Bug-for-bug compatibility [was RE: SimH DECtape vs. Tops-10 [was RE: Writing emulators [Was: Re: VCF PNW 2018: Pictures!]]]

allison allisonportable at gmail.com
Fri Mar 2 13:22:20 CST 2018

On 03/02/2018 06:44 AM, Maciej W. Rozycki via cctalk wrote:
> On Wed, 28 Feb 2018, Paul Koning via cctalk wrote:
>> With the VAX, this got cleaned up to a significant extent, and ditto 
>> with Alpha.  In both cases, an internal validator tool was created to 
>> verify that, at least from the point of view of instruction execution, a 
>> new machine worked the same as an existing reference machine.  But this 
>> seems to be quite an unusual notion in the history of computer hardware 
>> development generally.  Even when standard specifications exist that 
>> appear to spell out how an architecture is supposed to work, the reality 
>> is that two implementations will in general do it differently.  That is 
>> particularly likely to happen in cases of "no one will do this" -- like 
>> shifts by more than the word size, or other oddball stuff.
Its right after the VAX that DEC engineering started thinking about
It was clear that the PDP-11 was not one fully unified system as each
had its own unique quirks more so after the advent of the LSI-11 and F11
Of of the PDP-11 handbooks I have has a back page of differences in
set behavior between all of the known PDP-11s including T-11 and J11,

It was clear the VAX architecture had to have a clear definition that
was true
across all of VAXen yet to come.  This was brought about by the need to
VMS consistent and also later Ultrix.   Hence the DEC STD For VAX came to
being and the test for is it a VAX as well.  Exceptions were clearly
listed and
the software impacts were defined.    Of course since Alpha had to run VMS
(by then OpenVMS) and Ultrix or its heirs it also was spec'ed out fully
and for
all expected generations.

>  That's what architecture verification programs or AVP tests are for 
> nowadays.  Everything that's not undefined in the architecture is supposed 
> to work as defined.  This includes odd corner cases.  Formal definitions 
> are included in the architecture specification.
Now we do in the world.  But a good example of variations on a theme is ARM.
Though there are some controls is an earlier ARM 16bit  a subset of an ARM
64 bit?

>   Maciej

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