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Newsgroups: comp.lang.ada
Date: Fri, 10 Jun 2022 05:23:52 -0700 (PDT)
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Subject: Re: Using pointers with inline assembly in Ada
From: NiGHTS <nights@unku.us>
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Xref: reader02.eternal-september.org comp.lang.ada:63962
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On Friday, June 10, 2022 at 3:15:38 AM UTC-4, ldries46 wrote:
> Op 10-6-2022 om 7:24 schreef Rod Kay:
> On 10/6/22 07:30, NiGHTS wrote:=20
> declare=20
>           type ff is array (0 .. 10) of Unsigned_32;=20
>           pragma Pack(ff);=20
>           Flags : aliased ff :=3D (others =3D> 0);=20
>           Flag_Address : System.Address :=3D Flags'Address;=20
> begin=20
>          Asm (   "movl %0, %%eax" &=20
>                  "movl $1, (%%eax)" ,=20
>                  Inputs   =3D> System.Address'Asm_Input ("g", Flag_Addres=
s),=20
>                  Clobber  =3D> "eax",=20
>                  Volatile =3D> true=20
>          );=20
>          Put_Line ("Output:" & Flags(0)'Img);=20
> end;=20
>=20
>=20
>    If you are on a 64 bit machine, then I expect 'pragma Pack' might be t=
he problem, as 2 consecutive array elements will fir into a single address.=
=20
>=20
>=20
>    Also, possibly use ...=20
>=20
>    Flag_Address : System.Address :=3D Flags (Flags'First)'Address;=20
>=20
>=20
> Regards.
> There maybe a a way around this problem, if there is a compiler that can =
create assembly code. I have used that method in the past with Pascal progr=
ams where I wanted to avoid internal checks on errors that could not occur =
in that part of the program.
>=20
> Warning: Programming the way you want to means that your program will mea=
n that your program can become depending on the system you are working on. =
Meaning for instance not only recompiling when migrating from Windows to Li=
nux but also reprogramming that part of your work

Ada does a good job generating efficient machine code, and there are lots o=
f ways to get around compiler checks (unlike Pascal). In this situation I w=
as trying to make use of a very specific set of uncommon CPU instructions b=
ut required passing an array to it that allows side-effects. I'm a bit peev=
ed that there is so little in the way of documentation for the Asm command.