Combined paging and segmentation

COMBINED PAGING AND
SEGMENTATION

CONTENTS

 A small review of Paging
 Segmentation
 Combined Segmentation and
Paging
 Examples

PAGING
allows the physical
address space of a
Extends to any process to be
levels non-contiguous
Pages

PTBR and PTLR PAGING
Page Frames

CPU generated
Logical Address PageTable
FrameTable
(One of the (One for each
Page# Offset Process)
Main Memory)

EXAMPLE-LOGICAL TO PHYSICAL ADDRESS
TRANSLATION IN PAGING
16-bit logical address

6-bit page # 10-bit offset #

0 0 0 0 0 1 0 1 1 1 0 1 1 1 0

Page Table

0 0 0 0 1 0 1
1 0 0 0 1 1 0

0 0 0 1 1 0 0 1 1 1 0 1 1 1 1 0

16-bit Physical address

ADVANTAGES DISADVANTAGES

• No external Fragmentation • Internal fragmentation
• Simple memory • Page tables may consume
management algorithm more memory.
• Swapping is easy (Equal • Multi level paging leads to
sized Pages and Page memory reference overhead.
Frames)

HOW DOES USER VIEW MEMORY??

• An important part of the memory management is that become
unavoidable with paging is the separation of the user’s view of
the memory and the actual physical memory
• The users view is mapped to the physical memory
• Thus the differentiation comes between the logical and
physical memory.
Do users view memory as linear
array of bytes some containing
instruction and other
containing data???

No…Rather they would see it as
collection of segments…….

SEGMENTATION
It is a memory-management scheme that supports
user view of memory.

A program is a collection of segments which is a
logical unit which may be of unequal size.
• Each segment may be code of a single procedure, the data of an array, or the
collection of all local data values used by a particular module.

Here main memory is not partitioned.

When user program is compiled , compiler
automatically constructs segments.
• The program segments are specified by programmer to the
compiler (Decision made by the programmer)

SO HOW IS IT FOR C COMPILER???

Code

Global Variables

Heap
C COMPILER
The standard C library

Stack

LOGICAL ADDRESSING IN SEGMENTATION

Segment number
Logical Address space
Offset

The mapping of the logical address to the physical address is done with the help of
the segment table.
the length of the segment
SEGMENT TABLE

Segment Limit Segment Base Other bits

A bit is needed to determine if the segment is already
starting address of the in main memory (P)
corresponding Another bit is needed to determine if the segment has
segment in main mem been modified since it was loaded in main memory (M)

• The segments of a program can be placed anywhere in the
main memory.
• For each process in each execution, there is one segment
address table.
• 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

EXAMPLE-LOGICAL TO PHYSICAL ADDRESS
TRANSLATION IN SEGMENTATION
16-bit logical address

12-bit offset #

0 0 0 1 0 0 0 1 1 1 0 1 1 1 0

Segment Table
Length Base
0 0 0 1 0 1 1 1 0 1 1 1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0
1 0 1 1 1 1 0 0 1 1 1 1 0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 0

12-bit offset # 16-bit seg base

0 0 1 0 0 0 1 1 0 0 0 1 0 0 0 0

16-bit Physical address

Logical address External
Segment # Offset fragmentation
Seg 1
(code) Seg 2 Physical
(data) memory
Seg 3
as in paging:
(stack)
valid, modified, Seg 3
Logical Memory protection, etc. (stack)

MMU Segment table Seg 1
Base Limit Other (code)
offset <
no limit ? STBR
yes STLR
memory Seg 2
access fault (data)
Segment Base + Offset
physical address
0x00

ADVANTAGES OF SEGMENTATION

• No internal fragmentation
• Segment tables consume less memory than page tables ( only
one entry per actual segment as opposed to one entry per page
in Paging method)
• Because of the small segment table, memory reference is easy.
• Lends itself to sharing data among processes.
• Lends itself to protection.
• As the individual lines of a page do not form one logical
unit, it is not possible to set a particular access right to a page.
• Note that each segment could be set up an access right

PROTECTION AND SHARING

• Segmentation lends itself to the implementation of protection
and sharing policies
• Each entry has a base address and length so inadvertent
memory access can be controlled
• Sharing can be achieved by segments referencing multiple
processes
• Two processes that need to share access to a single segment
would have the same segment name and address in their
segment tables.

