content/courses/Ada_For_The_Embedded_C_Developer/chapters/04_Embedded…

[samek-h]

A pull request created in response to #1304 that fixes the incorrect order of the rdtsc instruction output registers.

Reproduced using following program:

with Ada.Text_IO; use Ada.Text_IO;
with Interfaces; use Interfaces;
with System.Machine_Code; use System.Machine_code;

procedure Rdtsc is
   function Get_Processor_Cycles return Unsigned_64 is
      Low, High : Unsigned_32;
      Counter   : Unsigned_64;
   begin
      Asm ("rdtsc",
           Outputs =>
             (Unsigned_32'Asm_Output ("=a", High),  -- Incorrect
              Unsigned_32'Asm_Output ("=d", Low)),  -- Incorrect
           Volatile => True);

      Counter :=
         Unsigned_64 (High) * 2 ** 32 +
         Unsigned_64 (Low);

      return Counter;
   end Get_Processor_Cycles;
begin
   Put_Line ("tsc =" & Get_Processor_Cycles'Image);
end Rdtsc;

The resulting rdtsc value is seemingly random, not increasing:

$ taskset -c 0 ./rdtsc
tsc = 14729149879850896313
$ taskset -c 0 ./rdtsc
tsc = 7769122674801904570
$ taskset -c 0 ./rdtsc
tsc = 17445020729480317882
$ taskset -c 0 ./rdtsc
tsc = 8327487355497745339
$ taskset -c 0 ./rdtsc
tsc = 1353739439335017404

The same program, but using the correct order:

with Ada.Text_IO; use Ada.Text_IO;
with Interfaces; use Interfaces;
with System.Machine_Code; use System.Machine_code;

procedure Rdtsc is
   function Get_Processor_Cycles return Unsigned_64 is
      Low, High : Unsigned_32;
      Counter   : Unsigned_64;
   begin
      Asm ("rdtsc",
           Outputs =>
             (Unsigned_32'Asm_Output ("=a", Low),
              Unsigned_32'Asm_Output ("=d", High)),
           Volatile => True);

      Counter :=
         Unsigned_64 (High) * 2 ** 32 +
         Unsigned_64 (Low);

      return Counter;
   end Get_Processor_Cycles;
begin
   Put_Line ("tsc =" & Get_Processor_Cycles'Image);
end Rdtsc;

The resulting values are now increasing:

$ taskset -c 0 ./rdtsc
tsc = 13377973989561
$ taskset -c 0 ./rdtsc
tsc = 13381489237427
$ taskset -c 0 ./rdtsc
tsc = 13384125133819
$ taskset -c 0 ./rdtsc
tsc = 13386827400627
$ taskset -c 0 ./rdtsc
tsc = 13389239550049

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