Qbus split I&D?

Paul Koning paulkoning at comcast.net
Tue Mar 17 14:27:22 CDT 2015

> 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.


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