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I've done some searching on the internet and several forums on it but they all resort to explaining what ROM is, it retains its memory even when there there is no power to it. I know what ROM is but what I want to know is how does it actually work.

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ROM typically stores data as wires that are either physically connected or physically disconnected.

In the classic diode matrix ROM, there are row wires and column wires and for each combination of row wire and column wire, there can either be a diode connecting them or not be a diode connecting them.

In the picture below, an 81316F 16K ROM from 1965 with the lid removed, you can see the row and column decoding logic around the perimeter. In the middle are two massive arrays that contain diodes at some intersections and nothing at others:

enter image description here

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  • Ahh "...wires that are either physically connected or physically disconnected". That really fixes things in my head, I was stuck wondering what sort of sorcery would be used with capacitors to make them retain their charge for long periods. Thanks @David Schwartz Apr 27, 2017 at 4:18
  • This is from the the 1960s and 70s!
    – harrymc
    Apr 27, 2017 at 7:38
  • @harrymc what do you mean? Apr 28, 2017 at 6:01
  • Well, have you seen any wires anywhere in a modern computer? The technology today is all solid-state.
    – harrymc
    Apr 28, 2017 at 7:00
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    @harrymc I honestly don't know what you're on about. The individual components inside modern semiconductors are in fact connected to each other with wires, typically made of aluminum or copper (at least, last I checked) and deposited on the semiconductor base. Nobody I know would call them "cables". Read the sections "metal layers" and "interconnect" in the page I linked to. (How do you think two transistors in a chip that don't abut are connected to each other?) Apr 28, 2017 at 7:50
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LMGTFU - There is no single answer to the question as there are different types of ROM which work differently. You might also be intested in Memristors (next generation ROM)

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Most rewritable ROM (a contradiction) today is Flash. It stores the data as an electrical charge on a floating gate, which is embedded in Silicon dioxide, one of the best insulators there are. So the charge can't leak away from the gate, and can be held for tens of years. If you switch the power off the charge will still be there.

The Wikipedia article Flash memory explains how that works :

In flash memory, each memory cell resembles a standard MOSFET, except that the transistor has two gates instead of one. On top is the control gate (CG), as in other MOS transistors, but below this there is a floating gate (FG) insulated all around by an oxide layer. The FG is interposed between the CG and the MOSFET channel. Because the FG is electrically isolated by its insulating layer, electrons placed on it are trapped until they are removed by another application of electric field (e.g. Applied voltage or UV as in EPROM).

More information as regarding the current flavors of Flash is to be found in the above Wikipedia article.

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  • "So the charge can't leak away from the gate, and can be held for tens of years." That's the problem with this answer - it is inaccurate. What if you need longer than 10 years? Unfortunately, charges can leak away, especially in high radiation environment, or space applications, or at higher temperature. So a mask-based ROMs (aka MROMs) are used instead, which work on the same basic principle as described by David Schwartz. -1, sorry. Jul 6, 2017 at 3:24

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