Memory Management Tanenbaum Ch' 4

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Memory Management (Tanenbaum Ch. 4)
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The Memory Hierarchy
Access Time
Capacity
lt 1 KB
1 nsec
Registers
1 MB
On-chip Cache
2 nsec
64-512MB
Main Memory
10 nsec
5-50GB
Magnetic (Hard) Disk
10 msec
Magnetic Tape
20-100GB
100 sec
Other types of memory ROM, EEPROM, Flash RAM
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Basic Memory Management
(Palm computers)
(MS-DOS)
BIOS

• An operating system with one user process

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Multiprogramming with Fixed Partitions

• Separate input queues for each partition
• Single input queue

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Problems with Fixed Partitions?
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Problems with Multiprogramming?

• Relocation the mechanism for fixing memory
  references in memory
• Cannot be sure where program will be loaded in
  memory address locations of variables, code
  routines cannot be absolute
• Protection one process should not be able to
  access another processes memory partition
• Solutions
• Relocation during loading
• Program Status Word (protection)
• Base and limit (protection and relocation)

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Swapping

• Dynamic partitions
• Allocate as much memory as needed by each process
• Swap processes out to disk to allow more
  multi-programming

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Swapping - example

• Memory allocation changes as
• processes come into memory
• leave memory
• Shaded regions are unused memory

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How much memory to allocate?

• (a) Allocating space for growing data segment
• (b) Allocating space for growing stack data
  segment

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Issues in Swapping

• When a process terminates
• Compact memory?
• Move all processes above the hole down in memory.
• Can be very slow 256MB of memory, copy 4 bytes
  in 40ns ? compacting memory in 2.7 sec
• Almost never used
• Result OS needs to keep track of holes.
• Problem to avoid memory fragmentation.

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Swapping Data Structure Bit Maps

• Part of memory with 5 processes, 3 holes
• tick marks show allocation units
• shaded regions are free
• Corresponding bit map

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Properties of Bit-Map Swapping

• Allocation unit is k bits
• bitmap uses M/k bits. Could be quite large.
• E.g., allocation unit is 32 bit
• Bit map uses 1/33 of memory
• Searching bit-map for a hole is slow

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Swapping Data Structure Linked Lists

• Variation keep a list of blocks (processP,
  holeH)