Memory : operating system ( Btech cse )

Silberschatz, Galvin and GagneOperating System Concepts – 9th
Edition
Chapter 9: Main Memory

9.2 Silberschatz, Galvin and GagneOperating System Concepts – 9th
Edition
Chapter 9: Memory Management
Background
Swapping
Contiguous Memory Allocation
Segmentation
Paging
Structure of the Page Table
Example: The Intel 32 and 64-bit Architectures
Example: ARM Architecture

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Objectives
To provide a detailed description of various ways of
organizing memory hardware
To discuss various memory-management techniques,
including paging and segmentation
To provide a detailed description of the Intel Pentium,
which supports both pure segmentation and segmentation
with paging

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Background
A program must be brought (from disk) into memory and
placed within a process for it to be run
A program can be written in machine language, assembly
language, or high-level language.
Main memory and registers are the only storage entities that a
CPU can access directly
The CPU fetches instructions from main memory according to
the value of the program counter.
Typical instruction execution cycle – fetch instruction from
memory, decode the instruction, operand fetch, possible
storage of result in memory.

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Background (Cont.)
Memory unit only sees a stream of one of the following:
address + read requests (e.g., load memory location
20010 into register number 8)
address + data and write requests (e.g., store content
of register 6 into memory location 1090)
Memory unit does not know how these addresses were
generated
Register access can be done in one CPU clock (or less)

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Background (Cont.)
Completing a memory access may take many cycles of
the CPU clock. In such a case the processor needs to
stall since it does not have the data required to complete
the instruction it is executing.
Cache sits between main memory and CPU registers to
deal with the “stall” issue.
Protection of memory is required to ensure correct
operation:
User process cannot access OS memory
One user process cannot access the memory of
another user process.

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Memory Protection
A base register (holding the smallest legal physical address
of a program in memory) and a limit register (specifies the
size of the program) define the boundary of a program in
memory.
CPU must check that every memory access generated in user
mode is between the base and base+limit for that user

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Hardware Address Protection

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Address Binding
A program residing on the disk needs to be brought into
memory in order to execute. Such a program is usually
stored as a binary executable file and is kept in an input
queue.
In general, we do not know a priori where the program is
going to reside in memory. Therefore, it is convenient to
assume that the first physical address of a program always
starts at location 0000.
Without some hardware or software support, program must
be loaded into address 0000
It is impractical to have first physical address of user
process to always start at location 0000.
Most (all) computer systems provide hardware and/or
software support for memory management,

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Address Binding (Cont.)
In general, addresses are represented in different ways at
different stages of a program’s life
Addresses in the source program are generally symbolic
 i.e., variable “count”
A compiler typically binds these symbolic addresses to
relocatable addresses
 i.e., “14 bytes from beginning of this module”
Linker or loader will bind relocatable addresses to
absolute addresses
 i.e., 74014
Each binding maps one address space to another
address space

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Binding of Instructions and Data to Memory
Compile time: If memory location known a priori,
absolute code can be generated; must recompile code if
starting location changes
Load time: If memory location is not known at compile
time and no hardware support is available, relocatable
code must be generated
Execution time: Binding delayed until run time if the
process can be moved during its execution from one
memory segment to another
Need hardware support for address maps (e.g., base
and limit registers)
Address binding of instructions and data to memory
addresses can happen at three different points in time:

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Multistep Processing of a User Program

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Logical vs. Physical Address Space
The concept of a logical address space that is bound to a separate
physical address space is central to proper memory management
Logical address – generated by the CPU.
Physical address – address seen by the memory unit
Logical and physical addresses are:
The same in compile-time and load-time address-binding schemes;
They differ in execution-time address-binding scheme. In that case
the logical address is referred to as virtual address.
We use Logical address and virtual address interchangeably
Logical address space is the set of all logical addresses generated
by a program
Physical address space is the set of all physical addresses
corresponding to a given logical address space.

