Sign magnitude, one's complement, two's complement
Paul Koning
paulkoning at comcast.net
Mon Aug 24 08:51:55 CDT 2015
> On Aug 22, 2015, at 10:31 PM, Chuck Guzis <cclist at sydex.com> wrote:
>
> On 08/22/2015 06:26 PM, Paul Koning wrote:
> than the usual rule).
>>>
>>> I recall the "integer multiply" feature (i.e. optional) available
>>> on the 6000. IXi Xj*Xk, but it didn't provide any more precision
>>> than the usual unnormalized double-precision multiply DXi Xj*Xk,
>>> but saved some time spent fiddling with exponent fields.
>>
>> ??? Never heard of any such thing. IXi Xj*Xk is a defined opcode,
>> but it's simply a synonym for Dxi Xj*Xk.
>
> Well, we were always the guinea pig for QSEs in SSD. It's not described in the Cyber 70 docs, but we had Cybers fitted with the option. It did make its way into the 170s however:
>
> In 60456100A, re: the 42 instruction (page 4-24, second column):
>
> "This instruction is used in multiple-precision floating-point calculations. This instruction also provides for integer multiplication capabilities where both operands have an exponent value of plus or minus zero and neither coefficient has been normalized. The integer result sent to Xi is *48 bits with a 60-bit sign extension* (emphasis mine)."
Ok, but that is simply the standard 6000 series double multiply. That's not an optional feature; the behavior you describe is how it always works and why IXX*X is a synonym for DXX*X. All that's going on here is that the exponent field remains zero, so it acts as an extended sign field.
paul
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