Java Glossary : orthogonal
Last updated 2004-06-28 by Roedy
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Glossary : O words : orthogonal.
- orthogonal
- Literally, at right angles, hence independent or irrelevant to. The term is
commonly used in describing computer instruction sets. With an orgothogonal
instruction set, such as the Motorola 68000, you can use any register for any
purpose. With a non-orthogonal set such as the Intel Pentium, each register has
special properties. E.g. only CX can be used for counting string loops. BX can
be used as an index register, but AX cannot. Only DX:AX can hold a dividend.
Orthogonal instuction sets result in bulkier hand-generated assembler code, but
are easier for compilers to generate optimally fast, compact code. More
generally, in programming languages, orthogonality means design so that changes
in one thing don't effect another. The example they give a user interface and
database -- you should be able to swap the database without changing the
interface or make changes to the interface without affecting the database.
Here are some examples of non-orthogonality in C:
-
C has two kinds of built in data structures, arrays and records (structs).
Records can be returned from functions, but arrays cannot.
-
A member of a struct can have any type except void or a structure of the same
type.
-
An array element can be any data type except void or a function.
-
Parameters are passed by value, unless they are arrays, in which case they are
passed by reference.
-
a+b usually means that they are added, unless a is a pointer the value of b may
be changed before the addition takes place.
Non-orthogonality means exceptions to the general language rules, which make it
harder to learn. It means that you cannot combine language features in all
possible ways. Excessive orthogonality makes it possible to say silly things in
the language that complicate the compilers.
