CSC 350 - Operating Systems

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CSC 350 - Operating Systems
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CSC 350 - Operating Systems
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CSC 350 - Operating Systems

• Creeds or Chaos?

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Memory Management I
II. Memory Management I

• manage the scarce resource of memory

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Memory Management I
C. Dynamic partitions MMS

• Partitions are created dynamically on the fly

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Memory Management I
Analogy

• Partitions are like suitcases

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Memory Management I
3. OS concept - allocation

• programs must be assigned to memory
• note if not enough contiguous RAM, program must
  wait

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Memory Management I

• programs must be assigned to memory
• how? (tradeoffs)

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Memory Management I
a. first-fit

• the first (lowest) partition big enough for the
  program will be allocated

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a. first-fit

• adv fast allocation

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b. best-fit

• the best (causing least fragmentation) partition
  will be allocated

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b. best-fit

• adv best utilization

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3. OS concept - allocation

• tables 2.2 and 2.3

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Memory Management I
3. OS concept - allocation

• suitcase analogy

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3. OS concept - allocation example problem 4,
p.39

• Job / Program size
• Block / Partition size

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Memory map
Jobs programs on storage
block 1 block 2 block 3
610K
850K
J1 740K J2 500K J3 700K
700K
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Memory Management I
Memory map
a. best fit
block 1 block 2 block 3
J2 500K
610K
J1 740K
850K
J3 700K
700K
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Memory Management I
Memory map
b. first fit low - highsearch
block 1 block 2 block 3
J2 500K
610K
J1 740K
850K
J3 700K
700K
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Memory Management I
Memory map
b. first fit high-low search
block 1 block 2 block 3
610K
J1 740K
850K
J2 500K
700K
J3 700K wait
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3. OS concept - allocation

• group exercise -)modified problem 4, p.39

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Memory Management I

• modified problem 4, p.39
• Job Size Block Size
• 1 740K 1 610K
• 2 500K 2 850K
• 3 600K 3 500K

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3. OS concept - allocation

• anotheranalogy

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4. OS - deallocation

• programs must be removed from memory when
  finished

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4. OS - deallocation easy for fixed partitions

• more difficult for dynamic partitions

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scenarios

• a. deallocated memory is not adjacent to free
  memory

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Memory Management I

• a. deallocated memory is not adjacent to free
  memory

allocated
To be deallocated
allocated
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Memory Management I
4. OS - deallocation

• scenarios
• b. deallocated memory is adjacent to one free
  memory partition

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Memory Management I

• b. deallocated memory is adjacent to one free
  memory

free
To be deallocated
allocated
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Memory Management I
4. OS - deallocation

• scenarios
• c. deallocated memory is between two free memory
  partitions

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Memory Management I

• c. deallocated memory is between two free memory
  blocks

free
To be deallocated
free
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Memory Management I
D. Relocatable Dynamic partitions MMS

• 1. Based upon Dynamic Partitions MMS with
  additional feature

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Based upon Dynamic Partitions MMS with additional
feature

• At times, OS will compact memory

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Red allocated partitions Green free space
(unallocated RAM)
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2. Tradeoffs

• a. advantages
• b. disadvantages

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review CSAO sections 2.2 and 2.3
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3. OS needs h/w assistance

• bounds register
• relocation register

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B1 20K B2 10K B3 30K
Prog K Prog B Prog S
now, Program B terminates,
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B1 20K B2 10K B3 30K
Prog K Prog S
now, Program B terminates, OS deallocates
Partition 2
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B1 20K B2 30K
Prog K Prog S
now, Program B terminates, OS deallocates
Partition 2 and compacts (relocates)
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B1 20K B2 30K
Prog K Prog S
Prog S Bounds Register 30K Relocation Register
-10K