?SYNTAX ERROR

?SYNTAX ERROR is the message the Commodore 64 BASIC interpreter emits when a line of code violates the language’s grammatical rules. The leading question mark is part of the standard Commodore error format (KERNAL + BASIC V2), not a typographical quirk.

How it works #

The C64 BASIC interpreter processes code line by line, either at runtime or when a statement is entered in direct mode. When it encounters an unrecognizable token — a misspelled keyword, an unbalanced parenthesis, a misplaced operator — it halts and prints the message followed by the offending line number:

?SYNTAX ERROR IN 100
READY.

The line number shown is where the interpreter detected the problem, not necessarily where it originated. An incomplete expression on line 90 can surface as an error on line 100, where the interpreter tries to consume tokens that are no longer available.

Operational context #

When debugging BASIC programs on the C64, ?SYNTAX ERROR is often a misleading starting point: the reported line must be read alongside the preceding lines, especially with multi-token constructs like IF...THEN, FOR...NEXT, or command chains separated by :. The absence of a symbolic debugger makes the entire process manual. This behavior — the error reported somewhere other than its root cause — is a pattern that recurs across sequential parsing systems, from compilers to SQL engines, and is a reminder not to trust reported line numbers at face value.