yet another pdp-11 in fgpa

Don North ak6dn at
Tue Jun 22 01:30:20 CDT 2010

Johnny Billquist wrote:
> "Walter F.J. Mueller" <W.F.J.Mueller at> wrote:
>> The reason why I picked a 11/70 and not a J11 as target is because my 
>> goal is a 11/74. I've implemented the IIST already and tested against 
>> the IIST Diagnostic I could find in XXDP (riiab0).  A dual core will 
>> fit into a single xc3s1200e of the NEXYS2 board. The work needed is 
>> quite clear and doable (changes on cache, mmu, and cpu core for asrb).
>> However, I've no plans to implement the CIS, so it will always be a 
>> subset of a 11/74. But for sure fun to do and run.
> 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?
> The 11/74 definitely don't need CIS though. I don't think any 
> prototype 11/74 even had it. It was planned for the next generation of 
> the machine, that never got built. Anyway, it was to be an option for 
> the CPU as far as I know. Just as FPP.

Here's a front panel from a 'real 11/74', the uniprocessor one with 

Note the CIS uADDRS and other CIS status lights selectable on the rotary 
switch on the right.

The marketing dinks appropriated the 11/74 moniker for the wanna-be 
11/70mp program because '11/74' sounded better for a 4 way MP system.

After the 11/74 CIS COBOL benchmarks came in, the native PDP-11 CIS 
implementation on the 11/74 blew the 11/780 benchmarks away. DEC 
marketing did not like this; it made selling business customers the just 
released 11/780 more difficult. The decision was made to kill the 11/74 
CIS option the day we released it to manufacturing in favor of the new 
VAX system.

Johnny is right, the 11/74 option was a multiboard set (just like the 
FPP option) that could be plugged into the new 11/74 backplane (which 
was an 11/70 backplane with all the CPU and FPP slots pushed down by 
four to make room for the CIS option in the first slots).

I wish I had saved more documentation (print sets, microcode listings, 
etc) but to my knowledge none of this survives for the 11/74 CIS. Only 
my front plex panel which I took as a souvenir.

I was one of the three engineers who wrote the microcode for the CIS 
option subsystem (total of 4K words of 96bit wide horizontal microcode).


> IIST is needed for RSX to be happy (the only OS that supports the 
> 11/74), and you also need to implement parts of the memory bus 
> behaviour with interlocking. You can ignore the MK11 box CSRs, even 
> though it will look a little funny, but you do need separate DL11s for 
> each CPU core, along with the rest of the I/O bus, or else things will 
> probably not work. The 11/74 is a shared memory machine, but not 
> shared I/O bus.
>     Johnny

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