FOSSIL is a standard protocol for allowing serial communication for telecommunications programs under the DOS operating system.
Before FidoBBS could start spreading, it was seen that a uniform method of communicating with serial interface hardware was needed if the software was going to be used on other machines.
Software using the FOSSIL standard could communicate using the same interrupt functions no matter what hardware it was running on.
FOSSIL drivers have also been implemented to support other communications hardware by making it "look like a modem" to the application.
Internal ISDN cards (that did not use serial ports at all) often came with FOSSIL drivers to make them work with software that was originally intended for modem operation only.