[Info-vax] VAX vs. MV/8000 [was Re: Hard links on VMS ODS5 disks]

[abrsvc]

On Friday, August 25, 2023 at 8:33:10 PM UTC-4, Arne Vajhøj wrote:
> On 8/24/2023 6:05 AM, Johnny Billquist wrote: 
> > On 2023-08-23 04:16, Arne Vajhøj wrote: 
> >> On 8/22/2023 9:36 PM, Rich Alderson wrote: 
> >>> =?UTF-8?Q?Arne_Vajh=c3=b8j?= <ar... at vajhoej.dk> writes: 
> >>>> VAX + VMS                               Eclipse MV + AOS/VS 
> >> ...>> ---------                               ------------------- 
> >> 
> >>>> 16 bit compatibility mode               16 bit compatibility mode 
> >>> 
> >>> Wrong!  See below. 
> >> 
> >>> Unlike the PDP-11 mode bit in the VAX, the MV/8000 used the same 
> >>> method as the 
> >>> Eclipse to add 32 bit intructions to the mix:  A previously unused 
> >>> set of NOPs 
> >>> was turned into the prefixes for the 32 bit instruction set. 
> >> 
> >> So you say that the Eclipse MV was "Eclipse compatible" instead 
> >> of having "Eclipse compatibility mode"? 
> > 
> > I think there was a famous quote about this: 
> > "We will *not* have a compatibility mode bit like the VAX did!".
> But if we look at this from the high level. 
> 
> Both DEC and DG considered it important to be able to 
> run old 16 bit applications on their new 32 bit system. 
> 
> Both DEC and DG was willing to add support for the 
> 16 bit instructions in the hardware (as opposed to 
> DEC a little over a decade later when the 64 bit support 
> for 32 bit applications was software based: VEST). 
> 
> DEC chose to have the VAX switch mode between 32 and 
> 16 bit instructions (similar to what AMD did a couple 
> of decades later). 
> 
> DG chose to support both instruction sets at the 
> same time. 
> 
> Both DEC and DG achieved the business goal of being 
> able to run both old 16 bit applications and new 
> 32 bit applications. 
> 
> DG approach seems more elegant as it must have 
> allowed new 32 bit code to call old 16 bit code 
> within the same program. If someone knew how to 
> ensure that everything got set up properly (somewhat 
> similar to VMS code today using 64 bit pointers needing 
> to call code using 32 bit pointers - you better have 
> the data in P0 and not P2). 
> 
> Arne
VEST was not intended to be a vehicle for 32 programs to be transformed into 64 bit.  It was intended for VAX programs to run under Alpha without the need for having source code access. Many customers were happy that this worked as well as it did because of their lack of original source.  VESTed programs still had access only to 3 bit space. There are sites today still running VESTed images on Alpha systems.

Dan