Re: VAX floating point formats & GCC patches

[Anders Magnusson <ragge%ludd.ltu.se@localhost>]

Den 2016-04-09 kl. 05:56, skrev Toby Thain:
> On 2016-04-08 11:33 PM, Jake Hamby wrote:
> > > On Apr 6, 2016, at 18:04, Paul Koning <paulkoning%comcast.net@localhost> wrote:
> > >
> > > > On Apr 6, 2016, at 7:54 PM, Maciej W. Rozycki 
> > > > <macro%linux-mips.org@localhost> wrote:
> > > >
> > > > On Wed, 6 Apr 2016, Mouse wrote:
> > > >
> > > > > > > > I would expect -msoft-float if it existed to be entirely IEEE
> > > > > > > > format and [...]
> > > > > > > The floating-point format used is down to the ABI -- [...]
> > > > >
> > > > > Well, to an extent.  I expect you would have trouble finding a 
> > > > > compiler
> > > > > that generated VAX assembly/machine code but with IEEE floats.
> > > > > -msoft-float (if it existed) would handle the arithmetic, but you'd
> > > > > also have to generate constants and the like to match.
> > > >
> > > > It's easier than you think it would seem.  You can actually 
> > > > configure GCC
> > > > to use the IEEE floating point with the VAX target -- all you need 
> > > > is to
> > > > set REAL_MODE_FORMAT appropriately for the relevant formats (SFmode,
> > > > DFmode, etc.) at run time.  Then you can flip between the formats 
> > > > desired
> > > > with a command-line option -- be it `-msoft-float' or whatever.
> > >
> > > Yes, it could indeed be done, and it would be an interesting science 
> > > project.  You'd end up with a non-standard ABI, producing binary 
> > > data incompatible with any real VAX.  But it could be done.
> > >
> > > I'm not sure it would be all that useful.  Given the fact that there 
> > > aren't any non-FPU VAXen, it doesn't seem worth the trouble.
> > >
> > > It's interesting that clearly some subset options were only 
> > > theoretical (float) while others were commonly used (POLYx, decimal 
> > > instructions, PDP11 compatibility mode especially).  I suspect that 
> > > comes from the fact that doing without float instructions is a major 
> > > hardship for a general purpose computer, while the other CISC 
> > > instructions that clutter up the VAX instruction set are far less 
> > > important.  Things like POLY and CRC really don't add much value.
> > >
> > >     paul
> >
> > Are we sure there wasn't some MicroVAX model somewhere that was sold
> with only the CPU chip and FP emulation? The Digital Technical Journals
> seem to imply that you could get one just the CPU chip and no FPA, and
>
> I haven't tried to verify this, but I think the FPA board in the 
> 11/750 is optional? (I own an 11/750 but have never had it running.)
>
> Hope someone more knowledgeable can confirm (or I could check with 
> other 11/750 owners in the retrocomputing community).
Yep, the FPA bord was optional (and uncommon).  But that was just a 
hardware accelerator; all FP instructions were implemented in microcode.

-- Ragge


> --Toby
>
> VMS & Ultrix would emulate the FP instructions through the reserved
> operand exception. But it's possible that they never actually configured
> many systems that way because it would be so slow.
> > ...
> >
> > Regards,
> > Jake