Happy Birthday VAX 11/780 (influence of)

Johnny Billquist bqt at softjar.se
Thu Oct 28 16:24:50 CDT 2010

On 2010-10-28 23:02, Rich Alderson<RichA at vulcan.com> wrote:

> From: Brad Parker
> Sent: Thursday, October 28, 2010 10:12 AM
> On Oct 27, 2010, at 11:31 AM, William Donzelli wrote:
>>>> >>>  You could insert a small paragraph here about the role of unix and how
>>>> >>>  unix and the pdp-11 and vax interacted.
>>> >>  Are you saying that the PDP-11 and VAX were the first machines where
>>> >>  the hardware and software were both considered and designed together?
>>> >>  Once again, I think a lot of people would not agree with that.
>> >  No, I didn't say that at all.  I was just saying that the it would be
>> >  interesting to explore how the pdp-11 and VAX architectures influenced
>> >  the design of unix, and how unix, in turn, influenced software
>> >  development as a whole.  I thin the two are interrelated.
> From what Thompson and Ritchie have written about the origins of Unix,
> the PDP-11's architecture had very little to do with how it was created.
> The original PDP-7 Unix (specific I/O ports addressed in I/O instructions,
> 18-bit words, word addressing) was taken up and ported to the PDP-11 with
> few or no user-visible changes.

Indeed. Well, I don't know about few or no user-visible changes, but 
there is no denying that Unix started on a PDP-7, and not a PDP-11.

> Because the VAX offered a virtual memory capability (hi, Johnny!:-), it
> did change the way Unix developed, but so did other ports (Interdata, for
> example, and even the IBM Series/1).

Hi. :-)
I hope you do remember that early versions of Unix (even on the VAX) did 
not do demand paging. Are you saying that they didn't have virtual 
memory either, then? :-)

BSD3 anyone?

As for what PDP-11 might have innovated, we have covered the memory 
mapped I/O at some length now, and it appear that the PDP-11 might 
atleast have a half claim to fame there. But, as some pointed out, the 
x86 do not use memory mapped I/O (and shared memory with a graphics 
subsystem is not the same thing). Most RISC machines did/do use memory 
mapped I/O anyway, but I digress...

I have not seen anyone comment any of the other things I listed as 
possible firsts on the PDP-11.
Can anyone come up with an earlier machine that used condition codes?
How about general registers with addressing modes, which is totally 
orthogonal? How about having the PC as a general register?

I don't know of any machines before the PDP-11 that had these. 
Admittedly, the only one of these attributes the x86 inherited (from 
wherever) is condition codes, but I think it might be interesting to 
hear the collective wisdom on some more details than just memory mapped 


Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol

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