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There are only 3 forms of bit storage, historically.

1) Poking holes in things ( tape, DVD, etc ) 2) Magnets ( core memory, drum memory, tape, disks ) 3) Circuits ( DRAM, NAND, etc )

Examples of all three of these still exist.

What's interesting about XPoint is it is literally a fourth form that has never been commercially available: melting a substance and cooling it quickly or slowly, forming either a crystal or amorphic solid, which then has different properties. We don't know what the substance is, but it's cool that we now have this 4th thing.

> melting a substance and cooling it quickly or slowly, forming either a crystal or amorphic solid, which then has different properties

This is exactly how rewritable optical media works.

There are only 3 forms that really survive to this day. There are a couple of notable obsolete ones:

4) Delay lines. I would definitely not categorize these as circuits. The bits are stored as pressure waves in motion.

5) Electrostatic charge. Talking about the Williams tube. The bits are stored as residual charge on a phosphor surface.

... and magnetic bubble memory, which is a very odd hybrid somewhere between tape and delay lines.

Also, phase-change optical (as in rewritable CDs/DVDs).

Also also, printing stuff out at high bit-density with good ECC, and scanning it back in again.

Also also also, chemically encoded bits (as in the DNA-based storage that was demonstrated recently).

There are more, not in common use or very scalable (e.g., mechanical switches).

I think it's a bit conspicuous how you mention good ECC for printing stuff out, since good ECC is a staple of many forms of storage (especially hard drives, optical drives, tape) and we don't mention e.g. "storing bits on spinning rust with good ECC and reading it back in again".

I think the problem with paper is thot it's not really part of the computer any more. You need a human to take the paper out of the printer, store it, and put it back in the scanner. At least with tape and optical, we have robots to do that for us.

I hadn't thought of that. Your average "crappy thumb drive" probably has more ECC than you'd need on a physical piece of paper. You're right.

Although when crappy thumb drives fail the whole thing tends to be unreadable or doesn't even show up as a USB device.

Generally no amount of ECC will help the default failure mode of these.

Is #5 that much different from dynamic RAM, with bits stored as capacitive charge?

Memristors would technically be another form of storage but after the hype HP have been a bit quiet on this for the last 3 years.

MO drives heated the medium to the Curie point. 3D XPoint isn't all the revolutionary.

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