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Session 29 - April 16, 2003
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AGENDA:
0 Admin
1 Ch. 4.1: Basic memory management
2 Ch. 4.2: Swapping

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0 - ADMIN
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Everybody thoroughly confused about the test?  :-)

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1 - CH 4.1 BASIC MEMORY MANAGEMENT
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What are the characteristics of the ideal comp. memory?
* how close to that do we get?
* what are problems?

Describe the memory hierarchy
* In the context of the hierarchy, what would the
  memory manager do?

Memory management approaches
* keep processes in memory all the time
* move processes back and forth between main memory and
  disk during execution (swapping and paging).

Monoprogramming without swapping or paging
* What does mono programming mean?
* Fig 4-1 p. 310
* pros & cons

Multiprogramming with fixed partitions
* What does multiprogramming mean?
* Fig 4-2 p. 311
* pros & cons

Relocation
* different jobs run at different addresses
  - how can the program be coded to reference the
    correct address for where it will be run, esp.
    when it is not known where it will be run?


* could modify all instructions as loaded into memory
  to have the correct address
  - this is ugly, but can be done
  - what information would be needed to make this work?

Protection problem
* programs can access any memory address
* Tanenbaum: "In multiuser systems, it is undesirable
  to let processes read and write memory belonging to
  other users." Hmm, delicately understated.
* Proposed solutions?

  PSW key (process status word)
  Base and limit registers
  - solves relocation problem as well

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2 - CH 4.2 SWAPPING
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With timesharing systems/personal computers, not always
  enough memory to hold everything that wants to run.
* Possible solutions?

  - don't allow more to run than have memory
  - swapping (simplest)
  - virtual memory (not simplest)

Swapping
* Fig 4-3 p. 314
* Don't use fixed partitions - each partition is
  exactly the right size.
  - Possible problems?

    What if program needs more memory while running?
    - Fig 4-4 p. 315
    What about creating unusable "holes" in memory?
    - could do memory compaction, but what is problem?

Memory fragmentation: internal and external.
* We are dealing with external fragmentation when deal
  with holes in memory. Wasted space within memory
  block allocated to a process is internal
  fragmentation.



Memory management with bit maps
* Fig 4-5 p. 316
* What is the ideal size block?

  - size of bitmap vs. wasted space in block

* cost of finding correct size contiguous chunk
  using the bitmap

Memory management with linked lists
* Fig 4-6 p. 317
* Allocation algorithm suggestions?

  - first fit
  - next fit - continues search from where last ended
  - best fit
  - worst fit
  - quick fit

COS 421 - Lecture Notes #29		SPRING 2003	
COS421-lect-2003-04-16.doc	Page 1	Printed 16.04.03