C8hardcaml

c8hardcaml

⡿⠿⢏⣉⡹⠿⢏⣉⡹⠿⢏⣉⡹⠿⢏⣉⡹⠿⢏⣉⡹⠿⢏⣉⡹⠿⢏⣉⡹⠿⢏⣉⡹⠿⢏⣉⡹⠿⢏⣉⡹⠿⢏⣉⡹⠿⢏⣉⡹⠿⢏⣉⡹⠿⢏⣉⡹⠿⢏⣉⡹⠿⢏⣉⣹
⡇⠀⠈⠉⠁⠀⠈⠉⠁⠀⠈⠉⠁⠀⠈⠉⠁⠀⠈⠉⠁⠀⠈⠉⠁⠀⠈⠉⠁⠀⠈⠉⠁⠀⠈⠉⠁⠀⠈⠉⠁⠀⠈⠉⠁⠀⠈⠉⠁⠀⠈⠉⠁⠀⠈⠉⠁⠀⠈⠉⠁⠀⠈⠉⢹
⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢸
⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢸
⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢀⣀⣀⣀⣀⣀⣀⣀⡀⠀⢀⣀⣀⣀⣀⣀⣀⣀⡀⠀⢀⣀⡀⠀⠀⠀⢀⣀⡀⠀⢀⣀⣀⣀⣀⣀⣀⣀⡀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢸
⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢸⣿⡿⠿⠿⠿⠿⠿⠇⠀⢸⣿⡿⠿⠿⠿⢿⣿⡇⠀⢸⣿⡇⠀⠀⠀⢸⣿⡇⠀⢸⣿⡿⠿⠿⠿⠿⠿⠇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢸
⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢸⣿⡇⠀⠀⠀⠀⠀⠀⠀⢸⣿⡇⠀⠀⠀⢸⣿⡇⠀⢸⣿⡇⠀⠀⠀⢸⣿⡇⠀⢸⣿⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢸
⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢸⣿⡇⠀⠀⠀⠀⠀⠀⠀⢸⣿⡇⠀⠀⠀⢸⣿⡇⠀⠈⢹⣷⡆⠀⢰⣾⡏⠁⠀⢸⣿⣇⣀⣀⣀⣀⣀⡀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢸
⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢸⣿⡇⠀⠀⠀⠀⠀⠀⠀⢸⣿⣷⣶⣶⣶⣾⣿⡇⠀⠀⢸⣿⡇⠀⢸⣿⡇⠀⠀⢸⣿⡿⠿⠿⠿⠿⠿⠇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢸
⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢸⣿⡇⠀⠀⠀⠀⠀⠀⠀⢸⣿⡏⠉⠉⠉⢹⣿⡇⠀⠀⠸⢿⣇⣀⣸⡿⠇⠀⠀⢸⣿⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢸
⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢸⣿⣇⣀⣀⣀⣀⣀⡀⠀⢸⣿⡇⠀⠀⠀⢸⣿⡇⠀⠀⠀⠸⢿⣿⡿⠇⠀⠀⠀⢸⣿⣇⣀⣀⣀⣀⣀⡀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢸
⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠸⠿⠿⠿⠿⠿⠿⠿⠇⠀⠸⠿⠇⠀⠀⠀⠸⠿⠇⠀⠀⠀⠀⠸⠿⠇⠀⠀⠀⠀⠸⠿⠿⠿⠿⠿⠿⠿⠇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢸
⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢸
⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢸
⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢸
⡷⠶⢆⣀⡰⠶⢆⣀⡰⠶⢆⣀⡰⠶⢆⣀⡰⠶⢆⣀⡰⠶⢆⣀⡰⠶⢆⣀⡰⠶⢆⣀⡰⠶⢆⣀⡰⠶⢆⣀⡰⠶⢆⣀⡰⠶⢆⣀⡰⠶⢆⣀⡰⠶⢆⣀⡰⠶⢆⣀⡰⠶⢆⣀⣸
⠓⠒⠚⠛⠓⠒⠚⠛⠓⠒⠚⠛⠓⠒⠚⠛⠓⠒⠚⠛⠓⠒⠚⠛⠓⠒⠚⠛⠓⠒⠚⠛⠓⠒⠚⠛⠓⠒⠚⠛⠓⠒⠚⠛⠓⠒⠚⠛⠓⠒⠚⠛⠓⠒⠚⠛⠓⠒⠚⠛⠓⠒⠚⠛⠛ 

⡏⠉⠉⠉⠉⠉⠉⠉⠉⠉⠉⠉⠉⠉⠉⠉⠉⢉⣉⣉⣉⣉⡉⢉⣉⣉⣉⣉⡉⢉⣉⣉⣉⣉⣉⡉⢉⣉⣉⣉⣉⡉⢉⣉⣉⣉⣉⡉⠉⠉⠉⠉⠉⠉⠉⠉⠉⠉⠉⠉⠉⠉⠉⠉⢹
⡇⠰⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠆⠀⢈⡉⠉⠉⠉⠁⢈⡉⠉⠉⢹⡇⢈⡉⠉⠉⠉⢹⡇⢈⡉⠉⠉⠉⠁⢈⡉⠉⠉⠉⠁⠀⠰⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠆⢸
⡇⠀⠰⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠆⠀⠀⠸⠷⠶⠶⢶⡆⢸⣷⣶⣶⣾⡇⢸⣷⣶⣶⣶⣾⡇⢸⡇⠀⠀⠀⠀⢸⡷⠆⠀⠀⠀⠀⠀⠰⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠆⠀⢸
⡇⠰⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠆⠀⢰⣶⣶⣶⣾⡇⢸⡏⠉⠉⠉⠁⢸⡏⠉⠉⠉⢹⡇⢸⣷⣶⣶⣶⡆⢸⣷⣶⣶⣶⡆⠀⠰⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠆⢸
⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠈⠉⠉⠉⠉⠁⠈⠁⠀⠀⠀⠀⠈⠁⠀⠀⠀⠈⠁⠈⠉⠉⠉⠉⠁⠈⠉⠉⠉⠉⠁⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢸
⡇⠀⠀⠀⠀⠀⠀⢰⡆⢰⡶⠶⠶⠶⢶⡆⢰⣶⡆⠀⠀⠀⢰⡆⠀⢰⡶⠶⠶⢶⡆⠀⢰⡶⠶⠶⠶⢆⡀⢰⡶⠶⠶⠶⠆⢰⡶⠶⠶⠶⢶⡆⢰⡶⠶⠶⠶⠶⠆⠀⠀⠀⠀⠀⢸
⡇⠀⠀⠀⠀⠀⢀⣸⡇⢸⣇⡀⠀⠀⢸⡇⠸⢿⡇⠀⠀⢰⡾⠇⢰⣾⡷⠶⠶⠾⢷⡆⢸⣷⡆⠀⠀⢸⡇⢸⣷⡶⠶⠆⠀⢸⡷⢶⡶⠶⠾⠇⠸⠷⠶⠶⢶⣶⡆⠀⠀⠀⠀⠀⢸
⡇⠀⠀⠀⠀⠀⢸⣿⡇⢸⣿⡇⠀⠀⢸⡇⠀⠸⢷⡆⢰⡾⠇⠀⢸⣿⡇⠀⠀⠀⢸⡇⢸⣿⡇⠀⠀⢸⡇⢸⣿⡇⠀⠀⠀⢸⡇⠸⠷⢶⣶⡆⠀⠀⠀⠀⢸⣿⡇⠀⠀⠀⠀⠀⢸
⡇⠀⠀⠀⠀⠀⠸⠿⠇⠸⠿⠇⠀⠀⠸⠇⠀⠀⠸⠷⠾⠇⠀⠀⠸⠿⠇⠀⠀⠀⠸⠇⠸⠿⠷⠶⠶⠎⠁⠸⠿⠷⠶⠶⠆⠸⠇⠀⠀⠸⠿⠇⠰⠶⠶⠶⠾⠿⠇⠀⠀⠀⠀⠀⢸
