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

[Dan Cross]

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.

>The advantage is instruction size. NOVA instructions are one word (16 
>bits). Eclipse instructions are 1 or 2 word, and MV instructions are 1 
>to 4 words. They are actually pretty elegant about how you can mix them.
>
>If you need a 32 bit result, you use an MV instruction.
>
>If you need a 16 bit (or less) result, you can use any of the instructions.
>
>In general, MV instructions can reference the whole 4GB memory space 
>directly.
>
>NOVA and Eclipse instructions are still 16 bit, but 32 bit aware.
>
>PC relative branches can use NOVA or Eclipse instructions if the branch 
>destination is in the range of the shorter offset in the instruction. 
>Otherwise, MV instructions are used for "long" branches.
>
>NOVA and Eclipse instructions running on an MV can directly address the 
>first 32KW of the current ring.
>
>16 bit results in registers are extended to 32 bits.

Again, this is almost exactly what x86 does today.  x86 is
otherwise a dumpster fire of bad design, but the approach they
took with the 64-bit extensions is spiritually closer to what
DG did with Nova, Eclipse, and MV than what DEC did.

	- Dan C.