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From: "Steve" <nospam_steved94@comcast.net>
Newsgroups: comp.lang.ada
References: <20619edc.0310221056.4c92d10c@posting.google.com>
Subject: Re: Is there a better (Ada) way?
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Xref: archiver1.google.com comp.lang.ada:1482
Date: 2003-10-23T03:12:59+00:00
List-Id: <comp.lang.ada>

I don't know if its really more "Ada like" but here is the approach I would
take.

Rougly following your conventions I would add a type definition:

  type SReg16_T is range -2**15..2**15-1;
  for SReg16_T'size use 16;

Then use a type conversion for a sign extend:
  SReg16_T( PB.Offset5 )

And an instance of unchecked conversion to convert to a SReg16_T to a
Reg16_T:

function To_Reg16_T is new Ada.Unchecked_Conversion( SReg16_T, Reg16_T );

Then the expression in your example would read:

R16 := R16 + To_Reg16_T( SReg16_T( PB.Offset5 ) );

Another approach would be to use a lookup table indexed by the offset giving
the amount to change the index.  Something like:

  type RegOff_T is array( Offset5_T ) of Reg16_T;
  Reg_Offset : constant RegOff_T :=
  (
    -16 =>   16#FFF0#,
    -15 =>   16#FFF1#,
    -14 =>   16#FFF2#,
    -13 =>   16#FFF3#,
    -12 =>   16#FFF4#,
    -11 =>   16#FFF5#,
    -10 =>   16#FFF6#,
    -9  =>   16#FFF7#,
    -8  =>   16#FFF8#,
    -7  =>   16#FFF9#,
    -6  =>   16#FFFA#,
    -5  =>   16#FFFB#,
    -4  =>   16#FFFC#,
    -3  =>   16#FFFD#,
    -2  =>   16#FFFE#,
    -1  =>   16#FFFF#,
     0  =>   16#0000#,
     1  =>   16#0001#,
     2  =>   16#0002#,
     3  =>   16#0003#,
     4  =>   16#0004#,
     5  =>   16#0005#,
     6  =>   16#0006#,
     7  =>   16#0007#,
     8  =>   16#0008#,
     9  =>   16#0009#,
     10 =>   16#000A#,
     11 =>   16#000B#,
     12 =>   16#000C#,
     13 =>   16#000D#,
     14 =>   16#000E#,
     15 =>   16#000F#
  );

Then the expression in your example would read:

R16 := R16 + Reg_Offset( PB.Offset5 );

If you actually want to try to run the simulator, I'd try a few approaches
and pick the one that is faster.

BTW:  There is a nice module built in to Ada95 called "Interfaces" it
defines stuff like Unsigned_16, etc.  I prefer to use that over defining my
own similar types.

Steve
(The Duck)


"Mike Silva" <snarflemike@yahoo.com> wrote in message
news:20619edc.0310221056.4c92d10c@posting.google.com...
> I've started writing a little CPU simulator, just for fun and to be
> doing *something* in Ada, and I want to make sure I'm not writing "C
> in Ada" when it comes to all the bit, bitfield and register
> manipulations.
>
[snip]
>    ....
>    R16 := R16 + Reg16_t( Integer( PB.offset5 ) mod Reg16_t'Modulus );
> -- the above line works, but is it good Ada, or C-in-Ada?