17 Mar
2015
17 Mar
'15
12:27 p.m.
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:
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.
paul
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").