Qbus split I&D?

[Guy Sotomayor]

> On Mar 17, 2015, at 1:04 PM, John Wilson <wilson at dbit.com> wrote:
> 
> On Tue, Mar 17, 2015 at 12:39:21PM -0700, Guy Sotomayor wrote:
>> The parts that I've been looking at have unlimited write endurance and >20
>> year data retention.
> 
> I'd be slightly worried that there are conditions being assumed here, even
> if the MRAM data sheet doesn't seem to say so.  F-RAM claimed unlimited
> writes, but under slightly cherry-picked conditions that might be exceeded
> after all.  You could certainly write the whole memory from beginning to
> end all day long, but whaling on just a small area (like the stack) could
> be trouble.  Obviously I hope I'm wrong -- MRAM *looks* wonderful!
> 
> I used battery-backed SRAM in the Unibus RAM card I prototyped a while back
> (and just recently got working and have been finishing the XMOS firmware
> for).  The SRAM is cheap and fast and draws hardly any current (so it's
> nothing like a lead-acid BBU setup -- AAs are fine and a CR2032 might even
> last a while, so I've got one dangling off for testing to see if Rev B
> should have coin cell holders), and once the battery is there, it's a nice
> excuse for a TOY clock (my favorite KDJ11E feature that all PDP-11s would
> benefit from).

Since I'm using the MRAM to store not only PDP-11 boot ROMs but also
the J1 code that makes the whole thing work, I'm really hesitant about using
anything that isn't truely non-volatile.

TTFN - Guy