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1 Overview of Fortran 90

The diagram in Figure 1 shows the major components of Fortran 90 [1],[2]. The size of each slice of this ``pie" is roughly proportional to the number of syntax rules needed to describe the features associated with that slice, and hence is a measure of the structural complexity of those features. (These measures should not, however, be taken as an indication of conceptual or semantic complexity nor of implementation effort-syntactic complexity may or may not be related to these other forms of complexity.)

Fortran 77
Fortran 90 is a super-set of Fortran 77-all standard Fortran 77 programs are standard Fortran 90 programs. Fortran 90 therefore encompasses and is completely compatible with the existing Fortran 77 computational science infrastructure.

Source Form
To Fortran 77's `fixed' source form Fortran 90 adds another source form, called `free' source form, in which there are no column dependencies. In free source form comments need not start in column 1 and column 6 is not reserved for continuation; continuation in free source form is indicated by a trailing ampersand, on the `first' line. In both source forms an exclamation point , `!', may be used to initiate end-of-line comments (e.g., following a statement on that line) and a semicolon may be used to separate two statements on the same line. As in Fortran 77, Fortran 90 names (of variables, procedures, etc.) begin with a letter and contain letters and digits; in addition, names may have up to 31 characters, may contain underscore, `_', characters, and may contain both upper and lower case letters.

Figure 1: Fortran 90.