The Patch was a unique all-hardware approach to implementing lower case on a TRS‑80 Model I. It was sold by CECDAT Inc. for a price starting at $69.97.
The normal approach to adding lower case involved a hardware modification plus a software driver. Radio Shack never added a lower case driver to the Model I ROM, so the separate driver was required to enable generation of lower case characters. The lower case driver was stored in RAM, so it needed to be loaded again from cassette or disk every time the Model I was used.
The Patch combined an ordinary lower case hardware modification with a “patch” to the Model I ROM that added a lower case driver. Probably the best description of how it worked comes from one of their advertisements:
“The Patch” is not only a lower case modification, but also a complete system offering many of the enhancements you would like available on your TRS‑80. How is this possible? “The Patch” is a “smart chip” which monitors the same address lines as the ROM in your TRS‑80 Model I Level II. “The Patch” recognizes certain addresses, turns off the Level II ROM, then substitutes its’ own byte of data. This “Patching” is what allows ALL these enhancements with NO software overhead, thus NO software conflict. In fact, “The Patch” is the ONLY lower case modification that is compatible with both BASIC and machine language programs AND the ONLY all hardware lower case modification.
In addition to lower case, The Patch also provided:
- a keyboard debounce routine
- a block cursor
- the ability to directly type all characters from the keyboard
- the ability to use the SHIFT key as a CAPS LOCK key (for $30 extra)
- a shortened leader on cassettes (for $10 extra)