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Modern flash memory uses multiple voltage levels to cram two or more bits into each cell, in order to increase density.

This technique is not used in DRAM (perhaps because it would compromise speed?) but it turns out to be workable for ROM, indeed to have been used for microcode ROM several decades ago: http://www.righto.com/2018/09/two-bits-per-transistor-high-density.html?m=1

I get the impression from that article that after the 8087, the technique was not subsequently used. Has it been used in more recent times? If not, why not? Double density would seem to be a big win.

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That technique is only viable for die cast (non-programmable) ROM. PROM is more flexible.

A lot more components have to work correctly for it to work, reducing yields. Each transistor in the ROM must fall within its margin, the resistor must be correctly sized, the comparators have to work correctly.

Flash memory is being developed using multiple levels with more bits per cell according to wikipedia (https://en.wikipedia.org/wiki/Multi-level_cell)

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    \$\begingroup\$ Not just being developed, widely deployed. Pretty much any gigagbyte or above flash you run into in a current consumer product is going to be multi-level, unless it's old or you pay extra. In fact it's so prevalent that the primary alternative now available is not actual single level, but a multi-level chip with modified controller which stores a single bit in what are designed and basically fabbed as multi-level cells (ie, pseudo-SLC) \$\endgroup\$ – Chris Stratton Oct 1 '18 at 14:57

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