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Message from discussion Exact integer-valued floats
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w...@mac.com  
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 More options Sep 24 2012, 11:29 am
Newsgroups: comp.lang.python
From: w...@mac.com
Date: Mon, 24 Sep 2012 11:29:12 -0400
Local: Mon, Sep 24 2012 11:29 am
Subject: Re: Exact integer-valued floats
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On Sep 22, 2012, at 7:06 PM, Dave Angel <d...@davea.name> wrote:

> On 09/22/2012 05:05 PM, Tim Roberts wrote:
>> Dennis Lee Bieber <wlfr...@ix.netcom.com> wrote:
>>> On 22 Sep 2012 01:36:59 GMT, Steven D'Aprano wrote:
>>>> For non IEEE 754 floating point systems, there is no telling how bad the
>>>> implementation could be :(
>>>    Let's see what can be found...

>>>    IBM 360: Same as Sigma-6 (no surprise; hearsay is the Sigma was
>>> designed by renegade IBM folk; even down to using EBCDIC internally --
>>> but with a much different interrupt system [224 individual interrupt
>>> vectors as I recall, vs the IBM's 7 vectors and polling to find what
>>> device]).
>> The Control Data 6000/Cyber series had sign bit and 11-bit exponent, with
>> either a 48-bit mantissa or a 96-bit mantissa, packed into one or two
>> 60-bit words.  Values were not automatically normalized, so there was no
>> assumed 1 bit, as in IEEE-754.

> And it's been a long time (about 39 years), but as I recall the CDC 6400
> (at least) had no integer multiply or divide.  You had to convert to
> float first.  The other oddity about the CDC series is it's the last
> machine I've encountered that used ones-complement for ints, with two
> values for zero.

Well, for what it is worth… the DEC Laboratory INstrument Computer (LINC-8, sort of a forced acronym because I believe they were built to specs issued by Lincoln Labs.) and the later DEC PDP-12 (which incorporated the LINC-8 instruction set, along with the PDP-8 basic set) also did ones-complement integer arithmetic.  I never used a LINC-8, but I worked for several years around PDP-12s.  (They also had a build-in CRT and a MUX'd analog-to-digital converter with CPU instructions for driving both directly.)  As I remember, they maxed out at 32k (12-bit) words of RAM.  I don't know when they were discontinued, but there were still some PDP-12s in use as late as 1988 when I lost track.

-Bil


 
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