The second type of AND-NOT gate can be repeated in a grid pattern to form a ROM. We store bits in the ROM by adding or removing ears from each gate. Each gate stores one bit of data: a ‘0’ if it has an ear by its north-easterly output, a ‘1’ if it has no ear there.
Watch the progress of an electron arriving from below. On entering the ROM, it starts to move north-west through the gates. Each time it hits a gate with a missing ear, an electron is emitted from that gate’s north-east output. This newly-emitted electron moves north-east through the ROM array and finally leaves the north-east edge.
The picture below shows how the ROM can be accessed continuously.
The ROMs shown in the pictures here have ten inputs and seven outputs. They are used in the computer as look-up tables to drive seven-segment displays.
If accesses to the ROM always follow a fixed sequence it can be more simply implemented as a loop of suitable length carrying the desired bit pattern.
Next, a flip-flop.
This page most recently updated Thu 26 Jul 08:28:27 BST 2018
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