The Zapple Monitor was a firmware-based product developed by Roger Amidon[1] at Technical Design Laboratories (also known as TDL).
[2] The Zapple monitor was a primitive operating system which could be expanded and used as a Basic Input/Output Services (BIOS) 8080- and Z80-based computers.
Because of the simple structure of the program, consisting of a vector table (one for each letter) and a small number of subroutines, and because the source code was readily available, adding or modifying Zapple was straightforward.
The dominant operating system of the era, CP/M, required the computer manufacturer or hobbyist to develop hardware specific BIOS.
The general structure of Zapple lives on in the code of many older programmers working on embedded systems as it provides a simple mechanism to test the hardware before moving to more advanced user interfaces.