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4.33 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.

4.33.1 Simple Constraints		Basic use of constraints.
4.33.2 Multiple Alternative Constraints		When an insn has two alternative constraint-patterns.
4.33.3 Constraint Modifier Characters		More precise control over effects of constraints.
4.33.4 Constraints for Particular Machines		Special constraints for some particular machines.