1. Chapter 8 Operating 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. Too many processes in too little memory
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 can’t 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. Scheduling: OS Review 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. Power PC – Motorola & IBM
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. Segmentation may be disabled Two level page table lookup
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