| Assembler Error and Warning Messages

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`as` may write warnings and error messages to the standard error file (usually your terminal). This should not happen when a compiler runs `as` automatically. Warnings report an assumption made so that `as` could keep assembling a flawed program; errors report a grave problem that stops the assembly.
Warning messages have the format

    file_name:NNN:Warning Message Text

(where NNN is a line number). If a logical file name has been given (see `.file`) it is used for the filename, otherwise the name of the current input file is used. If a logical line number was given (see `.line`) then it is used to calculate the number printed, otherwise the actual line in the current source file is printed. The message text is intended to be self explanatory (in the grand Unix tradition).
Error messages have the format

    file_name:NNN:FATAL:Error Message Text

The file name and line number are derived as for warning messages. The actual message text may be rather less explanatory because many of them aren't supposed to happen.