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From: John Levine <johnl@taugh.com>
Newsgroups: comp.arch
Subject: Re: is Vax addressing sane today
Date: Wed, 11 Sep 2024 15:53:06 -0000 (UTC)
Organization: Taughannock Networks
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According to Thomas Koenig  <tkoenig@netcologne.de>:
>John Levine <johnl@taugh.com> schrieb:
>> It appears that Anton Ertl <anton@mips.complang.tuwien.ac.at> said:
>>>>The future of programming languages is type safe with checks, you need to
>>>>get on that bandwagon early.
>>>
>>>MIPS got on that bandwagon early.  It has, e.g., add (which traps on
>>>signed overflow) in addition to addu (which performs modulo
>>>arithmetic).  It has been abandoned and replaced by RISC-V several
>>>years ago.
>>
>> S/360 had signed and unsigned adds in the 1960s, with optional
>> trapping for signed overflow. OS/360 let you catch the traps and z
>> still does but it is not my impression that many programs did or do.
>
>With trapping, I understand.  Without trapping - what is the
>difference on a two's complement machine?

Different condition codes.  Signed add:

0 Sum is zero
1 Sum is less than zero
2 Sum is greater than zero 
3 Overflow

Unsigned add:

0 Sum is zero (no carry)
1 Sum is not zero (no carry)
2 Sum is zero (carry)
3 Sum is not zero (carry)

-- 
Regards,
John Levine, johnl@taugh.com, Primary Perpetrator of "The Internet for Dummies",
Please consider the environment before reading this e-mail. https://jl.ly