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

[Arne Vajhøj]

On 8/27/2023 8:45 AM, Dan Cross wrote:
> In article <uces30$12gsl$1 at dont-email.me>,
> Chris Scheers  <chris at applied-synergy.com> wrote:
>> Arne Vajhøj wrote:
>>> 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).
>>
>> Actually, DG's approach allows mixing NOVA, Eclipse, and MV instructions
>> in the same instruction stream.  Actually, it not only allowed, it is
>> normal to do this.
> 
> Ah, this is precisely what x86 does, again underscoring how it
> is different from the approach that DEC chose with PDP-11
> compatibility on the VAX.

That is not how x86-64 work.

You cannot mix code.

A 64 bit program cannot use 32 bit libraries.

The build will detect it and reject it. But if that check was
disabled then the result would be almost guaranteed to crash.

If I understand the peoples description of Eclipse MV and AOS/VS
correct then it 32 bit code can call 16 bit code.

That is one of the benefits of not having the mode bit.

The fact that maybe 98% of the instructions in 32 bit x86
code would work fine interpreted as 64 bit x86-64 code
and only maybe 2% would return wrong results or crash
is about as relevant as the background color used when
running the program.

Almost compatible is not compatible.

Arne