Sign magnitude, one's complement, two's complement
Chuck Guzis
cclist at sydex.com
Sat Aug 22 17:27:11 CDT 2015
On 08/22/2015 02:23 PM, Sean Conner wrote:
>
> For my own morbid curiosity, and because it came up on another
> mailing list I'm on [1], what machines commercially avaialble were
> sign magnitude and one's complement? Every machine I've encountered
> was two's complement (okay, IEEE 754 [2] is a sign magnitude format
> but I'm talking about integer implementations here, not floating
> point). I've only found reference to one sign magnitude computer
> (the IBM 7090, release in 1959) and a few one's complement machines
> (mostly the PDP series from DEC).
>
> Where there others? And honestly, are there any machines that use
> anything other than two's complement today?
There were, as noted, several ones complement systems.
Every decimal computer that I can think of, or ones, at least that have
a decimal instruction vocabulary are sign-magnitude. That is, a
negative one is represented as -1 in some form, not in its nine's or
ten's complement.
For another pure binary sign-magnitude system, consider one of the first
minicomputers (although the term hadn't been then invented)--the Packard
Bell PB 250. I'm pretty certain there were several others.
Floating point can engender some interesting representations. Consider
the exponent field on the aforementioned CDC 6000 series. It's a
"biased by 2000 octal) system--and the assumed binary point of the
mantissa is to the right of the LSB. So, 2000 0000 0000 0000 0001 octal
= 1 exactly.
--Chuck
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