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Chapter 8 Operating System Support (Continued)

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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

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