Notes on Memory management-
Note:- These notes are just bullet points for refreshing the knowledge on Memory management. This is not written with the intention to explain any concept or for reference.
1. Main memory and the registers build
into the processors itself are storage that CPU directly access.
2 . Registers are generally accessible
within one CPU cycle of CPU clock.
3. Protection on memory space is
provided by two registers-
·
Base
registers(Relocation register)- holds the smallest legal physical address.
·
Limit
address- specifies the size of range
4. Protection of memory is accomplished
by having the CPU hardware compare every address (generated in user mode, ie
generated by user code) is within the above two register address. Any attempt
to access any memory outside the range is marked as fatal error.
5. The runtime mapping from virtual to
physical address is done by hardware device called Memory Management Unit(MMU)
6. Dynamic loading – a routine is not
loaded into memory until it is called.
·
Relocatable
linker table – loads the desired
routines into the memory and updates the program address tables to reflect this
change.
7. Static Linking – In which the system
language libraries are treated like any other object module and combined by the loader into the
binary program image.
In dynamic linking –the linking is postponed until execution time.
8. Version information of library is included
in both the program and library. More than one version of library can be loaded
into the memory and each program uses its own version information to decide which
copy of library to use.
9. Roll in & roll out- the process
of swapping in and swapping out process for execution.
10. Relocation registers = base registers of the process.
Physical address = logical address + base(Relocation) register value.
11. Transient Operating system code –
The OS code that is not required too frequently.
12. Variable partition scheme (M.V.T) =
Satisfy the request of size ‘n’ from the list of available free holes.
There are many suggestions to above problem,
·
First
Fit
·
Best
fit
·
Worst
fit
13. External fragmentation – it exists
when there is enough total memory space to satisfy the request but the
available spaces are not contiguous.
14. Internal fragmentation – it is the
unused memory that is internal to the partition. This occurs when we take the
approach of breaking the physical memory into fixed sized partition and allocate
in unit based on block size.
15. Compaction – shuffling the memory
contents as to place all the free memory space in once large block.
16. Backing store – the memory where the
swapped out process is kept.
17. Paging is the memory management
scheme that permits physical address space of the process to be non contiguous.
·
It
avoids external fragmentation.
·
Compaction
is not needed.
·
Also
saves the memory management problem in backing store.
18. Frames – Breaking physical memory
into fixed size of blocks called frame.
Pages – breaking logical memory into
fixed size of blocks called pages.
Page size is defined by the hardware.
19. Frame table –it is the data structure
that has entry for each physical page frame, indicating that whether the later
is free or allocated and if allocated to which page of which process
20. For protection - protection bits are
used that is associated with each frame.
We have access type buts and valid
and invalid buts associated in page table.
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