Tölvumál - 01.04.1981, Blaðsíða 2

2
tölvum^l'
Short Presentation by Michael Jackson on 13th April 1981
for the Icelandic Society for Information Processing
Structured Programming, like Modular Programming, raises
central questions of design. It is not enough to say that
a program should be constructed from DO-WHILE and IF-THEN-
ELSE components: the central question remains ’what struc-
ture is right for the given problem?1.
A true design method must answer this question. It must
do so by decomposing the design activity into a step-by-
step procedure which the designer uses to reach the right
structure. The JSP method of program design bases the
program structure on the structure of the data which it
processes. The design procedure is:-
- Data step: define the structures of the data which
are input to and output from the program.
- Program step: combine the data structures into a
single structure which will be the structure of the
program.
- Operations step: list the executable operations to be
carried out by the program, and allocate each one to
its proper place in the program structure.
- Text step: transcribe the design into a textual form
that can be converted into machine code, adding the
conditions to components that require them.
JSP design ensures that the structure of the program will
accurately reflect the structure of the data, and hence of
the problem that is being solved. This is the reason for
the greatly reduced cost of maintenance experienced by JSP
users. High maintenance costs are a direct result of a
failure to produce a program whose structure matches the
structure of the problem: an apparently small and simple
change in the problem specification causes a large and com-
plex change in the program. This can be avoided only by
correct program structure, and by the adoption of a design
method which leads to that correct structure.
Technical details of JSP may be found in:
Principles of Program Design; M A Jackson; Academic
Press; 1975.
JSP - En Praktisk M.etod för Programkonstruktion; Leif
Ingevaldsson; Studentlitteratur; 1977.
There are also various technical and management papers avail-
able from:
Michael Jackson Systems Limited, 101 Hamilton Terrace,
London NW8 9QX, England;
Michael Jackson Systems International Limited, Karlagacan
10, S 416 61 Göteborg, Sweden.