PDP-11/70 cache memory

Ethan Dicks ethan.dicks at gmail.com
Mon Dec 29 18:30:01 CST 2008

On Mon, Dec 29, 2008 at 6:02 PM, Guy Sotomayor <ggs at shiresoft.com> wrote:
> If there's enough interest, I'll try for it since I'm doing a unibus memory
> board (a multifunction board) with an FPGA for everything but the memory and
> interfaces.  I'd have to look at the interface from the cache boards, but I
> might be tempted to use SSRAMs since they can be obtained in wide bit widths
> (36 bits) in a single chip and aren't too difficult to interface to (timing
> wise).  I know that they come in 512K x 36 since I'm considering that for
> another project.  So that would only require 2 parts for the entire memory
> array (4MB).


>  If ECC were required, there'd be a bit more involved and I'd
> have to look to see what was available.

My personal thought was that one could get away with simulated ECC
(enough to fool what the OS is expecting) or I do know that one can do
some form of ECC with 36 bits per word - I have an Alpha board that
does it (36-bit-wide 72-pin SIMMs).

> However, I'm swamped with my real job and haven't had a lot of time (read
> none) to spend on this stuff.


> I've already done the schematic capture for
> the Unibus interface and level shifters and I'm about 15-20% done coding up
> the verilog for the FPGA.  I've done all of the high level design, memory
> timing and unibus address decode.  The rest is the internal interconnect,
> serial, unibus state machine and configuration state machine plus the
> individual unibus devices (memory, KW11L, ROMs, 2 SLUs).


Speaking only for myself, a modern Unibus board is more interesting
than a board that only works in 11/70s.  The number of 11/70s running
in hobbyists' hands is tiny compared to the overall number of lesser
Unibus machines.  And since many of those lesser machines might not
have a full load of memory, the impact of changing from old to new is
greater.  My 11/70s have full MK11s - the idea of running without an
MK11 is about space (second rack) and power consumption and
maintainability.  If I had run across an 11/70 CPU only that had lost
the MK-11 some time back, I might feel differently, but I think the
ratio of 11/70s with no memory to ones that are complete is rather

I look forward to hearing more about, then buying, your universal
memory board.  I'm not sure I could afford a modern PEP-70
replacement, let alone two.


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