantum or time slice.

time quantum must be large than the context switch as context switch time is included in the time slice.

q) where we can use multilevel-queue-scheduling?
a) where we can classify the processes easily.some common categories are:- interactive processes,background(batch) processes,
system processes.

q) what is logic for multilevel queues?
a) separate processes in separate queues, each having its own scheduling needs.like interactive processe need RR and batch
processes can be done using FCFS.

note:- now ready queue is splitted into separate queue,on basis of type of process, and there must be scheduling among queues
also.

q) what is technique used in Solaris2?
a) higher order queues need be done before a lower level queue.if control in lower queue and a process come in higher order
, preemtion is required.

note:- time slice can also be used between the queues.

q) how is multilevel-feedback-queue scheduling different from multilevel-queues?
a) here the processes can move between the queues. here logic is to separate processes on basis of cpu burst.if process uses
too much cpu time, it is moved to lower queue.

q) what we do for load sharing in multi-processor scheduling?
a) use only 1 queue and schedule on to available processor. here each processor can select a process from queue for itself.
or a master-slave relationship is used.here 1 processor schedules other processors.other processors only execute user-code.


q) how process is submitted in hard real-time systems?
a) here process is given along with statement of amount of time it needs to complete or do I/O. here the scheduler then
admits process with guarantee that it will complete on time or it will reject it as impossible. it is called resource
reservation.

note:- here the scheduler must know that how much time each o.s function takes.

q) how to keep dispatch latency low in case of soft-real time processes?
a) to preempt system calls.in long duration system calls, preemption points are introduced,where check is made to see if
higher priority process wants to run. preemption points need to placed at safe positions. but only few preemption points
can be added to a system process.

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Galvin Operating system

6:19 AM

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Here we are giving chapter wise notes of operating system based on operating system concepts by galvin

O.S STRUCTURES

q) what if 2 processes are associated with same program?
a) they are considered separate.

q) tell some impotant funcions of o.s?
a) process management,main memory management,file management,i/o system management, secondary storage management.

q) which protocol needs a clients to login before using server's resources and which does not need this?
a) that require log in are:- ftp,nfs(network file system).
do not require log in:- http

q) what is protection?
a) it is a mechanism for controlling access of programs,processes to resources defined by computer system.

q) what is command interpreter?
a) it is one of the most important system program of o.s, which is interface b/w user and o.s .some o.s include it in their kernel,others run it when job is initiated.

q) what is shell?
a) a program that reads and interprets control statements(commands given to o.s) is executed automatically.this program is called command line interpreter or shell.

q) tell some important o.s services?
a) it provides environment for execution of programs. some services are:-
1) program execution:-
2) i/o operations:- user can't control these operations, o.s need to carry out these.
3) file-system manipulation:-
4) communications:- it may be implemented by shared memory or message passing.
5) error detection:-
6) resource allocaton:-
7) accounting:- it may be used to bill users or by researchers to reconfigure the system.
8) protection:-

q) what are system calls?
a) these provide inteface b/w the process and o.s, and are generally available as assembly language instructions.

q) name some languages where u can make system calls directly?
a) c,c++,perl,assembly language.

q) give example of certain system calls?
a) display message on screen,open files, closing files, terminate normally or abnormally,read from or write to file,etc.

note:- we need to specify certain information that will be passed to o.s as parameter with system calls.

q) what are the various methods to pass parameters to o.s by system calls?
a) there are 3 methods:-
1) pass parameters in registers.
2) if parameters more then parameter stored in block in memory and address to block passed as parameter in register (linux use this).
3) some use stack.

q) what are different categories of system calls?
a) process control,file management,device management,information maintainance and communications.

q) in case of processes tell a common system call?
a) create process.

q) name some system calls dealing with the coordination of system processes?
a) wait time,wait event,signal event.

q) name for file system calls?
a) create,delete,read,write,reposition,get/set file attributes.

note:- device also behave as files only, system calls like request,release,read,write etc are there.

q) name some information maintainance system calls?
a) system call that return date and time,number of current users,version of o.s,about all processes etc.

q) system calls in case of communications?
a) get hostid,get processid. what these 2 return are passed as parameters to openconnection,close connection calls.then we have read message ,write message.

