Fortran implementations have a fair amount of freedom given them by the
standard as far as how much storage space is used and how much precision
and range is offered by the various types such as `LOGICAL(KIND=1)`

,
`INTEGER(KIND=1)`

, `REAL(KIND=1)`

, `REAL(KIND=2)`

,
`COMPLEX(KIND=1)`

, and `CHARACTER`

.
Further, many compilers offer so-called *`n` notation, but
the interpretation of `n` varies across compilers and target architectures.

The standard requires that `LOGICAL(KIND=1)`

, `INTEGER(KIND=1)`

,
and `REAL(KIND=1)`

occupy the same amount of storage space, and that `COMPLEX(KIND=1)`

and `REAL(KIND=2)`

take twice as much storage space as `REAL(KIND=1)`

.
Further, it requires that `COMPLEX(KIND=1)`

entities be ordered such that when a `COMPLEX(KIND=1)`

variable is
storage-associated (such as via `EQUIVALENCE`

)
with a two-element `REAL(KIND=1)`

array named R, R(1)
corresponds to the real element and R(2) to the imaginary
element of the `COMPLEX(KIND=1)`

variable.

(Few requirements as to precision or ranges of any of these are
placed on the implementation, nor is the relationship of storage sizes of
these types to the `CHARACTER`

type specified, by the standard.)

g77 follows the above requirements, warning when compiling
a program requires placement of items in memory that contradict the
requirements of the target architecture.
(For example, a program can require placement of a `REAL(KIND=2)`

on a boundary that is not an even multiple of its size, but still an
even multiple of the size of a `REAL(KIND=1)`

variable.
On some target architectures, using the canonical
mapping of Fortran types to underlying architectural types, such
placement is prohibited by the machine definition or
the Application Binary Interface (ABI) in force for
the configuration defined for building gcc and g77.
g77 warns about such
situations when it encounters them.)

g77 follows consistent rules for configuring the mapping between Fortran
types, including the *`n` notation, and the underlying architectural
types as accessed by a similarly-configured applicable version of the
gcc compiler.
These rules offer a widely portable, consistent Fortran/C
environment, although they might well conflict with the expectations of
users of Fortran compilers designed and written for particular
architectures.

These rules are based on the configuration that is in force for the
version of gcc built in the same release as g77 (and
which was therefore used to build both the g77 compiler
components and the `libg2c`

run-time library):

`REAL(KIND=1)`

- Same as
`float`

type. `REAL(KIND=2)`

- Same as whatever floating-point type that is twice the size
of a
`float`

—usually, this is a`double`

. `INTEGER(KIND=1)`

- Same as an integral type that is occupies the same amount
of memory storage as
`float`

—usually, this is either an`int`

or a`long int`

. `LOGICAL(KIND=1)`

- Same gcc type as
`INTEGER(KIND=1)`

. `INTEGER(KIND=2)`

- Twice the size, and usually nearly twice the range,
as
`INTEGER(KIND=1)`

—usually, this is either a`long int`

or a`long long int`

. `LOGICAL(KIND=2)`

- Same gcc type as
`INTEGER(KIND=2)`

. `INTEGER(KIND=3)`

- Same gcc type as signed
`char`

. `LOGICAL(KIND=3)`

- Same gcc type as
`INTEGER(KIND=3)`

. `INTEGER(KIND=6)`

- Twice the size, and usually nearly twice the range,
as
`INTEGER(KIND=3)`

—usually, this is a`short`

. `LOGICAL(KIND=6)`

- Same gcc type as
`INTEGER(KIND=6)`

. `COMPLEX(KIND=1)`

- Two
`REAL(KIND=1)`

scalars (one for the real part followed by one for the imaginary part). `COMPLEX(KIND=2)`

- Two
`REAL(KIND=2)`

scalars. `numeric-type``*`

`n`- (Where
`numeric-type`is any type other than`CHARACTER`

.) Same as whatever gcc type occupies`n`times the storage space of a gcc`char`

item. `DOUBLE PRECISION`

- Same as
`REAL(KIND=2)`

. `DOUBLE COMPLEX`

- Same as
`COMPLEX(KIND=2)`

.

Note that the above are proposed correspondences and might change in future versions of g77—avoid writing code depending on them.

Other types supported by g77
are derived from gcc types such as `char`

, `short`

,
`int`

, `long int`

, `long long int`

, `long double`

,
and so on.
That is, whatever types gcc already supports, g77 supports
now or probably will support in a future version.
The rules for the `numeric-type`*`n` notation
apply to these types,
and new values for `numeric-type`(KIND=`n`) will be
assigned in a way that encourages clarity, consistency, and portability.