yet another pdp-11 in fgpa

Don North ak6dn at
Tue Jun 22 13:43:14 CDT 2010

On 6/22/2010 12:34 AM, E. Groenenberg wrote:
>> 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).
> How was this achived? By reducing the Massbus card space by 4 boards
> so 3 interface sections were available? Or by redesigning the processor
> in such a way that there were less boards needed?
> Ed

Removed one of the MASSBUS controllers.

The CPU board set was essentially the same, slot for slot. There were 
several boards that needed rev/ECO to provide hooks for the CIS option, 
but all in all these were minor updates and were backward compatible.

IIRC the main change was to add additional microcode to the base 11/70 
microengine (from 256 to 512 words) by doubling the PROM bit density. 
This code did all the base instruction dispatch and operand fetch for 
the CIS unit.


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