In an assembler instruction using asm , you can specify the operands of the instruction using C expressions. This means you need not guess which registers or memory locations will contain the data you want to use. You must specify an assembler instruction template much like what appears in a machine description, plus an operand constraint string for each operand. Each operand is described by an operand-constraint string followed by the C expression in parentheses. A colon separates the assembler template from the first output operand and another separates the last output operand from the first input, if any.
Commas separate the operands within each group.
The total number of operands is limited to ten or to the maximum number of operands in any instruction pattern in the machine description, whichever is greater. If there are no output operands but there are input operands, you must place two consecutive colons surrounding the place where the output operands would go. The input operands need not be lvalues. The compiler cannot check whether the operands have data types that are reasonable for the instruction being executed. It does not parse the assembler instruction template and does not know what it means or even whether it is valid assembler input.