Chapter 8 Operating System Support Continued

1
Chapter 8Operating System Support (Continued)

• Paging
• Virtual memory
• Segmentation
• Illustrations in the Pentium Power PC

2
Paging

• Split memory into equal sized, small chunks -page
  frames
• Operating System maintains list of free frames
• Split programs (processes) into equal sized small
  chunks - pages
• Allocate the required number page frames to a
  process
• - A process does not require contiguous
• page frames
• - Each process has its page table

3
Allocation of Free Frames
4
Logical and Physical Addresses - Paging
5
Paging Implementation

• Demand paging
• Do not require all pages of a process in memory
• Bring in pages as required
• Page fault
• Required page is not in memory
• Operating System must swap in required page
• May need to swap out a page to make space
• Perhaps select page to throw out based on recent
  history

6
Thrashing

• Too many processes in too little memory
• Operating System spends all its time swapping
• Little or no real work is done
• Solutions
• Good page replacement algorithms
• Reduce number of processes running
• Add more memory

7
Virtual Memory

• We do not need all of a process in memory for it
  to run - We can swap in pages as required
• So - we can now run processes that are bigger
  than total memory available!
• Main memory is called real memory
• User/programmer can see much bigger memory space
  - virtual memory
• Tables can become huge that cant fit into memory
  need multiple level tables yech!

8
Alternate Inverted Page Table Structure
9
Translation Lookaside Buffer

• Every virtual memory reference causes two
  physical memory access
• Fetch page table entry
• Fetch data
• Use special cache for page table(s)

10
TLB and Cache Operation (special Cache for
tables)
11
Segmentation

• Paging is not (usually) visible to the programmer
• Segmentation is visible to the programmer
• Usually different segments allocated to program
  and data
• May be a number of program and data segments,
  e.g. to support protection levels, priority
  levels, organization, flexibility, etc.

12
Advantages of Segmentation

• Simplifies handling of growing data structures
• Allows programs to be altered and recompiled
  independently, without re-linking and re-loading
• Lends itself to sharing among processes
• Lends itself to protection
• Some systems combine segmentation with paging

13

OS Review

• Scheduling
• uni-programming
• multi-programming
• time-sharing
• long-term scheduler (queue of all jobs
  potentially schedulable)
• short-term scheduler (queue of processes that
  are ready to execute)
• medium-term scheduling (queue of jobs that can
  reside in memory)
• blocked monitoring (queue of processes blocked
  for resources)
• new ready running blocked exit state
  machine
• Memory management
• partitioning
• paging frames, pages, page fault, page table,
  logical/physical addr
• virtual memory inverted page table,
  Translation Lookaside Buffer
• segmentation

14
Pentium and Power PC

• Pentium Intel
• Power PC Motorola IBM
• A 32 bit memory address space is 4 G Bytes
• A 46 bit memory address space is 64 T Bytes
• 1.25 terabytes has been claimed as the capacity
  of a human being's functional memory (according
  to Raymond Kurzweil).
• A Holographic Versatile Disc (HVD) can hold up to
  3.9 terabytes.
• One hour of uncompressed Ultra High Definition
  Video (UHDV) consumes approximately 11.5
  terabytes of data.
• The U.S. Library of Congress has claimed that "as
  of December 31, 2005, the Library has collected
  more than 40 terabytes of data."
• A Protein-coated disc (PCD) can hold 50 terabytes
  of data.
• A 64 bit memory address space is ? (Who cares!)
• The point is that it is not clear to me why we
  care for some at least some years to come.

15
Pentium II (Uses hardware for segmentation
paging)

• Unsegmented, unpaged
• virtual address physical address
• Used in Low complexity, High performance systems
• Unsegmented, paged
• Memory viewed as paged linear address space
• Protection and management via paging (Ex
  Berkeley UNIX)
• Segmented, unpaged
• Collection of local address spaces
• Protection to single byte level, Translation
  table needed is on chip when segment is in
  memory, provide predictable access times
• Segmented, paged
• Segmentation used to define logical memory
  partitions subject to access control
• Paging manages allocation of memory within
  partitions (Ex Unix System V)

16
Pentium II Address Translation Mechanism

• Segment uses 2 bits to provide 4 levels of
  protection, typically
• 0 OS kernel, 1 OS, 2 apps needing
  special security, 3 general apps

17
Pentium II Paging

• Segmentation may be disabled
• In which case linear address space is used
• Two level page table lookup
• First, page directory
• 1024 entries max
• Splits 4G linear memory into 1024 page groups of
  4Mbyte
• Each page table has 1024 entries corresponding to
  4Kbyte pages
• Can use one page directory for all processes, one
  per process or mixture
• Page directory for current process always in
  memory
• Use TLB holding 32 page table entries
• Two page sizes available 4k or 4M

18
PowerPC 32-bit Address Translation
19
PowerPC Memory Management Hardware

• 32 bit paging with simple segmentation
• or 64 bit paging with more powerful segmentation
• Or, both do block address translation
• Map 4 large blocks of instructions 4 of memory
  to bypass paging
• e.g. OS tables or graphics frame buffers
• 32 bit effective address
• 12 bit byte selector
• 4kbyte pages
• 16 bit page id
• 64k pages per segment
• 4 bits indicate one of 16 segment registers
• Segment registers under OS control

20
PowerPC 32-bit Memory Management Formats