DISADVANTAGES
• External fragmentation.
• Costly memory management algorithm
• Unequal size of segments is not good in the case of swapping.

So, why can’t we combine the ease of sharing and
protection we get from segments with efficient
memory utilization we get from pages ????

COMBINED SEGMENTATION AND
PAGING

In a combined
paging/segmentation system a Segmentation is visible to the
user’s address space is broken programmer
up into a number of segments.
Each segment is broken up
into a number of fixed-sized
pages which are equal in Paging is transparent to the
length to a main memory programmer
frame

IMPLEMENTING SEGMENTATION
• Each process has:
one segment table.
several page tables : one page table per segment.

Segment number : used to index the
segment table who’s entry gives the starting
address of the page table for that segment.

Logical Page number : used to index that page table
Address space to obtain the corresponding frame number

Offset : used to locate the word within the
frame.

ADDRESSES IN A SEGMENTED PAGING
SYSTEM
 The segment number indexes into the segment table which
yields the base address of the page table for that segment.
 Check the remainder of the address (page number and offset)
against the limit of the segment.
 Use the page number to index the page table. The entry is the
frame. (The rest of this is just like paging.)
 Concat the frame and the offset to get the physical address.

ADDRESS TRANSLATION
Seg Page
ment table
limit base

yes Physical
CPU
s so < so memory

Logical no
address p po

Memory +
trap
Page table (for
segment)

f po
f

ADVANTAGES
• Reduces memory usage as opposed to pure paging
– Page table size limited by segment size
– Segment table has only one entry per actual segment
• Share individual pages by copying page table entries.
• Share whole segments by sharing segment table entries, which
is the same as sharing the page table for that segment.
• Most advantages of paging still hold
– Simplifies memory allocation
– Eliminates external fragmentation.
• In general this system combines the efficiency in paging with
the protection and sharing capabilities of the segmentation.

DISADVANTAGE

• Internal fragmentation still exists…

Page 1 Process requests a 6KB address range
Page 2 (4KB pages)

internal
fragmentation

EXAMPLE

The Intel Pentium
Supports both segmentation and segmentation
with paging. CPU generates logical address given
to segmentation unit which in turn is handed over
to paging unit which generates physical address
in main memory

EXAMPLE 1
• Assume that a task is divided into four equal-sized segments
and that the system builds an eight-entry page descriptor table
for each segment. Thus, the system has a combination of
segmentation and paging. Assume also that the page size is 2
Kbytes.
a. What is the maximum size of each segment?
b What is the maximum logical address space for the task?
c. Assume that an element in physical location 00021ABC is
accessed by this task. What is the format of the logical address
that the task generates for it? What is the maximum physical
address space for the system?

SOLUTION

A. (8 entries in the page table) x 2K = 16K.

B. (16 K) x 4 segments per task = 64K.

C. The physical address is 32 bits wide total, so the frame
number must be 21 bits wide. Thus 00021ABC is represented
in binary as:
Frame Offset
0000 0000 0000 0010 0001 1 | 010 1011 1100

D. The maximum physical address space is 232 = 4 GB

EXAMPLE-2
• This question refers to an architecture using segmentation with
paging. In this architecture, the 32-bit virtual address is divided into
fields as follows:
4 bit segment number
12 bit page number
16 bit offset

Find the physical address corresponding to each of the following
virtual addresses (answer "bad virtual address" if the virtual address
is invalid)..

1. 00000000
2. 20022002
3. 10015555

Segment table Page table A Page table B
0 Page table A 0 CAFE 0 F000

1 Page table B 1 DEAD 1 D8BF

x (rest invalid) 2 BEEF X (rest invalid)

3 BA11

X

SOLUTION:

1. CAFE0000.
2. Bad virtual address.
3. D8BF5555.

REFERENCES….

• Silberschatz,Galvin,Gange – Operating
System Concepts (7th Ed)
•William Stalling – Operating System
(6th Ed)

Combined paging and segmentation

Combined paging and segmentation

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