Using the GNU Compiler Collection (GCC): Constraints

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#### 6.45.3 Constraints for `asm` Operands

Here are specific details on what constraint letters you can use with `asm` operands. 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 `asm`, unless ‘`<`’ 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.