Chapter 2 Memory Management,
Early Systems
CIS106
Microcomputer Operating Systems
Gina Rue
CIS Faculty
Ivy Tech State College Northwest Region 01
Memory Manager
Four types of memory allocation schemes
• single-user systems
• fixed partitions
• dynamic partitions
• re-locatable dynamic partitions
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Memory Manager
• Memory performance is dependent on
two things:
– how much memory is available
– how it is optimized while jobs are being
processed
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Single-user
Contiguous Scheme
First memory allocation scheme
• early computers had limited memory
• allocated as much contiguous space
– if the job was too large, it couldn’t execute
• jobs processed sequentially
– OS algorithm used to allocate memory
• did not support multiprogramming
See Single User Algorithm p.18
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Fixed-Partitions
• First attempt to allow for
multiprogramming
• created within main memory - one
partition for each job
• to protect job’s memory space critical
– algorithm used to store jobs in memory, size
of job must match size of partition
– major drawback: internal fragmentation is
unused space within the fixed partition
See Table & Fig. 2.1 p.20
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Dynamic Partitions
• available memory kept in contiguous
blocks
• jobs are given only as much memory as
they request when loaded for processing
• first-come first-served priority basis
– subsequent allocation of memory creates
fragments
– called external fragmentation, free memory
blocks between allocated memory
See Fig. 2.2 p.22
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Best-Fit vs First-Fit Allocation
New jobs come into the system, the free
partitions must be allocated
• first-fit
first partition fitting the requirement, low to high
order memory list
• best-fit
closest fit, smallest partition fitting the
requirements, smallest to largest memory list
For both schemes, memory manager keeps
lists of free & used partitions (free/busy) by
size or location See Table 2.2 & 2.3 p.24
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Best-Fit vs First-Fit Allocation
Which is best?
• For many years, no answer
• best-fit resulted in a better “fit”, it also
resulted in smaller “free” space (sliver)
• In recent years, access times have improved
so that the scheme that saves more
resources & memory space is the best choice
• Research continues to find the optimum
allocation scheme
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Deallocation
Memory space must be released
• fixed partitions - when a job is complete, the
memory manager resets status of memory block
where the job was stored to “free”
• dynamic partitions - more complex algorithm tries
to combine free areas of memory wherever possible
3 alternative situations systems must prepare for
deallocation
– block is adjacent to another free block
– block is between two free blocks
– block is isolated from other free blocks
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Relocatable Dynamic Partitions
Both fixed & dynamic leave “slivers” of
memory left over
Solution - Memory Manager relocates
programs to gather together all empty
blocks & compact them into one large
enough to accommodate jobs waiting
compaction - every program in memory
must be relocated so they are contiguous
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Relocatable Dynamic Partitions
What goes on behind the scenes when
relocation and compaction take place?
What keeps track of how far each job has
moved from its original location?
• Special-purpose registers are used to help
with relocation
• bound register-stores highest or lowest
location in memory accessible by each
program
• relocation register-contains the value that
must be added to each address referenced in
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the program
Relocatable Dynamic Partitions
What lists have to be updated?
• After relocation & compaction, both free
lists and busy lists are updated
• Each job will have a new address except for
those that already reside in the lowest
memory locations
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Relocatable Dynamic Partitions
In effect,
by compacting & relocating,
the Memory Manager optimizes the use
of memory thus improves throughput -one of the measures of the system
performance. The crucial factor is the
timing of the compaction….
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Relocatable Dynamic Partitions
Compaction - when & how often should it
be done?
– When a certain % of memory becomes busy,
75 percent
– Only when there are jobs waiting to get in
– After a prescribed amount of time
The best choice depends on the job mix to
optimize both processing time & memory
use while keeping overhead low
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Summary
• 4 memory management techniques
– single-user, fixed partitions, dynamic
partitions, relocatable partitions
– all 4 require that the entire program
• loaded into memory
• be stored contiguously
• remain in memory until the job is completed
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Summary
• 4 memory management techniques
(schemes)
– sufficient for first 3 generations of
computers which processed jobs in batch
mode
– turnaround time was delayed to hours or
even days between job submission and
completion
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