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From: Simon Wright <simon@pogner.demon.co.uk>
Newsgroups: comp.lang.ada
Subject: Re: Ada and ASM
Date: Sat, 6 Sep 2003 08:25:00 +0100
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Date: 2003-09-06T08:25:00+01:00

> I want to replace the ASM call by Ada syntax. Assuming one had a
> package Asm, then I would like something like the following
> 
> A,B:Asm.register_32;
> A:=10_000;
> Asm.Move(From=>A To=>B);
> 
> The assembly operations are expressed as true Ada subprograms.

All very well if all you wanted to do was move things about. But
machines have all sorts of features, not all of which have
counterparts on each architecture to be supported. What about reading
the high-resolution clock? (rdtsc on Pentium, mftb on PowerPC).

   function Clock return Time is
      type Half is (Low, High);
      Lower, Upper : Interfaces.Unsigned_32;
      Results : array (Half) of Interfaces.Unsigned_32;
      Result : Time;
      for Result'Address use Results (Low)'Address;
   begin
      System.Machine_Code.Asm
        ("rdtsc" & ASCII.LF & ASCII.HT &
           "movl %%eax, %0"& ASCII.LF & ASCII.HT &
           "movl %%edx, %1",
         Outputs => (Interfaces.Unsigned_32'Asm_Output ("=g", Lower),
                     Interfaces.Unsigned_32'Asm_Output ("=g", Upper)),
         Clobber => "eax, edx");
      Results := (Low => Lower, High => Upper);
      return Result;
   end Clock;

is an Intel version, the PPC version (which I can't post) is quite
different in structure. For a start, you have to read the timestamp in
two bites! (on 32-bit implementations).

The major reason for using asm must be precisely to accommodate these
sorts of differences.