Linked by Thom Holwerda on Mon 27th Feb 2017 23:25 UTC
Apple

The Apple engineers were smart when they were building their MessagePads. The MessagePad required an immense 8MB of storage for the Newton operating system. In the 90's, flash memory was extremely expensive, so they had to use ROM chips that were mass-produced and could never be updated. But they knew that they would have changes to their firmware until release day, and they would need to able to fix bugs even after the machine was sold.

They came up with three solutions.

[...]

If all else fails, MessagePads have their ROM chips sitting on a daughter board, a small additional cicuit board that is fitted into a common (at that time) connector and can be changed without tools after opening the case.

[...]

Anyway, wouldn't it be fantastic to create a souped-up ROM board? 8MB Flash and 8MB NewtonOS, also in Flash, being able to patch it, fix it, extend it, have fun. Maybe have even more that 16MB if that is possible. Is it possible? How can we find out?

An early draft of the licensee information for this ROM card exists, but it is not detailed enough to build such a card. Before starting a patch wire solution, I wanted to know how the original board worked, and then fill in the missing information in that draft.

Well, I went all the way and reverse engineered the entire ROM board. Here are my findings.

Amazing work.

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