The Texas Instruments TI 59 family of programmable scientific calculators used interchangeable ROM cartridges that could be installed in a slot at the back of the calculator. The calculator came with a module that provides several standard mathematical functions including solution of simultaneous equations. Other modules were specialized for financial calculations, or other subject areas, and even a "games" module. Modules were not user-programmable. The Hewlett-Packard HP-41C had expansion slots which could hold ROM memory as well as I/O expansion ports.