Qbus split I&D?

Guy Sotomayor ggs at shiresoft.com
Tue Mar 17 14:39:21 CDT 2015

> On Mar 17, 2015, at 12:27 PM, Paul Koning <paulkoning at comcast.net> wrote:
>> On Mar 17, 2015, at 3:03 PM, Christian Gauger-Cosgrove <captainkirk359 at gmail.com> wrote:
>> On 16 March 2015 at 23:42, Eric Smith <spacewar at gmail.com> wrote:
>>> Not including parity or ECC, it takes two devices to fill the entire
>>> 4MB address space of the PDP-11/70. Either parity or ECC will require
>>> another one additional device, which won't be fully utilized.
>>> Ordinary SRAM is cheaper, but $110.16 for enough RAM chips to max out
>>> a PDP-11/70 doesn't seem all that expensive, unless you're comparing
>>> to DDR SDRAM DIMMs for PCs.
>> True, SRAM would be cheaper, and you can find faster SRAM and DRAM
>> than the currently available MRAM (if I recall correctly). But then
>> you lose the benefits of MRAM in the first place: It's non-volatile,
>> like core memory, and doesn't need battery backup (as you'd need if
>> you wanted to make SRAM "non-volatile"). 
> MRAM is non-volatile, sure.  I’m not sure its write limit is high enough to be used as a substitute for main memory.  In any case, what PDP-11 operating systems use the non-volatility of memory?  I know of one: RSTS-11.  But RSTS/E dropped that (it reboots on powerup instead).  That makes sense, given that semiconductor memory appeared fairly early in the PDP-11 product life, and none of that came with battery backup.  In other words, only some of the models in some configurations offered non-volatile memory, which made it fairly uninteresting for operating systems to support.

The parts that I've been looking at have unlimited write endurance and >20 year data retention.  I'm using MRAM in my MEM11 board (seems the previous FRAM vendor was bought out and no longer produce some of the parts I was planning on using) for emulation of main memory, ROMs and RF11 disks.  They're also plenty fast enough at 35ns access times.

TTFN - Guy

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