22 Aug
2015
22 Aug
'15
3:27 p.m.
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