Here we introduce the syntax of asm assembler instructions with an x86 example to shift a value 8 bits to the right:
asm ("shrl $8, %0" : "=r" (answer) : "r" (operand) : "cc");
- The first section contains an assembler instruction and its operands. In this example, shrl right-shifts the bits in its first operand.
- Its first operand is represented by %0.
- Its second operand is the immediate constant $8.
- The second section specifies the outputs.
- The instruction's one output will be placed in the C variable answer, which must be an lvalue. The string "=r" contains an equals sign indicating an output operand and an r indicating that answer is stored in a register.
- The third section specifies the inputs. The C variable operand specifies the value to
shift.
- The string "r" indicates that it is stored in a register but omits an equals sign because it is an input operand, not an output operand.
- The fourth section indicates that the instruction changes the value in the condition
code cc register.
