MEM11A questions

Paul Koning paulkoning at
Tue Mar 8 14:48:10 CST 2016

> On Mar 8, 2016, at 3:44 PM, Mike Ross <tmfdmike at> wrote:
>>> ...
>> No, it will only have 128KW.  Since it’s a single FRAM part, I’m not going to
>> try and “stock” different flavors (especially since I’ll be having the boards
>> assembled at the board house).
> But you will do something to allow memory to be switched in and out? I
> presume this thing wouldn't work in an 11/20 unless it was strapped
> back to 32KW? Or will the 11/20 be happy and simply ignore the
> unaddressable memory beyond 32KW?

The Unibus has 18 address lines.  This should work so long as the memory doens't respond to I/O page addresses (the top 4kW of the address space).  That's a requirement for any Unibus memory on any PDP11.  The difference between 11/20 and 11/05 vs. the others is that the lack of MMU means that the CPU can't generate addresses in the range 28 to 124 kW.  (I/O devices can, though, which would be rather interesting.)


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