q) what are deamons?
a) most processes that will be recieving connections are special purpose deamons(system programs). they execute wait for connection call and gets awakened when connection is made.

q) what are system programs?
a) these are generally user interfaces to system calls.

q) how many types of system programs are there?
a) 1)file management:- these programs create ,delete,copy,rename,manipulate files and directories.
2)status information:- date,time,number of users,available memory etc.
3)file modification:-
4)programming language support:- compilers,assemblers,interpreters.
5)loaders,linkage editors.
6)help to browse web pages,send e-mails,send messages to other's screen,file transfer etc.

note:- most important system program for o.s is command interpreter.

q) in how many ways the commands can be implemented in o.s?
a) 2 ways:-
1)code for command is within the interpreter program.we can jump to that part , set parameters and make appropriate system call.
2)this is used by unix. command only tells name of file to be loaded and executed.parameter is passed to it.

note:- unix system consist of 2 parts:- kernel and system programs.

q) what is there in kernel?
a) kernel has device drivers, it provides file system,cpu scheduling,memory management and other functions through sytem calls.

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Galvin Operating system

6:17 AM

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Here we are giving chapter wise notes of operating system based on operating system concepts by galvin

COMPUTER SYSTEM STRUCTURES

q) tell about generaL purpose computer system?
a) it consists of cpu and number of device cntollers connected through common bus, that provides access to memory.

q) what is job of memory contoller?
a) to ensure orderly access to shared memory,ie provides synchonization.

q) when computer is powered which is first program that runs?
a) bootstrap loader(present in ROM). it loads o.s into memory .

q) what operating system do then?
a) it then start executing the first process, such as "init" and waits for some event to take place.

note:- modern o.s are interuppt driven.interuppt by hardware is called interuppt only.software do this by executing a special operation called system call.

q) what is interrupt vector?
a) given only predefined number of interrupts, a table of pointers to interrupt routines can be used . it is placed in low memory. this table of pointers is called interrupt vector.it is indexed by unique device numbers.

q) name some o.s that use interrupt vector scheme to dispatch interrupts?
a) ms-dos and unix.

q) what do device contoller has generally?
a) buffer storage and set of special registers.

q) give an example of interrupt by i/o device?
a) suppose cpu asked device controller to perform read operation. after read operation is performed and data is in device controllers's buffer, it makes interrupt to the cpu.

q) what is device status table?
a) it is the way by which o.s keeps track of many i/o requests at same time. it keeps information like device type,address,state. if device is busy then a queue is formed for that device entry.

q) if a interrupt arrives and cpu is in middle of some instruction, then what happens?
a) interrupt is held pending till execution of instruction completes.after that address of instruction about to be executed is saved then interrupt is handled by transferring control to appropriate service routine.

q) when a terminal sends input to computer ,who sends interrupt to cpu?
a) the serial port to which terminal is connected.

q) what do interrupt service routine do , when a character input interrupt is there?
a) takes character from device to buffer. sets pointer and flag for other parts of operating system for processing the buffer.

note:- fast devices like disk,tape etc transmit information close to memory speed, then cpu does not get much time for execution.

q) what is solution to above problem?
a) dma(direct memory access) for high speed i/o devices.

q) tell something about dma?
a) here device controller sends dierctly the entire block of data to or from its own buffer storage to memory.

q) is in dma cpu is nowhere?
a) only 1 interrupt per block is generated . whereas low-speed devices generate 1 interrupt per byte.

q) what is role of device driver here?
a) it sets the dma contoller register to use appropriate source and destination addresses and transfer length.

q) what is cycle stealing?
a) since memory can transfer only 1 word at a time , dma steals memory cycles from cpu. this cycle stealing slows down the cpu execution.

q) how cpu access ram or main memory?
a) through load and store instructions.

q) what is memory -mapped i/o?
a) here ranges of memory addresses are set aside and are mapped to device registers.

q) what is programmed i/o?
a) when cpu constantly loops to see control bit, to see whether device is ready or not. this mode of operation is called programmed i/o.

q) to what is r/w head of disk attached?
a) to disk arm.

q) define transfer rate, positioning time?
a) 1) Tranfer rate:- rate at which data flows b/w drive and computer.

