Question
[As we all no doubt know](https://stackoverflow.com/questions/3952123/representation-of-negative- numbers-in-c/3952262#3952262), the ISO C standard (and C++ as well, I think, though I'm more interested on the C side) allows three underlying representations of signed numbers:
- two's complement;
- ones' complement; and
- sign/magnitude.
Wikipedia's entry states that sign/magnitude is used on the IBM 7090 from the 60s, and that ones' complement is used by the PDP-1, CDC 160A and UNIVAC 1100, all of which date back to the 60s as well.
Are there any other implementations of C (or underlying hardware) with these alternative representations, that have come out a little more recently than fifty years ago (and what are they)?
It seems a little wasteful to keep something in a standard for machines no longer in existence.
Answer
The most recent example I can find is the UNISYS 2200 series, based on UNIVAC, with ones-complement arithmetic. The various models were produced between 1986 and 1997 but the OS was still in active development as late as 2015. They also had a C compiler, as seen here.
It seems likely that they may still be in use today.
