yet another pdp-11 in fgpa

Johnny Billquist bqt at
Sun Jun 27 13:26:22 CDT 2010

"Walter F.J. Mueller" <w.f.j.mueller at> wrote:

> Johnny Billquist <bqt at> wrote:
>  > I wasn't aware that any prototypes ever were produced and came as
>  > far as being functional. I thought it was just paper work that
>  > had bee done.
> The 11/74 wasn't marketed, as pointed out in this thread, but a
> few systems were build by DEC. A picture of such 11/74 system
> was made available by Tim Shoppa, see
> You'll nicely see the four CPUs.

Yes, I know of these systems. However, that is not an 11/74 on that 
picture, but an 11/70mP. There is a difference...
As pointed out, the 11/70mP was marketed as an 11/74, but it's a 
different CPU.

The easiest way to see that this is a picture of an 11/70mP is by 
looking at the lower rotary switch, which only have four positions, and 
not eight (which the 11/74 have). So no CIS on this machine.

The only 11/74 picture I've seen so far is the silk screen panel picture 
posted a few days ago. Unfortunately I've already forgotten the name 
(I'm lousy with names, sorry) of the person who posted it, and who also 
worked on the 11/74 CIS microcode.

The machine on that picture is probably CASTOR:: by the way.

> arcarlini at wrote:
>  > I've read somewhere that it [PDP-11/74] was full of flat ribbon
>  > cables  and would have been a beast to maintain.
> Well, you see on the picture that there was quite a bit of
> flat ribbon cables :).

Yes, when you run multiple CPU configurations, you do get many flat cables.
However, if you were to run the 11/74 in a single CPU configuration, it 
would not have had more flat cables than an 11/70. So it seems strange 
if that would have been the reason for cancelling it.

On the other hand, I think that CIS never was popular, or in any demand, 
for the PDP-11, so if that (and multiprocess capability) was the only 
things the 11/74 brought to the table to differ it from tne 11/70, then 
I can understand why they would cancel it if they had problems with the 
multiprocessor part.

> The 'PDP 11/70 Multiprocessor Technical Manual (Preliminary Version)'
> is on bitsavers, see
> and gives you a feeling of the modules and interconnects.

Yes. And all evidence I ever see is that the only systems DEC actually 
did get working was the 11/70mP. That they actually got the 11/74 (CPU) 
beyond paperwork was what was news to me.

> Johnny Billquist <bqt at> wrote
>  > You do know that the J11 is already designed for mP usage, except that
>  > DECs testing of that was even more secret than the 11/74?
> Sure, the mP instructions WRTLCK and the TSTSET of the J11 are
> documented, and allow cleaner way to implement spin locks than
> the 'asrb hack' of the 11/74. In a future version of the w11a I'll
> probably also implement the instructions supported by 11/34 and
> J11 and make the 'processor profile' selectable at start time, much
> like in the simh simulator.

That would be nice. While RSX don't use it, you could use it for Unix, 
or other systems.
The J11 also have the cache bypass bit in the PDR, btw.
What instructions are you thinking about when you say 11/34 btw?

>  > But FPP is among the most important things in there as well, I'd say.
>  > Lots of software who won't be happy without it.
> That's true from a performance point of view. However
>   - you can run RSX and Fortran without FPP (did this 30 years ago
>     when the FPP broke on the 11/45 I was working with...).

True. But you cannot run almost anything else. I don't think I've seen 
any other language that do not require the FPP.

>   - you can run 2.11BSD without FPP (I'm doing that each time I
>     boot 2.11BSD).

Yes, that was fixed recently, right?

> So a FPP is very good to have, but not my highest priority. Also,
> it's  quite a project to design, implement and verify it, with the
> verification, as usual, being the most time consuming part.


>  > By the way. You don't have to worry about cache coherency. The
>  > PDP-11/74 do not do that. Cache coherency is managed by software
>  > on the PDP-11 (well, in RSX, since that's the only system that
>  > supports the hardware). In short, the real hardware do not implement
>  > any sort of cache coherency  in hardware.
> I know, but I'll go for a cache with full cache coherency. That will
> make it a lot easier to try a MP hack for 2.11BSD. And RSX, if I ever
> get to it, will not mind.

True. Definitely will not break anything. But it will make your life 
more complicated. :-)

> Johnny Billquist <bqt at> wrote:
>  > Good thinking.
>  > But I'm surprised by some numbers here. The J11 at 20 MHz is
>  > only slightly faster than an 11/70. In fact, if you can throw
>  > the 11/70 into running all from cache, it might even be slightly
>  > faster than an 11/9x.
>  > Or so I seem to remember from looking at the numbers back when
>  > I last was digging into this.
>  >
>  > Maybe I'm mixing some numbers up here... What I do remember for
>  > sure is that the 11/9x machines run at 20 MHz, and that they are
>  > not more than maybe 1.2 times the speed of an 11/70 in general.
> oops, you are right, I was mixing numbers here and forgot about
> a factor of four. Here the revised arguments:
> See , there
> is a table comparing 11/70 with various J11 systems:
>                        11/23 11/53 11/73 11/83 11/93
>                        ----- ----- ----- ----- -----
>    CPU                 F-11  J-11  J-11  J-11  J-11
>    Microcycle(ns)      300   267   267   222    222
>    Clock (MHz)         ?     15    15    18     18
>    Performance         0.2   0.5   0.7   1.2    1.4
>    (11/70 = 1)	
>    Cache               no    no    yes   yes    no
>    Floating-Pt         opt   no    no    yes    yes
>    Coprocessor
> My mistake was to forget that the J11 needs 4 clock cycles per
> microcycle (MC), that's why 18 MHz clock leads to 222 ns MC period.
> In the end the J11 (222ns MC) is faster than the 11/70 (150 ns MC)
> because it has a better micro architecture (three stage pipeline
> vs. instruction prefetch only). The J11 is roughly a factor two
> better compared to 11/70 in terms of MC efficiency.

Hmm, I seem to remember checking numbers a while ago and came to the 
conclusion that if the 11/70 would have all memory as cache, it would 
outperform the J11. Atleast at 18MHz, maybe also 20MHz.
(Btw, that table is slightly wrong, as the 11/9x eventually made it to 

I have a processor handbook which gives instruction speeds of 11/70 
instructions with cache hits and without, which tells you how fast the 
machine would be if you were all cache. The 11/70 suffers because memory 
is so slow.


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