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Memory-Management Unit (MMU)
Hardware device that at run time maps virtual addresses to
physical address
Many methods possible, covered in the rest of this chapter
The user program deals with logical addresses; it never sees the
real physical addresses
Execution-time binding occurs when reference is made to
location in memory
Logical address bound to physical addresses

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Dynamic relocation using a relocation register
To start, consider simple scheme where the value in the base
register is added to every address generated by a user process
at the time it is sent to memory
Base register now called relocation register
MS-DOS on Intel 80x86 used 4 relocation registers

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Dynamic Loading
Until now we assumed that the entire program and data
has to be in main memory to execute
Dynamic loading allows a routine (module) to be loaded
into memory only when it is called (used)
Results in better memory-space utilization; unused routine
is never loaded
All routines kept on disk in relocatable load format
Useful when large amounts of code are needed to handle
infrequently occurring cases (e.g., exception handling)
No special support from the operating system is required
It is the responsibility of the users to design their
programs to take advantage of such a method
OS can help by providing libraries to implement
dynamic loading

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Dynamic Linking
Dynamically linked libraries – system libraries that are
linked to user programs when the programs are run.
Similar to dynamic loading. But, linking rather than loading
is postponed until execution time
Small piece of code, stub, used to locate the appropriate
memory-resident library routine
Stub replaces itself with the address of the routine, and
executes the routine
Operating system checks if routine is in processes’ memory
address
If not in address space, add to address space
Dynamic linking is particularly useful for libraries
System also known as shared libraries

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Contiguous Allocation
Main memory must support both OS and user processes
Limited resource -- must allocate efficiently
Contiguous allocation is one early method
Main memory is usually divided into two partitions:
Resident operating system, usually held in low memory
with interrupt vector
User processes are held in high memory
Each process contained in single contiguous section of
memory

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Contiguous Allocation (Cont.)
Relocation registers used to protect user processes from each
other, and from changing operating-system code and data
Base register contains value of smallest physical address
Limit register contains range of logical addresses – each
logical address must be less than the limit register
MMU maps logical address dynamically
Can then allow actions such as kernel code being
transient – comes and goes as needed. Thus, kernel can
change size dynamically.

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Hardware Support for Relocation and Limit Registers

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Multiple-partition allocation
Variable-partition -- sized to a given process’ needs.
Hole – block of available memory; holes of various size are
scattered throughout memory
When a process arrives, it is allocated memory from a hole large
enough to accommodate it
Process exiting frees its partition, adjacent free partitions
combined
Operating system maintains information about:
a) allocated partitions b) free partitions (holes)

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Dynamic Storage-Allocation Problem
How to satisfy a request of size n from a list of free holes?
First-fit: Allocate the first hole that is big enough
Best-fit: Allocate the smallest hole that is big enough;
must search entire list, unless the list is ordered by size.
 Produces the smallest leftover hole
Worst-fit: Allocate the largest hole; must also search
entire list, unless the list is ordered by size
 Produces the largest leftover hole
First-fit and best-fit are better than worst-fit in terms of speed
and storage utilization

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Fragmentation
External Fragmentation – total memory space exists to
satisfy a request, but it is not contiguous and therefore
cannot be used.
First fit analysis reveals that given N allocated blocks,
another 0.5 N blocks will be lost to fragmentation
 1/3 of memory may be unusable -> 50-percent
rule
Internal Fragmentation – allocated memory may be
slightly larger than requested memory.
Can happen if there is hole of size 15,000 bytes and a
process needs 14,900 bytes; Keeping a hole of size 100
bytes is not worth the effort so the process is allocated
15,000 bytes.
The size difference of 100 bytes is memory internal to a
partition, but not being used

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Fragmentation (Cont.)
Shuffle memory contents to place all free memory
together in one large block
Compaction is possible only if relocation is dynamic, and
is done at execution time
I/O problem -- cannot perform compaction while I/O is in
progress involving memory that is being compacted.
Latch job in memory while it is involved in I/O
Do I/O only into OS buffers
Reduce external fragmentation by compaction

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Non-contiguous Allocation
Partition the a program into a number of small units, each
of which can reside in a different part of the memory.
Need hardware support.
Various methods to do the partitions:
Segmentation.
Paging
paged segmentation.

