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From: John Ames <commodorejohn@gmail.com>
Newsgroups: comp.lang.forth,comp.arch
Subject: Re: adding overflow and carry to every GPR (was Re: OT: Epic RISC-V rant)
Date: Wed, 2 Sep 2026 08:23:15 -0700
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On Wed, 02 Sep 2026 02:04:51 GMT
MitchAlsup <user5857@newsgrouper.org.invalid> wrote:

> > The idea I had last time I dabbled with this stuff was to extend the
> > basic add/subtract instructions with an option to save the
> > carry/borrow value in another GPR, instead - but I never gave any
> > thought to how to represent overflow that way. Hmm.  
> 
> That is what CARRY does. The Rd register in CARRY supplies a register
> used to carry the additional values from calculation to calculation.
> The Immediate in CARRY selects which instruction consume Rd {i},
> which instructions generate the next Rd {o} and which do both {io}
> and which do neither {}.
> 
> For ADD, carry Rd contains only 1 bit, but for others, it contains
> "all the bits that did not make it into the standard result". This
> makes CARRY suitable for all integer multiprecision arithmetic and
> for significant quantities of FP arithmetic.
> [...]
> Multi-precisions shifts use carry

Yes, exactly - add/subtract set the designated overflow/carry register
to 1 on a carry/borrow (which can then be added/subtracted from the next
more significant word) but e.g. multiply can set it to the whole most-
significant half of the product, multi-place shifts can catch all the
bits that fall off one end in the other so that rotate becomes shift-
and-OR, etc. Very flexible, especially if you have the capability to
branch on the value of a GPR.