Q. 32 What functions are performed by memory management of operating systems ? State any four memory management systems.
Q. 33 Explain memory map of single user operating system.
Q. 34 What is partitioning ? Explain fixed and variable partitioning.
Q. 35 Give the disadvantages of fixed partition.
Q. 36 State the various steps involved in the allocation of a partition in case of fixed partition memory management.
Q. 37 Explain the single contiguous memory management systems with a suitable memory mapping diagram
Q. 38 What is paging ? Explain in detail. Explain page memory management system with a suitable page map Table (PMT)
Q. 39 What is segmentation?
CONCEPTS RELATED TO MEMORY MANAGEMENT
Q. 32 What functions are performed by memory management of operating systems ? State any four memory management systems.
Ans : 1) In general, the memory management modules performs following functions :
a) To keep track of all memory locations free or allocated and i allocated; to which process and how much.
b) To decide memory allocation policy i.e. which process should get how much memory locations Normally, this is achieve with the help of some special hardware.
c) Fo use various techniques and algorithms to allocate or deallocate memory locations.
Normally this is achived with the help of some special hardware.
2) The following are the memory management systems :
A) Contiguous, Real Memory Management System :
(a) Single contiguous
(b) Fixed partitioned
(¢) Variable partitioned
B) Non – contiguous, Real Memory Management System :
(a) Paging
(b) Segmentation
(c) Combined
C) Non – contiguous, Virtual Memory Management System :
(a) Virtual memory
Q. 33 Explain memory map of single user operating system.
Ans :
Memory map of single user OS
(1) The operating system like MS-DOS is single user O.S
(2) The memory map of such operating systems consists of program to be executed i.e. process, free memory available, command interpreter and kernel.
(3) The command interpreter of the single user operating system is invoked when the computer is started.
(4) This O.S loads program to be excuted in the main memory and assign as much memory as possible to it. It then sets instruction pointer and executes the program.
(5) It the program is terminated, then it is removed from memory
(6) The kernel of such O.S provides basic operating systems services, while the command interpreter interpretes the commands
Q. 34 What is partitioning ? Explain fixed and variable partitioning.
Ans : Certain operating systems use partitioned memory to allow multiprogramming. Partitioning means dividing main memory into various sections. These sections are called partitions.
There are two types of partitions.
I) Fixed partitions
II) Vanable partitions
I) Fixed partitions (Static Partition) :
(1) In fixed partitioning, partitions could be of different sizes. But once decided, their size can not be changed
2) In this method partitions are fixed at the time of system generation. At this time, system manager has to declare the partition time.
(3) Fixed partitions are also called as static partitions. On declaring fixed partitions, the operating system creates Partition Description Table (PDT).
II) Variable partitioning :
(1) In variable partitioning number of partitions and their sizes are variable.
(2) They are not defined at the time of system generation.
(3) These partitions are created by the operating system at run time they differ in size.
(4) The procedure to be followed for memory allocation is nearly same as that in case of fixed
partitions.
(5) At any time, any partition may be free or allocated to some process. Also, in variable
partitioning, starting address of partition is not fixed.
Q. 35 Give the disadvantages of fixed partition.
Ans : (i) Fixed partitioning suffers from internal fragmentation i.e. wastage of memory space within the partition e.g. suppose the partition size is 200k and a job is of 100k, then 100k of memory will be wasted.
(ii) Fixed partitioning reduces degree of multiprogramming.
(iii) It also restricts C.P.U. utilisation.
Variable partitioning overcomes these problems and hence it is widely accepted.
Q. 36 State the various steps involved in the allocation of a partition in case of fixed partition memory management.
Ans : When a process is to be allocated a partition, following take place :
i) The long term process scheduler of the PM decides which process to be brought in to the
memory.
ii) It then finds out the size of the program to be loaded by consulting the IM portion of the 0.S. The compiler keeps the size of the program in the header of the executable file.
iii) Then makes a request to the partition allocation routine of the MM to allocate a free partition, with the appropriate size.
The partition description table (PDT) is useful for this procedure.
(iv) With the help of the IM module it now loads the binary program in the allocated partiition.
(v) It then makes an entry of the partition ID in the PCB (process control block) before the PCB is linked to the chain of ready processes by using the PM module of the operating system.
(vi) The routine in the MM now redefines partition description table and marks the status of that partition as allocated. (ALLC)
(vii) The PM eventually schedules this process.
A partition description table (PDT) is shown in Fig. below :
Q. 37 Explain the single contiguous memory management systems with a suitable memory mapping diagram
Ans : Single contiguous memory management system :
(1) In this memory management system, the physical memory is divided into two contiguous areas. One is permanently allocated to the resident portion of the operating system and the remaining used for user process.
(2) As shown in figure operating system may be loaded at lower addresses i.e. 0 to P. At any time, only one user process is in the memory. This process is run to completion and then next process is brought in the memory.
Q. 38 What is paging ? Explain in detail.
OR
Explain page memory management system with a suitable page map Table (PMT)
Ans :
(a) Partitions suffer from external fragmentation because of available memory is not
contiguous.
(b) Paging permits a program’s memory to be non-contiguous allowing a program to be allocated
physical memory wherever it is available.
(c) Physical memory is broken into fixed-size blocks called Page Frames. Logical memory is
also broken into blocks of the same size called Pages.
(d) When a program is to be executed its pages are loaded into any available frames and the
page map tables defined to translate from user pages to memory frames.
(e) The page size is defined by hardware. It is typically power of 2.
The paging model of memory is shown as follows :
Every address generated by CPU is divided in two parts :- a page number (p) and a page
offset / displacement (d). The page number is used as an index into a PMT.
Q. 39 What is segmentation?
Ans : (1) Segments are logical divisions of programs and hence are normally of variable sizes.
(2) Segmentation 1s a memory management scheme which support user’s view of memory.
(3) Each segment has number and length.
(4) Each program in executable form can be considered to be consisting of ditferent segments
such as code, data and stack. Each of these can be further divided into new segments.
(5) A program normally contains main program, some subprograms and few predefined and
precompiled functions. Each of these belongs to different segments.
(6) An application programmer does not necessarily have to declare different segments in the program. If various segments in his programs does not explicitly, then the compiler does it by its own. Following are the jobs of compiler :
i) Recognize different segments in program.
ii) Number those segments.
iii) Define segment table
iv) Produce an executable image by assigning two dimensional addresses.
(7) Consider the examples as shown in figure Four segmeents numbered 0 to 3. The SMT (Segment Map Table) has separate entry for each segment givingthe size and base of segment.
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