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Program must be brought into memory andplaced within a process
for it to be executed.
Input queue collection of processes on the diskthat are waiting
to be brought into memory forexecution.
Select one process from input queue to load it
into memory for execution. After execution(terminates) memory
space becomes available.
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Address binding of instructions and data tomemory addresses can
happen at three differentstages:
Compile time: If memory location known a priori,absolute code
can be generated. Since it starts fromthat location.
Load time: Must generate reloadable code if memorylocation is
not known at compile time.
Execution time: Binding delayed until run time if theprocess can
be moved during its execution from onememory segment to another.
Need hardware supportfor address maps (e.g., base and limit
registers).
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Multistep processing of a user program
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The concept of a logical address space that is bound to
aseparate physical address space is central to propermemory
management.
Logical address : Generated by the CPU; also referred to
asvirtual address.
Physical address: Address seen by the memory unit.
Logical and physical addresses are the same in compile-
time and load-time address-binding schemes; logical(virtual) and
physical addresses differ in execution-timeaddress-binding
scheme.
The set of all logical address generated by a program.
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The run-time mapping from virtual to physicaladdresses is done
by the memory management unit(MMU), which is a hardware device.
The user program deals with logical addresses; itnever sees the
real physical addresses.
Two different types of addresses:
Logical: range 0 to max. Physical: range R+0 to R+max; where R
is a base value.
The user generates only logical addresses and thinksthat the
process runs in locations 0 to max.
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In MMU scheme, the value in the relocation register is addedto
every address generated by a user process at the time it issent to
memory.
Example: Dynamic relocation using a relocation register.
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Dynamic loading is used to obtain bettermemory space.
An unused routine will never been used.
It is particularly useful when large amount of
code.
It does not requires any special support fromthe OS.
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It is similar to Dynamic loading.
Here linking is postponed until the time ofexecution.
It avoids to copy the whole language librariesincluded in the
executable image.
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A process can be swapped temporarily out ofmemory to a backing
store, and then broughtback into memory for continued
execution.
Take Round robin scheduler as example, whentime quantum expires
the memory swap out theprocess and swap in another process.
Roll out, roll in In priority-based schedulingalgorithms;
lower-priority process is swappedout if higher-priority process
arrives and wantsservice, can be loaded and executed.
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A process that is swapped out will be swappedback into the same
memory space that itoccupied previously.
Normally CPU maintains the ready queue for thebacking store.
When CPU needs it calls the dispatcher.
If we want to swap, it should be completely idle. Like without
any pending i/o requests.
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Main memory usually is divided into two
partitions: For the resident operating system
For the user processes.
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The issues of memory protection is Protecting OS from user
processes. Protecting user processes from another process.
Relocation register: It contains the value of thesmallest
physical address.
Limit register: It contains the range of the logical
address. Each logical address must be less than the Limit
register and is dynamically adding the relocationregister. This
mapped address is sent to the memory.
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The memory can be divided into several fixed sizedcomponents is
called Partitions.
The block of available memory called as Hole.
This may be at various sizes scattered throughout thememory.
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To satisfy the request of size n from a list of free holes.There
are three approaches.
First fit: Allocates the first hole that is big enough.
Best fit: Allocates the smallest hole that is big enough,but it
searches the entire list. It produces the smallestleftover
hole.
Worst fit: Allocates the largest hole, it also searchesthe whole
list. It produces the largest leftover hole.
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Simply wasting of the memory. Memory that is internal to the
partition but is not used called Internal
fragmentation. Memory that is outside to the partition and is
too small to used by any
process is called External Fragmentation.
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Divide physical memory into fixed-sized blocks calledframes
(typically between 512 bytes and 8192 bytes).
Paging is a memory management technique whereDivision of logical
memory into blocks of same sizecalled pages. i.e., divide each
process into same size of frames.
Operating system maintains a page table for eachprocess. Page
table contains the frame location for each process
page. memory address = page number + offset
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Here the logicaladdress is dividedinto twocomponents.
1. Page number (p)2. Page offset(d)
Page number p is an index intoa page table that contains
baseaddress of each page in physicalmemory.
Page offset d is
a displacement,combined withbase address todefine the
physicalmemory addressthat is sent to the
memory unit.
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Page table is kept in main memory.
Page-table base register (PTBR) points to the page table.
Page-table length register (PRLR) indicates size of thepage
table.
In this scheme every data/instruction access requires twomemory
accesses. One for the page table and another for
the data/instruction. The two memory access problem can be
solved by the use
of a special fast-lookup hardware cache called associativememory
or Translation Look-aside Buffers (TLBs)
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Associative Lookup = time unit
Assume time to access memory is 1 microsecond
Hit ratio percentage of times that a page number isfound in the
TLB.
Hit ratio =
Effective Access Time (EAT)
EAT = (1 + )+ (2 + )(1 )= 2 +
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Memory protection implemented byassociating protection bit with
each frame
Valid-invalid bit attached to each entry in thepage table: valid
indicates that the associated page is in the
process logical address space, and is thus a legalpage.
invalid indicates that the page is not in theprocess logical
address space
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Vvalid bit.Iinvalid bit.
