Bit Banged I2C on 6502: A Beginner's Guide.
Anders Nielsen demonstrates how to bit-bang I2C using 6502 assembly language in his 65duino project. Since there is no dedicated I2C interface hardware, the programmer must take care of all the details of the I2C protocol in software. Nielsen shows how to generate the I2C start, stop, and NACK conditions required to write to the SSD1306-based OLED. From these basic roots, a complete OLED library in assembly can be constructed.