toby at telegraphics.com.au
Sun May 22 08:23:01 CDT 2016
On 2016-05-22 3:10 AM, Chuck Guzis wrote:
> On 05/21/2016 07:34 PM, Toby Thain wrote:
>> Don't underestimate the headache of handling two zeroes.
> At least on the CDC iron, it was never much of a problem. The primary
> way you got -0 as a result (without resorting to boolean operations),
> was subtracting -0 from -0. The ZR/NZ operations worked on either sign.
> Note that, on the fullword 60-bit operations, the conditionals were
> based on the contents of a single register. To compare, you had to
> subtract or otherwise operate on two operands and branch on the content
> of the result. There were no condition codes--something that made
> instruction scheduling much simpler.
> The 18-bit index register conditionals (EQ, NE, GT...etc.) didn't
> usually see -0 as a problem.
> Ones complement did have advantages in bit twiddling--there were some
> nifty tricks not possible in two's complement. Of course the CDC 6000
> series was word-addressable--something that seemed eminently practical.
> As far as I'm aware, the possibility of two zeroes never impacted the
> performance of the CDC 6000 series in the real world.
I didn't mean performance - I meant programmer effort to ensure
robust/correct code. The Grishman book didn't reassure me, but thanks
for the additional info.
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