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Segmentation
Memory-management scheme that supports user’s view of memory
A program is a collection of segments -- a logical unit such as:
main program
procedure
function
method
object
local variables, global variables
common block
stack
symbol table
arrays
Each segment can to reside in different parts of memory. Way to
circumvent the contiguous allocation requirement.

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User’s View of a Program

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Two Dimensional Addresses

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Logical and Physical Memory

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Segmentation Architecture
Logical address consists of a two tuple:
<segment-number, offset>
Need to map a two-dimensional logical addresses to a one-
dimensional physical address. Done via Segment table:
base – contains the starting physical address where the
segments reside in memory
limit – specifies the length of the segment
Segment table is kept in memory
Segment-table base register (STBR) points to the
segment table’s location in memory
Segment-table length register (STLR) indicates
number of segments used by a program;
segment number s is legal if s < STLR

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Segmentation Hardware

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Example of Segmentation

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Paging
Physical address space of a process can be non-contiguous.
Process is divided into fixed-size blocks, each of which may
reside in a different part of physical memory.
Divide physical memory into fixed-sized blocks called frames
Size of a frame is power of 2 between 512 bytes and 16
Mbytes
Divide logical memory into blocks of same size as frames called
pages
Backing store, where the program is permanently residing, is
also split into storage units (called blocks), which are the same
size as the frame and pages.
Physical memory allocated whenever the latter is available
Avoids external fragmentation
Still have Internal fragmentation

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Paging (Cont.)
Keep track of all free frames
To run a program of size N pages, need to find N free frames
and load program from backing store.
Set up a page table to translate logical to physical addresses
Page table is kept in memory.
Page-table base register (PTBR) points to the page
table
Page-table length register (PTLR) indicates size of the
page table
Still have Internal fragmentation (more later)

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Address Translation Scheme
Assume the logical address space is 2m.
(How is m
determined?)
Assume page size is 2n
Address generated by CPU is divided into:
Page number (p) – used as an index into a page table
which contains base address of each page in physical
memory. Size of p is “m – n”
Page offset (d) – combined with base address to define
the physical memory address that is sent to the memory
unit. Size of d is “n”.

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Paging Hardware

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Paging Model of Logical and Physical Memory

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Paging Example
Assume m = 4 and n = 2 and 32-byte memory and 4-byte pages

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Internal Fragmentation
Calculating internal fragmentation
Page size = 2,048 bytes
Process size = 72,766 bytes
35 pages + 1,086 bytes
Internal fragmentation of 2,048 - 1,086 = 962 bytes
Worst case fragmentation = 1 frame – 1 byte
On average fragmentation = 1 / 2 frame size
So small frame sizes desirable?
But each page table entry takes memory to track
Page sizes growing over time
 Solaris supports two page sizes – 8 KB and 4 MB
By implementation process can only access its own memory

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Allocating Frames to a New Process
Before allocation After allocation

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TLB -- Associative Memory
If page table is kept in main memory every data/instruction access
requires two memory accesses
One for the page table and one for the data / instruction
The two memory access problem can be solved by the use of a
special fast-lookup hardware cache called associative memory or
translation look-aside buffers (TLBs)
Associative memory – parallel search
Address translation (p, d)
If p is in associative register, get frame # out
Otherwise get frame # from page table in memory
P a g e # F r a m e #

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TLB issues
TLB is typically small (64 to 1,024 entries)
On a TLB miss, the value of the (missed page-table and
frame-number), is loaded into the TLB for faster access
next time that address is used.
What if there is no free TLB entry? Replacement
policies must be considered
Some entries can be wired down for permanent fast
access
Some TLBs store address-space identifiers (ASIDs)
in each TLB entry – uniquely identifies each process to
provide address-space protection for that process
Otherwise need to flush TLB at every context switch

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Paging Hardware With TLB

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Effective Access Time
Associative Lookup = ε time unit
Can be < 10% of memory access time
Hit ratio = α
Hit ratio – percentage of times that a page number is found in
the associative registers; ratio related to number of associative
registers
Effective Access Time (EAT)
EAT = (1 + ε) α + (2 + ε)(1 – α)
= 2 + ε – α
Consider ε = 20ns for TLB search and 100ns for memory access
if α = 80%:
 EAT = 0.80 x 100 + 0.20 x 200 = 120ns
Consider more realistic hit ratio of α = 99%
 EAT = 0.99 x 100 + 0.01 x 200 = 101ns

