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From: "Martin Dowie" <martin.dowie@gecm.com>
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 <qrws6.3335$bL.360000@news6-win.server.ntlworld.com>
Subject: Re: RISC 
Date: Mon, 19 Mar 2001 09:43:18 -0000
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Date: 2001-03-19T09:43:18+00:00
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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.