What is a 'storage unit' - numeric or otherwise?

As per Fortran standards going all the way back to Fortran-66, REAL, INTEGER, and LOGICAL data types are defined as using 1 'numeric storage unit'. DOUBLE PRECISION and COMPLEX are defined as using 2 numeric storage units. No guidelines or requirements are imposed as to how big, in terms of numbers of bits or bytes, a numeric storage unit is. This is intentional to allow Fortran to be easily implemented on a wide variety of hardware. These days one numeric storage unit tends to be 32-bits to accomodate the IEEE floating point standard. However in the past, I've used computers where a single numeric storage unit was 16, 18, 24, 32, 36, 60, and 64 bits. Even 48 bit numeric storage units are not unknown. Note that even though DOUBLE PRECISION is required to occupy twice the storage as REAL, the standard does not require twice the precision in calculations. Thus, even if only 1 additional bit were actually used, an implementation would meet the requirements of the standard. Fortran-77 ...  more