CS 426 Operating Systems

1
CS 426 - Operating Systems

• Class 12
• February 15, 2000

2
Thought for the Day

• Human beings,
• who are almost unique in having the ability to
  learn from the experience of others,
• are also remarkable for their apparent
  disinclination to do so.
• -- Douglas Adams,
• Last Chance to See

3
Todays Cartoon
The cartoon that was here came from Randy
Glassbergens Cartoon of the Day site.
4
Todays Agenda

• Paper 1 Assignment
• Hand back Project 1
• Finish Chapter 8 Memory Management

5
Reading Assignment

• Chapter 9 Virtual Memory for Thursday (Dr. V.)

6
Paper 1 Assignment

• Compile run fork.C on Bama.
• Investigate all the included files and all the
  system calls.
• Write a short paper explaining
• What the program does
• How it does it (calls, etc.)
• Explain the output.
• Detailed instructions handout, web page.
• Due Feb 24th.
• Any questions on this project?

7
Warnings about Papers

• Note well English grammar, spelling,
• This will be graded strictly.
• Get help if you need it!
• At the least, use a spelling checker and a
  grammar checker!
• Be very careful about plagiarism!

8
Project 1

• Average 37/40
• Notes
• Questions 1, 2 3 only marked off if obviously
  wrong.
• Q 4a What do these plots represent?
• What is the idle process? The PVIEW process?
• Q 6a What applications are running?
• Q 6d Any plot in Performance Monitor like Memory
  Usage in Task Manager?

9
Chapter 8 Memory Management

• Background
• Logical versus Physical Address Space
• Swapping
• Contiguous Allocation
• Paging
• Segmentation
• Segmentation with Paging

10
Memory Protection

• 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.

11
Two-Level Page-Table Scheme
12
Two-Level Paging Example

• A logical address (on 32-bit machine with 4K page
  size) is divided into
• a page number consisting of 20 bits.
• a page offset consisting of 12 bits.
• Since the page table is paged, the page number is
  further divided into
• a 10-bit page number.
• a 10-bit page offset.
• Thus, a logical address is as follows

where pi is an index into the outer page table,
and p2 is the displacement within the page of the
outer page table.
13
Address-Translation Scheme

• Address-translation scheme for a two-level 32-bit
  paging architecture

14
Multilevel Paging and Performance

• Since each level is stored as a separate table in
  memory, covering a logical address to a physical
  one may take four memory accesses.
• Even though time needed for one memory access is
  quintupled, caching permits performance to remain
  reasonable.
• Cache hit rate of 98 percent yields
• effective access time 0.98 x 120 0.02 x
  520
• 128 nanoseconds.which is only a 28
  percent slowdown in memory access time.

15
Inverted Page Table

• 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.

16
Inverted Page Table Architecture
17
Shared Pages

• 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.

18
Shared Pages Example
19
Segmentation

• 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,
• local variables, global variables,
• common block,
• stack,
• symbol table, arrays

20
Logical View of Segmentation
1
2
3
4
user space
physical memory space
21
Segmentation Architecture

• Logical address consists of a two tuple
• ltsegment-number, offsetgt
• 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 tables location in memory.
• Segment-table length register (STLR) indicates
  number of segments used by a program
• segment number s is legal if s lt
  STLR.

22
Segmentation Architecture (2)

• Relocation.
• dynamic
• by segment table
• Sharing.
• shared segments
• same segment number in each process (hard!)
• Allocation.
• first fit/best fit
• external fragmentation

23
Segmentation Architecture (3)

• 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

24
Sharing of segments
25
Segmentation with Paging MULTICS

• The MULTICS system solved problems of external
  fragmentation and lengthy search times by paging
  the segments.
• Solution differs from pure segmentation in that
  the segment-table entry contains not the base
  address of the segment, but rather the base
  address of a page table for this segment.

26
MULTICS Address Translation Scheme
27
Segmentation with Paging Intel 386

• As shown in the following diagram, the Intel 386
  uses segmentation with paging for memory
  management with a two-level paging scheme.

28
Intel 30386 address translation
29
Comparing Memory-Management Strategies

• Hardware support
• Performance
• Fragmentation
• Relocation
• Swapping
• Sharing
• Protection