History: memory you cannot erase

Letters on the screen

The first terminals and home computers showed text as dots. Each character was a small grid, most often 5 × 7, and the font was stored in a separate chip — a character generator. In the Apple I (1976) it was the Signetics 2513 chip: in went the character code and the number of the scan line, out came the five dots of that line. The screen was drawn line by line, many times a second.

Ours is simpler: the screen is not scanned but lights up all at once, so the character generator gives out all 35 dots of a character at the same time.

The transistor that is not there

The contents of such a chip were set when it was made: on the last layer of the silicon, a transistor was connected in the right places and left out everywhere else. This was called mask ROM: the mask for a font or a program was ordered from the factory, and after that it could not be changed. Later came ROMs you could write yourself (with fusible links), ones you could erase with ultraviolet light through a little window in the package, and finally ones erased electrically — like a flash drive.

In all of them, a point works the same way: the column wire sits in one state by default, and a "programmed" point pulls it into the other. This is exactly what we build: a resistor holds the column at 1, and a transistor pulls it down to 0.

A matrix inside a processor

The same trick lives inside processors. In the MOS 6502 — the processor of the Apple II, Atari and NES — you can see a neat grid at the top of the chip: a table that uses the instruction code and the clock step number to decide which signals to turn on. Each of its points is a transistor on a pulled-up line, as in our matrix. If you look at a photo of the chip, you can see the table with your own eyes — just like our font.

Apple is a trademark of Apple Inc.; Signetics, MOS Technology, Atari and Nintendo are trademarks of their respective owners. This kit is not connected with them or approved by them; the names are mentioned to tell the story.