When did FORTRAN decide on signed integers?

Question

I have personally always been of the opinion that it would make sense for the default integer type to be unsigned, though it's been a long time since that would've been a live issue for debate; C in the 1970s was already defaulting to signed integers, nor was it the first language to do so.

I'm interested in exactly when, why and how the decision was first made. In assembly language, there isn't really a default; you always specify signed versus unsigned. So we should look at high-level languages (using here the classical definition of high-level as 'higher level than assembly'). The first significantly influential high-level language was Fortran. Modern Fortran standards mandate that compilers shall treat integer variables as signed unless otherwise specified.

When did Fortran decide this? Was the decision already made in the earliest Fortran compilers? Did any compilers on any machines, treat integers as unsigned?

Answer 1

FORTRAN was originally developed for the IBM 704 computer, which stored integers in sign-and-magnitude format. In the original documentation, it supports fixed-point variables, which used the machine’s native format, floating-point variables, and unsigned fixed-point constants, which were intended for line numbers and subscripts. These would be translated into offsets that fit into the 704’s index registers. Other than indexing by those three registers, the 704 had only a small handful of instruction for unsigned integer arithmetic,ACY and CAL (which could add a logical word to the accumulator). It was also possible to do multiplication by constants through a combination of shifts and additions, which was enough to compute the address of an array element, but not to perform other arithmetic.

John Backus wrote later, “We certainly had no idea that languages almost identical to the one we were working on would be used for more than one IBM computer, not to mention those of other manufacturers.”

There was no serious consideration of unsigned integer variables, because the architecture FORTRAN was designed to run on had no such native type. The decision was made for that technical reason. Considerations about how useful one would or would not have been for computational physics were not historically important to it.

Answer 2

Fortran was developed with scientific computing in mind. Negative values clearly occur quite frequently when doing scientific computing or, for that matter, in many other problem domains that the developers of the language might have considered.

Supporting unsigned integral would have had some value but the language would have still had to support signed integral types. Since almost all interesting unsigned values fit comfortably within a signed integer variable, they may have felt that they were able to support their target audience (scientific computing) while also providing a reasonable level of support for those who really did want to work with unsigned values. Alternatively, they may have simply decided to focus on signed arithmetic without making any real effort to consider what it might mean to support unsigned arithmetic simply because their target audience would almost all need a language that supported signed values.