Open Basic: Retro d100 Roleplaying

From the introduction:

As the name suggests, this is a no frills set of roleplaying rules. These rules have been designed with an eye toward achieving fidelity to one particular d100 rule set that was released in 1980, insofar as open licenses will allow. Having said that, some changes have been made in accordance with the designer’s personal preferences.

These rules assume a basic familiarity with roleplaying games and dispense with drawn out discussions of what a roleplaying game is, what the roles of various players are, and so on. Such sacrifices were made for the sake of brevity, although future iterations of these rules may revisit such topics.