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.
|• Simple Constraints:||Basic use of constraints.|
|• Multi-Alternative:||When an insn has two alternative constraint-patterns.|
|• Modifiers:||More precise control over effects of constraints.|
|• Machine Constraints:||Special constraints for some particular machines.|