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Memory Protection
Memory protection implemented by associating protection
bits with each frame to indicate if “read-only “ or “read-write”
access is allowed
Can also add more bits to indicate “execute-only” and
so on
Valid-invalid bit attached to each entry in the page table:
“valid” indicates that the associated page is in the
process’ logical address space, and is thus is a legal
page
“invalid” indicates that the page is not in the process’
logical address space
Or use page-table length register (PTLR)
Any violations result in a trap to the kernel

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Valid (v) or Invalid (i) Bit In A Page Table

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Shared Pages
Shared code
One copy of read-only (reentrant) code shared among
processes (i.e., text editors, compilers, window systems)
Similar to multiple threads sharing the same process space
Also useful for inte-rprocess communication if sharing of
read-write pages is allowed
Private code and data
Each process keeps a separate copy of the code and data
The pages for the private code and data can appear
anywhere in the logical address space

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Shared Pages Example

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Memory structures for paging can get huge using straight-
forward methods
Consider a 32-bit logical address space
Page size of 1 KB (210
)
Page table would have 4 million entries (232
/ 210
)
If each entry is 4 bytes -> Page table is of size 16 MB
 That amount of memory used to cost a lot.
 Do not want to allocate that contiguously in main
memory
What about a 64-bit logical address space?

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Page Table for Large address space
Hierarchical Paging
Hashed Page Tables
Inverted Page Tables

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Hierarchical Page Tables
Break up the logical address space into multiple page tables
A simple technique is a two-level page table
We then page the page table

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Two-Level Paging Example
A logical address (on 32-bit machine with 1K page size) is divided into:
a page number consisting of 22 bits
a page offset consisting of 10 bits
Since the page table is paged, the page number is further divided into:
a 12-bit page number
a 10-bit page offset
Thus, a logical address is as follows:
where p1 is an index into the outer page table, and p2 is the
displacement within the page of the inner page table
Known as forward-mapped page table

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Address-Translation Scheme

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64-bit Logical Address Space
Even two-level paging scheme not sufficient
If page size is 4 KB (212
)
Then page table has 252
entries
If two level scheme, inner page tables could
be 210
4-byte entries
Address would look like
Outer page table has 242
entries or 244
bytes

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64-bit Logical Address Space (Cont.)
One solution is to divide the outer page table. Various
ways of doing so. Example – three-level page table
Even with 2nd
outer page table, the outer-outer table is
still 234
bytes in size.
And possibly 4 memory access to get to one physical
memory location.
The next step would be four-level. But ….

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64-bit Logical Address Space (Cont.)
Hashed Page Table.
Clustered Page Tables
Inverted Page Table
Several schemes for dealing with very large logical
address space

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Hashed Page Table
Common in address spaces > 32 bits
The virtual page number is hashed into a page table
This page table contains a chain of elements hashing
to the same location
Each element contains:
1. The virtual page number
2. The value of the mapped page frame
3. A pointer to the next element
Virtual page numbers are compared in this chain searching
for a match
If a match is found, the corresponding physical frame
is extracted

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Hashed Page Table

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Clustered Page Tables
Similar to hashed but each entry refers to several pages
(such as 16) rather than 1
Especially useful for sparse address spaces (where
memory references are non-contiguous and scattered)
Variation for 64-bit addresses is clustered page tables

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Inverted Page Table
Rather than each process having a page table and keeping
track of all possible logical pages, track all the physical
pages
Use inverted page-table, which has one entry for
each real page of memory
An entry the inverted-page table consists of the virtual
address of the page stored in that real memory location,
with information about the process that owns that page.
What is maximum size of the inverted page-table?

