"This realistic simulator of a 4-function Texas Instruments calculator from 1974 runs the calculator's source code instruction by instruction by simulating the processor. The unusual processor has 11-bit opcodes, 44-bit BCD registers, and a 9-bit address bus. To use the simulator, slowly click keys on the calculator image and you can watch how the calculator performs operations step by step. Since the processor doesn't do multiplication or division, it does these operations by repeated addition or subtraction."
This is a video of a game which replicates Portal's physics system in 2 dimensions on the TI-83 graphic calculator. The game was developed by a 20 year old student studying game design. A download link is available here.