the reference is from
https://gcc.gnu.org/onlinedocs/gcc.pdf (5.0.0 pre-release），
section 6.43 （How to Use Inline Assembly Language in C Code）
and http://www.ibiblio.org/gferg/ldp/GCC-Inline-Assembly-HOWTO.html

syntax of basic asm statement:
asm [ volatile ] ( Assembler Instructions )

syntax of extended asm statement
asm [volatile] ( Assembler Template
: Output Operands
[ : Input Operands
[ : Clobbers ] ])

Alanpsc, thanks you!