2) positioning time:- it consists of seek time ie time taken to move r/w head to desired cylinder and
rotaional latency ie time for derised sector to rotate to disk head.

q) what is head crash?
a) the head files over the disk surface, there is danger of its coming in contact with disk. sometimes head may damage the disk surface. this is called head crash.

q) give example of removable disk?
a) floppy disk. they have only 1 platter.

q) why floppy disks rotate slowly than the hard disks?
a) bcoz in them head sits directly on the surface.

q) how disk is attached to computer?
a) by i/o bus. different kinds of buses are "enhanced integrated drive electronics"(eide), advanced technology attachment
(ata).

q) tell little about data transfer b/w computer and disk?
a) transfer carried out by electronic processors called contollers. host controller(computer side of bus). disk controller on disk drive. disks have built in cache. transfer takes b/w cache and host controller.

q) tell hierarchy of storage meduim in decreasing order of speed and cost?
a) registers, cache, main memory (all these use semiconductor technology) ,electronic disk,magnetic disk,magnetic tape.

note:- left to electronic disk are volatile and to right are non volatile. it can be both.

note:- electronic disk have DRAM as well as hidden magnetin disk with battery backup.

note:- data transfer b/w cache and registers and cpu is a hardware function with no o.s intervention. o.s plays role in
disk to memory.

q) what is cache coherency?
a) multiprocessors with their own caches . so if same data in all caches. an update to data in 1 cache need to be updated in all. it is called cache coherency and is a hardware problem. it does not come into scene in case where only 1 process runs at a time.

note:-before o.s came complete control was in the hands of the programmer.

q) what was name given to early operating systems?
s) resident monitors.

q) what is mode bit?
a) it is a bit added to hardware of computer to tell whether it is user mode or monitor mode.

q) when system starts then it is in which mode?
a) monitor mode.

q) why we use dual mode?
a) it protects o.s from errant users and also users from one another. some machine instructions that may cause problem are called privileged instrucions can execute only in monitor mode.

q) what is system call?
a) when user request execution of priveleged instruction, such request is called system call. it is treated by hardware as software interrupt.

note:- ms-dos for intel8088 didn't had this facility.recent intel pentium provide dual mode.

q) what is given addittional with system call?
a) type of service needed as parameter.addittional info can be passed to registers,stack etc.

q) tell some previlege instructions?
a) all i/o instructions and instructions that modify operation of timer.

q) what main thing we need to protect in case of memory protection?
a) interrupt vector and interrupt service routines.

q) what in cpu protection?
a) user may get struck in loop, and not transferring control to operating system.

q) what is used to achieve this protection?
a) timer is used. counter decrements every 1 milliseconds(say) and when it reaches 0,interrupt occurs.

note:- timer also plays a important role in time sharing.

q) what is context switch?
a) after time slice of 1 process has finished, o.s add N to record that for how long process executed, saves its state and make changes for another process to get executed.
note:- some o.s use timer to compute current time. some have separate hardware for it, independent of o.s.


q) what for lan came?
a) to substitute mainframe computers.

q) main media in lan?
a) twisted pair cables and fibre optics.

q) media in wan?
a) telephone lines,microwave links,satellites.

note:- in wan special communication processors define interfaces through which sites communicate.

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Galvin Operating system

6:13 AM

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Here we are giving chapter wise notes of operating system based on operating system concepts by galvin
Introduction


q) what is operating system?
a) manages the computer hardware..acts as intermediatery between user and hardware..provides environment for applications/ processes to work better.
---- resource allocator
---- control program

q) in how many parts a computer system can be divided?
a) four:-
1) hardware 2) o.s 3) application programs 4) users


q) early computer?
a) very large in size. were run only from console.user only used to give inputs manually.input devices were card readers and tape drives.output devices were line printers, tape drives and punch cards. batch programming was done. o.s of these batched system had only 1 task to do: transfer control automatically from 1 job to another.

q) What is job pool?
a) it is collection of jobs on disk,ready to come to main memory.

