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Here are specific details on what constraint letters you can use with
Constraints can say whether
an operand may be in a register, and which kinds of register; whether the
operand can be a memory reference, and which kinds of address; whether the
operand may be an immediate constant, and which possible values it may
have. Constraints can also require two operands to match.
Side-effects aren't allowed in operands of inline
‘<’ or ‘>’ constraints are used, because there is no guarantee
that the side-effects will happen exactly once in an instruction that can update
the addressing register.