Presented By

Firoz Mahmud
Assistant Professor
Dept. of Computer Science & Engineering
Rajshahi University of Engineering & Technology
 Background
 Swapping
 Contiguous Memory Allocation
 Paging
 Structure of the Page Table
 Segmentation
 Example: The Intel Pentium
 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.
 Program must be brought (from disk) into memory
and placed within a process for it to be run.
 Main memory and registers are only storage CPU can
access directly.
 Register access in one CPU clock (or less).
 Main memory can take many cycles.
 Cache sits between main memory and CPU registers.
 Protection of memory required to ensure correct
operation.
 A pair of base and limit registers define the
logical address space
 Address binding of instructions and data to memory
addresses can happen at three different stages:
 Compile time: If memory location known a priori,
absolute code can be generated; must recompile code if
starting location changes.
 Load time: Must generate relocatable code if memory
location is not known at compile time.
 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).
 Source program

Compiler or assembler Compile time

Other object Object module

modules
Linkage editor
System library
Load module Load time

Dynamically
loaded system Loader
library
In-memory binary Execution time
Dynamic linking
memory image (run time)
 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; also
referred to as virtual 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-time
address-binding scheme
 Hardware device that maps virtual to physical
address.

 In MMU scheme, the value in the relocation

register is added to every address generated by
a user process at the time it is sent to memory.

 The user program deals with logical addresses;

it never sees the real physical addresses.
 The size of a process is limited to the size of
physical memory.
 Routine is not loaded until it is called.
 Better memory-space utilization; unused routine is
never loaded.
 Useful when large amounts of code are needed to
handle infrequently occurring cases such as error
routine.
 No special support from the operating system is
required implemented through program design.
 Linking 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 needed to check if routine is in
processes’ memory address.
 Dynamic linking is particularly useful for libraries.
 System also known as shared libraries.
 A process can be swapped temporarily out of memory to a backing store, and
then brought back into memory for continued execution.

 Backing store – fast disk large enough to accommodate copies of all memory
images for all users; must provide direct access to these memory images.

 Roll out, roll in – swapping variant used for priority-based scheduling

algorithms; lower-priority process is swapped out so higher-priority process
can be loaded and executed.

 Major part of swap time is transfer time; total transfer time is directly
proportional to the amount of memory swapped

 Modified versions of swapping are found on many systems (i.e., UNIX, Linux,
and Windows)
 System maintains a ready queue of ready-to-run processes which have
memory images on disk
 Main memory usually into two partitions:
 Resident operating system, usually held in low memory with
interrupt vector.
 User processes then held in high memory.

 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.
 Multiple-partition allocation
 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.
 Operating system maintains information about:
a) allocated partitions b) free partitions (hole)

OS OS OS OS

process 5 process 5 process 5 process 5

process 9 process 9

process 8 process 10

process 2 process 2 process 2 process 2

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 ordered by size.
 Produces the smallest leftover hole.
 Worst-fit: Allocate the largest hole; must also search entire list.
 Produces the largest leftover hole.

First-fit and best-fit better than worst-fit in terms of speed

and storage utilization.
 External Fragmentation– Total memory space exists to satisfy a request, but it is
not contiguous.
 Internal Fragmentation – allocated memory may be slightly larger than requested
memory; this size difference is memory internal to a partition, but not being used.
 Suppose a new process need 260KB memory space to run, Space are available
but not contiguous.

Internal Fragmentation
External Fragmentation
300KB P1
P1
hole
hole 20KB
200KB P2
P2 512KB

180KB hole
180KB hole
P3
P3
185KB
hole
270KB
 Reduce external fragmentation by compaction
 Shufflememory 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
Latch job in memory while it is involved in
I/O
Do I/O only into OS buffers
 Logical address space of a process can be noncontiguous;
process is allocated physical memory whenever the latter is
available.
 Divide physical memory into fixed-sized blocks called frames
(size is power of 2, between 512 bytes and 8,192 bytes).
 Divide logical memory into blocks of same size called pages.
 Keep track of all free frames.
 To run a program of size n pages, need to find n free frames
and load program.
 Set up a page table to translate logical to physical addresses.
 Internal fragmentation.
 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.

 Page offset (d) – combined with base address to define the

physical memory address that is sent to the memory unit.

page number page offset

p d
 For given logical address space 2m and page size 2n
m-n n
 Frame number
0
Page 0
0 1 1 Page 0
Page 1 1 4
2
Page 2 2 3
3 7 3 Page 2
Page 3
4 Page 1
Logical memory Page table
5

7 Page 3

Physical memory
Before allocation After allocation
 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 the
page table.
 In this scheme 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).
 Some TLBs store address-space identifiers (ASIDs) in
each TLB entry – uniquely identifies each process to provide
address-space protection for that process.
 Associative memory – parallel search

Page # Frame #

Address translation (p, d)

 If p is in associative register, get frame # out
 Otherwise get frame # from page table in
memory
 Associative Lookup =  time unit
 Assume memory cycle time is 1 microsecond
 Hit ratio – percentage of times that a page number is
found in the associative registers; ratio related to number
of associative registers
 Hit ratio = 
 Effective Access Time (EAT)
EAT = (1 + )  + (2 + )(1 – )
=2+–
 Memory protection implemented by associating
protection bit with each frame

 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 a legal page.
 “invalid” indicates that the page is not in the
process’ logical address space.
 Shared code
 One copy of read-only (reentrant) code shared among
processes (i.e., text editors, compilers, window systems).
 Shared code must appear in same location in the logical
address space of all processes

 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
 Hierarchical Paging

 Hashed Page Tables

 Inverted Page Tables

 Break up the logical address space into
multiple page tables

 A simple technique is a two-level page

table
 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:

page number page offset

pi p2 d

where pi is an index into the

12outer10
page table,
10and p2 is the displacement within
the page of the outer 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

 Virtual page numbers are compared in this chain

searching for a match
 If a match is found, the corresponding physical
frame is extracted
 One entry for each real page of memory
 Entry consists of the virtual address of the page
stored in that real memory location, with
information about the process that owns that
page
 Decreases memory needed to store each page
table, but increases time needed to search the
table when a page reference occurs
 Use hash table to limit the search to one — or at
most a few — page-table entries
 Memory-management scheme that supports user view of memory
 A program is a collection of segments
 A segment is a logical unit such as:
main program
procedure
function
method
object
local variables, global variables
common block
stack
symbol table
arrays
 1

4
1

3 2
4

user space physical memory space

 Logical address consists of a two tuple:
<segment-number, offset>,
 Segment table – maps two-dimensional physical addresses;
each table entry has:
 base – contains the starting physical address where the
segments reside in memory
 limit – specifies the length of the segment
 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
 Protection
 With each entry in segment table associate:
validation bit = 0  illegal segment
read/write/execute privileges
 Protection bits associated with segments; code sharing
occurs at segment level
 Since segments vary in length, memory allocation is a
dynamic storage-allocation problem
 A segmentation example is shown in the following
diagram
 Supports both segmentation and segmentation with
paging
 CPU generates logical address
 Given to segmentation unit
 Which produces linear addresses
 Linear address given to paging unit
 Which generates physical address in main
memory
 Paging units form equivalent of MMU