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

[Maciej W. Rozycki]

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.

 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.

  Maciej