note:- job scheduling and cpu scheduling are different

q) when do o.s first thought for users means processes?
a) when concept of mutiprogramming came.

q) what is time sharing or multitasking systems?
a) here cpu executes multiple jobs by switching between them, and that also so fast that users interact with program when it is running.

q) how do we achieve time sharing concept?
a) using cpu scheduling and multiprogramming.

q) give example of desktop systems?
a) pc's running under windows and apple machintosh.

q) what is multiprocessor system?
a) also called parallel system and tightly coupled system.more than 1 processor in close communication with same clock,and bus or may be memory and i/o devices.

q) what is meant by graceful degradation?
a) suppose we have 10 processors. 1 fails then other 9 can share work of failed 1.thus system runs only 10% slower than totally failing.this ability to continue providing service propotional to level of surviving hardware is called graceful degradation. systems are called fault tolerant.

q) what is tandem system?
a) uses duplication of hardware and software both. 2 identical processors with own memory and connected by bus. 1 act as primary and other as backup.checkpoints are used to give information to backup processor.

q) explain smp?
a) symmetric multiprocessing. here each processor runs copy of o.s and communicate with each other as required.

q) define asmp?
a) asymmteric (here we have master slave relationship b/w processors)

q) give an example of smp?
a) Encore's verion of unix.

q) what issues in multiprocessing?
a) * i/o (data must reach appropriate processor)
* 1 is idle and other is overloaded.

q) what is distibuted system?
a) take use of networking. processors at different places and use different clocks and donot share memory also.

q) name protocols supported by windows and unix?
a) tcp/ip.

q) types of server systems?
a) 1) compute servers 2) File servers

q) what is distributed lock manager service?
a) clusters donot allow shared access to data on disk. for this distributed file system provide access control and locking to files to avoid conflicts.

q) what are real time systems?
a) here rigid time requirement is placed on the operation of processor or data flow. processing must be done in defined time constraint.

q) give area of application of real time systems?
a) medical imaging,scientific experiments,weapon systems,home appliance controller etc.

q) types of real time systems?
a) hard and soft.

q) features of hard?
a) * critical tasks done. * limited or no secondary memory , only ROM. *no virtual memory * no time sharing concept. * o.s is very simple

q) what are handheld systems?
a) include pda ie personal digital assistants. eg cellular phones.

q) limitations of handheld systems?
a) small memory, small display and slow processor due to small battery.

q) under what category do pc's come?
a) microcomputers.

note: clusters share a storage.




Exercise:-


q1) three main purpose of o.s?
a) 1) resouce allocator 2) control program 3) manage hardware 4) file system

q2)difference b/w operating system for mainframes and pc's?
a) pc's os were mainly concerned with convenience and responsiveness rather than cpu and device utilization.they lacked multiuser and multitasking.file protection ,new feature of operating system.

q) what are security problem related to multiprogramminG and multitasking environment?
a) more than 1 program in memory , so need mamory management and protection.need to take care of cpu scheduling,job synchronisation, and also take care of deadlock prevention.

q) some properties:-
a) type of operating system features

1) batch very big, run from console,input on tapes,card readers,o.s only used to transfer control from 1 job to next.

2)interactive direct communication b/w user and o.s or program.

3)time sharing executes multiple jobs by switching b/w them,user can interact with jobs.uses cpu scheduling and multiprogramming.virtual memory came.
disk management(as file system), take care of job synchronization and communication, memory management.

4)real time rigid time requirements are placed on the processor or flow of data.can be hard or soft.

5)parallel more than 1 processor with same clock and share same memory.concept of graceful degradation. can be smp or asmp

6)Distributed depend on network.mutiple processors but at distant places. they donot share same memory and clock.

7)clustered 2 or more individual systems coupled together.linked through lan and share storage. can be asymmetric or symmetric clustering.also we have
parallel clusteing. they have facility called dlm(distributed lock manager).

8)handheld small memory, slow processor,not having virtual memory concept,

note:- o.s waste resources in case of PC's where user convenience and responsiveness is major aim to achieve.

q) what is being used in multiprocessing along with main CPU?
a) microprocessors with its own memory.

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Galvin Operating system

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