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Chapter 6. Security in Open Source Software
pp. 353-453]. Static partitioning is very easy to implement, but it provides a very
inefficient usage of the main memory. In a fixed environment, like in embedded systems,
it can be used as a very simple but efficient memory management strategy due to the
systems constrains.
For example, the ARINC specification 653P1-2 [3], which is applicable for avionics software
standard interfaces, requires the usage of static partitioning for operating systems for the
corresponding embedded systems.
Dynamic partitioning: The memory M is divided in m = {1, 2, 3, . . . } partitions p j with
∑
dynamic sizes: ∀j : length(p j ) > 0, M = {p 0 , p 1 , . . . , p m−1 } while length(M) ≥
m−1
j=0 length(p j) [77, pp. 353-453]. Compared to static partitioning, dynamic partitioning
provides a more efficient usage of the main memory but needs in general more
computation time for its management.
6.1.2. Paging
For paging with virtual memory, the virtual memory V consists of the main memory M and
the swap space S: V = M ∪ S. This virtual memory V is divided in pages p k , which all have
the same size: ∀k : length(p k ) = s. Programs or processes are loaded in different but not
necessarily sequential pages depending on their required memory. Pages of a program can be
loaded on demand and do not need to be loaded completely at program start. A program can
access its memory in a sequential manner because the physical memory addresses are mapped.
Therefore, a program can only access pages that belong to it [77, pp. 353-453]. Paging with
virtual memory provides an effective way of memory management for multiprocessing operating
systems, but it is quite complex to implement.
6.1.3. Segmentation
For segmentation with virtual memory, also the virtual memory V consists of the main memory
M and the swap space S: V = M ∪ S. A program e ∈ {0, 1, 2, . . . , n} is divided in certain
number of segments s e,l (l ∈ {1, 2, 3, . . . m}), which have a dynamic size. This size can
be influenced by the developer of a program or the used compiler for high-level languages:
∀e : ∀l : length(s e,l ) > 0 while V ≥ ∑ n ∑ m
e=0 l=1 length(s e,l).
As in paging, the segments are not necessarily sequential and not all segments of a program
must be loaded at its start. Also, the physical memory is mapped to provide each program a
sequential access to its allocated memory / segments [77, pp. 353-453]. The main difference,
compared with the advantages of paging, is that a developer can influence the size of the
segments. This can also be used to define areas of shared data segments, where several programs
have access to. Its implementation is even more complex as for paging.
6.1.4. Segmentation combined with Paging
To gain the advantages from paging and segmentation together, another more sophisticated
strategy is to combine both strategies. This means that segmentation is used for programs
while the segments s e,l of each program e ∈ {0, 1, 2, . . . , n} are again divided in pages p e,l,k of
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