I/O routines returned error codes of 128-255 (8016-FF16) via the processor's Y register and setting the carry flag of the processor. Setting the carry flag is a neat trick since the caller can immediately branch-on-carry (BCC or BCS instructions) to an error routine, a brief, quick and relocatable 6502 instruction (2 bytes, 2 cycles), without having to test Y for the (we hope) normal case where there is no error.
As with other aspects of the CIO, error codes were common across devices but could be extended for particular devices. Error handlers could thus be written quite generically, to fail gracefully, maybe put out a message, ask the user whether to retry, propagate the error, and so on.
There were no user-friendly messages for standard error codes in the OS itself. They would be interpreted by the application.
Atari BASIC (and other languages) thus had the freedom to return error codes less than 128, and these meant different things in different languages. There was nothing to stop a perverse implementer using error codes of 128 or above, but no incentive to do so.
Other articles related to "error handling, error, errors":
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... The error handling of the functions in the C standard library is not consistent and sometimes confusing ... Most (but not all) functions raise exceptions on errors ...
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