The example in this section illustrates how to specify the source operands for assembly language statements. The program simply increments its input value by 1:
with Interfaces; use Interfaces; with Ada.Text_IO; use Ada.Text_IO; with System.Machine_Code; use System.Machine_Code; procedure Increment is function Incr (Value : Unsigned_32) return Unsigned_32 is Result : Unsigned_32; begin Asm ("incl %0", Inputs => Unsigned_32'Asm_Input ("a", Value), Outputs => Unsigned_32'Asm_Output ("=a", Result)); return Result; end Incr; Value : Unsigned_32; begin Value := 5; Put_Line ("Value before is" & Value'Img); Value := Incr (Value); Put_Line ("Value after is" & Value'Img); end Increment;
Outputs parameter to
that the result will be in the eax register and that it is to be stored in the
Inputs parameter looks much like the
Outputs parameter, but with an
Asm_Input attribute. The
"=" constraint, indicating an output value, is not present.
You can have multiple input variables, in the same way that you can have more than one output variable.
The parameter count (%0, %1) etc, now starts at the first input statement, and continues with the output statements. When both parameters use the same variable, the compiler will treat them as the same %n operand, which is the case here.
Just as the
Outputs parameter causes the register to be stored into the
target variable after execution of the assembler statements, so does the
Inputs parameter cause its variable to be loaded into the register before execution
Thus the effect of the
Asm invocation is:
The resulting assembler file (with
-O2 optimization) contains:
_increment__incr.1: subl $4,%esp movl 8(%esp),%eax #APP incl %eax #NO_APP movl %eax,%edx movl %ecx,(%esp) addl $4,%esp ret