Re: RISC

["Corey Ashford" <x@y.com>]

Why not use signed integer types?  You'd get overflow detection
for free, via a Constraint_Error exception.  For example:

type X is new Integer range -128 .. 127;
for X'size use 8;


"chris.danx" <chris.danx@ntlworld.com> wrote in message news:dwtw6.3343$MD6.241100@news6-win.server.ntlworld.com...
> mod types have made some parts of the machine simple but are making
> arithmetic operations a nightmare.
>
> I need to support 2s compliment integers (floats will not feature at all)
> hence mod types.  Taking not of X and add 1, then it's negative.  no probs!
> Problem is with sign and overflow on add, sub, mul, div.
>
> Each one must cope with signed nums and unsigned ones -- signal a carry or
> overflow.  I think you can just do add/sub and no worries.  Problems arise
> when i detect overflow, i tried thinking this out, and Martin Dowie pointed
> out a possible solution that might work but only for unsigned nums.  I don't
> know how to go about this signed ones.
>
> I really need to know how to detect overflow for multiplication and
> division.  I also need a way to detect it for add and sub.  Maybe Ada has an
> attribute for this or something,
>
>     e.g. ...
>             type X is mod 256;
>             ...
>             a :    x := 128;
>             a := x + x;
>             ...
>             if a'overflow then
>                 ...
>             end if;
>
> {i doubt it though!}
>
> Can anyone help,
>
>
> Thanks,
> Chris Campbell
>
> "Martin Dowie" <martin.dowie@gecm.com> wrote in message
> news:3ab5d2a4$1@pull.gecm.com...
> > I had a similar problem recently when I had to mimic unsigned 64 bits with
> > only
> > unsigned 32 bits available.
> >
> > For addition and subtract it is fairly easy. e.g.
> >
> > a = 255, b = 1
> > c = a + b
> > if c < a then overflow as it has wrapped
> >
> > similarly for subtraction with > instead of <.
> >
> > I only required +/- support, so I'm afraid I have no solutions for * or /
> > :-(
> >
> > The long-hand way for multiplications would be to add e.g.
> > a = 90, b = 3
> > c = 0
> > for I in 1 .. b loop
> >   c = c + a
> >   if c < a then overflow as it has wrapped
> > end loop
> >
> > not very efficient...
> >
> > Sorry, no ideas spring to mind for division...
> >
> > chris.danx <chris.danx@ntlworld.com> wrote in message
> > news:qrws6.3335$bL.360000@news6-win.server.ntlworld.com...
> > > Two quickies! I hope!
> > >
> > >[snip]
> > >
> > > I'm using mod types to simulate registers in the RISC machine.  I'm
> about
> > to
> > > implement arithmetic operations but i've thought of a problem.  How do i
> > > detect overflow?
> > >
> > > e.g.
> > >     say it was 1 byte reg
> > >
> > >     25*158 is too big.
> > >
> > >     12+254 is too big also.
> >
> >
> >
>