> On Jul 9, 2020, at 8:29 PM, Johnny Billquist <[email protected]> wrote:
>
> On 2020-07-10 02:19, Paul Koning wrote:
>> The VAX architecture seems to have been an explicit design effort. For the
>> Alpha this was even more obvious, where a monstrously large book (certainly
>> 500 pages, maybe double that) was written and reviewed in depth before
>> anything was cast into silicon. Not so for the PDP11, as you pointed out.
>
> It definitely was an explicit effort. I seem to remember seeing/reading
> somewhere at some point that this was because of what had happened on the
> PDP-11. So a lesson learned kind of thing.
At least DEC learned. Too many other companies had the same opportunity but
did not take it.
> I used to have an Alpha Architecture manual, but I lost it somewhere along
> the way. :-(
That's pretty amazing. I had one, but I had to give it back. It was a
restricted access numbered copy.
> ...
>> My suspicion is that the PDP-11 architecture handbook was an after the fact
>> effort. The date (1983) supports that notion. Also, it's a handbook, a fat
>> paperback like the processor and peripheral handbooks. I don't know of any
>> internal analog, like DEC Std 032 for VAX.
>
> Yeah, I would expect that it would an after the fact thing. But it would
> still be interesting to see any effort made by DEC to put it all in one book.
> As mentioned, the list (maybe in different revisions) do exist in multiple
> other handbooks and manuals. But then it's just an appendix without much
> further analysis.
>
> I think I also saw/read somewhere that different new PDP-11 implementations
> basically tried to look at what had previously been done, and tried to just
> match that, as there was no official definition of a PDP-11. But then they
> always did some deviation or other for the sake of efficiency, cost or just
> clean up.
I think at least one or two differences were deliberate and planned -- between
11/20 and the rest. That's what the existence of the "Z" error code in the
assembler implies. (I think that pops up if you do one of those R0,(R0)+
instructions or cases like that, where the whole thread started.) And the
change of RTI along with the introduction of RTT is clearly an intentional
change for optimization.
paul
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