Group:
program design
Topic:
decomposition of a system into levels
Topic:
design documentation
Topic:
families of programs
Topic:
localized understanding
Topic:
information hiding
Topic:
hierarchical structures
Topic:
object-oriented design
Topic:
virtual machine
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Summary
Parnas builds virtual machines from a 'uses' hierarchy, i.e., the structural decomposition of a system into its simplest components. One module (A) "uses" another (B) if using module B simplifies A's specification, the relationship is hierarchical, and B is useful without regard to A. "uses" is not who calls whom since the interrupt handler is used by everyone but called by no one. (cbb 5/80)
Subtopic: assumption 
| Quote: measure structure by identifying those units that make assumptions about other units [»chanRN_1974, OK]
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Subtopic: uses hierarchy
| Quote: should organize modules in a hierarchy over the 'uses' or 'depends on' relation; allows independent use of lower levels
| | Quote: in higher-order-software, a module controls invocation and ordering of its immediate sub-modules only [»hamiM3_1976]
| | Quote: the design of the 'uses' hierarchy should be a major design milestone [»parnDL5_1978]
| | Quote: a program is at the right 'uses' level if it doesn't need functions above it but does need ones below it [»parnDL5_1978]
| | Quote: good interfaces allow low-level details to change without effecting the higher levels [»rossDT_1970]
| | Quote: in a 'uses' hierarchy, low level operations may assume a data structure, next level may allow similar data structures, and top level doesn't know [»parnDL5_1978]
| | Quote: the top-down designer creates tree structures; at a level with no shared components [»jackMA_1975]
| | Quote: CADES design is a series of levels; each level an abstract machine [»pearDJ7_1973]
| | Quote: if A and B use each other, then split into more than one program; called sandwiching [»parnDL5_1978]
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Subtopic: uses subset
| Quote: each level of a 'uses' hierarchy is a testable and usable subset of the system [»parnDL5_1978]
| | Quote: for an application, identify the programs needed from the 'uses' hierarchy and include everything else that is used [»parnDL5_1978]
| | Quote: can test and use subsets of a level in the 'uses' hierarchy
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Subtopic: A uses B
| Quote: A uses B when it simplifies A, B doesn't use A, a useful subset includes B alone, and useful subsets with A must include B [»parnDL5_1978]
| | Quote: 'A uses B' if the correct functioning of A depends on a correct implementation of B [»parnDL5_1978]
| | Quote: a program may use another without invoking it; e.g., most programs depend on interrupt handlers working correctly [»parnDL5_1978]
| | Quote: a program may invoke a program without using it; e.g., if A's specifications only require that it invoke B on some condition [»parnDL5_1978]
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Subtopic: problems of uses hierarchy
Quote: lower levels can't use knowledge about higher levels; but undesired events need higher level information to determine the appropriate action
[»parnDL10_1976b]
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Related Topics
Group: program design (13 topics, 454 quotes)
Topic: decomposition of a system into levels (49 items)
Topic: design documentation (43 items)
Topic: families of programs (11 items)
Topic: localized understanding (43 items)
Topic: information hiding (50 items)
Topic: hierarchical structures (14 items)
Topic: object-oriented design (30 items)
Topic: virtual machine
(13 items)
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