Relocatable
Partitioned Allocation : The
fragmentation problem is removed by relocated
partitioned scheme. The
blocks (jobs) already in the main memory can be relocated to make
a hole (region) large enough for incoming information
the relocation of the blocks already stored in the main memory
accomplished by a technics called "compaction". The
blocks currently in the main memory combined into
a single block placed at one end of the memory this creates a
single available region of maximum possible size at other
end. The blocks can be relocated efficiently with a special
hard ware facility for this purpose relocation register issued
whose contents are automatically added to every address. This
register is used to reference memory after each compaction incoming
blocks assigned to the available region of the memory when a new block is
assigned in the available region compaction is again carried out.
Advantages
:
1) It removes
fragmentation problem.
Dis-advantages
:
1) When a job ends
the system may have to relocate all other jobs in order to re-compact.
2)
There is still a small amount of memory is wasted.
Paged Memory Allocation : Paging is another solution for the
fragmentation problem. In
this each jobs address space divided into equal
pieces called Pages. The memory is also divided into pieces of same
size called Page Frames with the help of suitable hard ware mapping
facility, any page can be placed into any Page Frame. The pages
remain logically continuous but the corresponding Page Frames are not
necessary continuous.
There is a
seperate register for each Job called Page Map Table these
registers may be special hard ware registers or reserved section of the
micro memory. If the page size is too large, it becomes a relocatable
partitioned memory. If the page size is too small many
page
registers (PMT'S) are required which increases the cost
of the computer systems. The paged memory allocation is shown in figure. In
the next example address space of job1 is divided into two
pages. Job2 is divided into three pages and Job3 consist of only
one page we are assuming each page size is 1000 B there is a page map
table for each Job which consists of page. Number and the location
of that page in memory , there is mapping that takes place with
each address i.e.., each address in the Job 's address space can be
transferred into an address in the physical memory.
In the above example
a Job may not be a multiple of 1000 B long. Then a portion of last page of that
Job will be wasted. This is called Internal fragmentation.
Advantages
:
1. It solves
the fragmentation problem without physically moving the pages in memory.
2. This allows a higher degree of
multiprogramming.
3. The compaction in
relocatable partitioned allocation is elimated.
Dis-Advantages
:
1. Page address
mapping hardware increase the cost of the computer system.
2. Extra core or
extra registers needed for page map tables.
3. There is a
possibility of internal fragmentation (or) page breakage may occur.
4. Some
memory will still unused if the number of available
page frames are not sufficient for the Job's page.