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Inverted Page Table Architecture

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Inverted Page Table (Cont.)
Decreases memory needed to store each individual page
table, but increases time needed to search the inverted
page table when a page reference occurs
Use hash table to limit the search to one — or at most a
few — page-table entries
TLB can accelerate access
But how to implement shared memory?
One mapping of a virtual address to the shared
physical address

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Swapping
A process can be swapped temporarily out of memory to a
backing store and then brought back into memory for
continued execution
Total physical memory space of all processes can
exceed the real physical memory of the system.
Backing store – fast disk large enough to accommodate
copies of all memory images for all processes; must provide
direct access to these memory images
System maintains a ready queue of ready-to-run processes
which are either in memory or have memory images on disk.
Major part of swap time is transfer time; total transfer time is
directly proportional to the amount of memory swapped

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Schematic View of Swapping

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Swapping (Cont.)
Does the swapped out process need to swap back in to
same physical addresses?
Depends on address binding method
 Plus must consider pending I/O to / from process
memory space
Modified versions of swapping are found on many systems
(i.e., UNIX, Linux, and Windows). A common variation:
Swapping is normally disabled
Swapping is started if the amount of free memory
(unused memory available for the operating system or
processes to use) falls below a given threshold.
Swapping is disabled again once memory demand
reduced below the threshold
Another variation. Swapping portions of processes--rather
than entire processes--to decrease swap time.
Typically, these modified forms of swapping work in
conjunction with virtual memory (covered soon).

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Context Switch Time including Swapping
If next processes to be located a CPU (say A) is not in
memory and there is not enough physical memory to
accommodate A, then we need to swap out one of the
processes in memory (say B) and swap in process A.
Context switch time can then be very high
100MB process swapping to hard disk with transfer rate of
50MB/sec
Swap out time of 2000 ms
Plus swap in of same sized process
Total context switch swapping component time of 4000ms
(4 seconds)
Can reduce context switch time by knowing how much memory
is really being used. System calls to inform OS of memory use:
request_memory() and release_memory()

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Context Switch Time and Swapping (Cont.)
Other constraints as well on swapping
Pending I/O – can’t swap out as I/O would occur to wrong
process
Or always transfer I/O to kernel space, then to I/O device
 Known as double buffering, adds overhead
Standard swapping not used in modern operating systems
But modified version common
 Swap only when free memory extremely low

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Swapping on Mobile Systems
Not typically supported. Mobil systems use Flash memory:
Limited number of write cycles
Poor throughput between flash memory and CPU on mobile
platform
Mobile systems use other methods to free memory if low
iOS asks apps to voluntarily relinquish allocated memory
 Read-only data thrown out and reloaded from flash if
needed
 Failure to free can result in termination
Android terminates apps if low free memory, but first writes
application state to flash for fast restart
Both operating systems support paging.

Silberschatz, Galvin and GagneOperating System Concepts – 9th
Edition
End of Chapter 9

Edition
Paging
Physical address space of a process can be non-contiguous;
process is allocated physical memory whenever the latter is
available
Avoids external fragmentation
Avoids problem of varying sized memory chunks (e.g.,
segments)
Divide physical memory into fixed-sized blocks called frames
Size of a frame is power of 2 between 512 bytes and 16
Mbytes
Divide logical memory into blocks of same size as frames called
pages
Backing store, where the program is permanently residing, is
also split into storage units (called blocks), which are the same
size as the frame and pages.
Still have Internal fragmentation

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Memory structures for paging can get huge using straight-
forward methods
Consider a 32-bit logical address space
Page size of 1 KB (210
)
Page table would have 4 million entries (232
/ 210
)
If each entry is 4 bytes -> Page table is of size 16 MB
 That amount of memory used to cost a lot.
 Do not want to allocate that contiguously in main
memory
What about a 64-bit logical address space?
Hierarchical Paging
Hashed Page Tables
Inverted Page Tables

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Two-Level Page-Table Scheme

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Inverted Page Table
Rather than each process having a page table and keeping
track of all possible logical pages, track all the physical
pages – using an inverted page-table.
The inverted page-table has one entry for each real page of
memory.
An entry the inverted-page table consists of the virtual
address of the page stored in that real memory location,
with information about the process that owns that page.
What is maximum size of the inverted page-table?

Memory : operating system ( Btech cse )

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