VT100.net has published an annotated disassembly of the original 8 KiB firmware for the DEC VT100 terminal. The project takes the ROM code apart and explains how it works, with links to the Technical Manual and the print set.

The VT100 shipped in 1978. It was DEC’s first ANSI-compatible terminal and the first to drop discrete logic for a microprocessor, the Intel 8080. The escape sequences were changed slightly for the cheaper VT101 and VT102, and those became the definition of “VT100 compatible” in DEC’s internal Video Systems Reference Manual for the next 15 years.

The site splits the work into sections: why the firmware was disassembled, how the disassembly was produced, the disassembly itself, a list of bugs and oddities found in the code, and Awnty, an 8080 coverage checker written for the job. There is also a page of related resources.

See more at vt100.net.