⡇⠀⢀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⡀⠀⢸
⡇⠀⢸⡏⠉⠉⠉⠉⠉⠉⠉⠉⢉⡉⠉⠉⠉⠉⢉⡉⢉⡉⠉⠉⠉⠉⢉⡉⠉⠉⠉⢉⡉⠉⠉⠉⢉⣉⣉⣉⣉⣉⡉⠉⠉⠉⠉⠉⠉⠉⠉⠉⢉⡉⠉⠉⠉⢉⣉⡉⠉⢹⡇⠀⢸
⡇⠀⢸⡇⠀⠀⠀⠀⠀⠀⠀⠀⢈⣱⡆⠰⢆⣰⣎⡁⢸⡇⠀⠀⠀⠀⢸⡇⠀⠀⠀⢸⡇⠀⠀⠀⢸⣏⡉⠉⠉⠉⢱⡆⠀⠀⠀⠀⠀⠀⠀⠀⢸⡇⠀⢀⡀⢸⣿⡇⠀⢸⡇⠀⢸
⡇⠀⢸⡇⠀⠀⠀⠀⠀⠀⠀⠀⢸⡏⢱⣶⡎⢹⣿⡇⠸⠷⢶⣆⣰⡶⠾⠇⠀⠀⠀⢸⡇⠀⠀⠀⢸⣿⡇⠀⠀⠀⢸⡇⠀⠀⠀⠀⠀⠀⠀⠀⢸⡇⠀⢸⡇⠈⢹⡇⠀⢸⡇⠀⢸
⡇⠀⢸⡇⠀⠀⠀⠀⠀⠀⠀⠀⠸⠇⠸⠿⠇⠸⠿⠇⠀⠀⠈⠹⠏⠁⠀⠀⠀⠀⠀⠸⠇⠀⠀⠀⠸⠿⠷⠶⠶⠶⠎⠁⠀⠀⠀⠀⠀⠀⠀⠀⠸⠷⠶⠎⠱⠶⠾⠇⠀⢸⡇⠀⢸
⡇⠀⠸⠷⢶⡶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⠶⢶⡶⠾⠇⠀⢸
⣇⣀⣀⣀⣸⣇⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣀⣸⣇⣀⣀⣀⣸
⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛

⡏⢉⡹⠏⠉⢉⡹⠏⠹⢏⡉⠉⠹⢏⡉⠉⠹⢏⡉⠉⠉⢉⡹⠏⠉⢉⡹⠏⠹⢏⡉⠉⠹⢏⡉⠉⠉⢉⡹⠏⠉⢉⡹⠏⠹⢏⡉⠉⠹⢏⡉⠉⠉⢉⡹⠏⠹⢏⡉⠉⠹⢏⡉⠉⢹
⡷⠎⠁⢀⡰⠎⠁⢀⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡰⠎⠁⢀⡰⠎⠁⢀⡀⠈⠱⢆⡀⠈⠱⢆⡰⠎⠁⢀⡰⠎⠁⢀⡀⠈⠱⢆⡀⠈⠱⢆⡰⠎⠁⢀⡀⠈⠱⢆⡀⠈⠱⢆⣸
⡷⢆⡀⠈⠁⢀⡰⠎⠁⢀⡰⠎⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠁⢀⡰⠎⠁⢀⡰⠎⠁⢀⡰⠎⠁⢀⡰⠎⠱⢆⡀⠈⠁⢀⡰⠎⠱⢆⡀⠈⠁⢀⡰⠎⠱⢆⡀⠈⢹
⡇⠈⠱⢆⡰⠎⠁⢀⡰⠎⠁⢀⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡰⠎⠁⢀⡰⠎⠁⢀⡰⠎⠁⢀⡰⠎⠁⢀⡀⠈⠱⢆⡰⠎⠁⢀⡀⠈⠱⢆⡰⠎⠁⢀⡀⠈⠱⢆⣸
⡷⢆⡀⠈⠁⢀⡰⠎⠱⢆⡀⠈⠁⢀⡰⠎⠱⢆⡀⠈⠁⢀⡰⠎⠱⢆⡀⠈⠱⢆⡀⠈⠁⢀⡰⠎⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠁⢀⡰⠎⠁⢀⡰⠎⠁⢀⡰⠎⢹
⡇⠈⠱⢆⡰⠎⠁⢀⡀⠈⠱⢆⡰⠎⠁⢀⡀⠈⠱⢆⡰⠎⠁⢀⡀⠈⠱⢆⡀⠈⠱⢆⡰⠎⠁⢀⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡰⠎⠁⢀⡰⠎⠁⢀⡰⠎⠁⢀⣸
⡷⢆⡀⠈⠁⢀⡰⠎⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠁⢀⡰⠎⠁⢀⡰⠎⠱⢆⡀⠈⠱⢆⡀⠈⠁⢀⡰⠎⠁⢀⡰⠎⠁⢀⡰⠎⠁⢀⡰⠎⠱⢆⡀⠈⠁⢀⡰⠎⠁⢀⡰⠎⢹
⡇⠈⠱⢆⡰⠎⠁⢀⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡰⠎⠁⢀⡰⠎⠁⢀⡀⠈⠱⢆⡀⠈⠱⢆⡰⠎⠁⢀⡰⠎⠁⢀⡰⠎⠁⢀⡰⠎⠁⢀⡀⠈⠱⢆⡰⠎⠁⢀⡰⠎⠁⢀⣸
⡷⢆⡀⠈⠁⢀⡰⠎⠁⢀⡰⠎⠱⢆⡀⠈⠁⢀⡰⠎⠱⢆⡀⠈⠱⢆⡀⠈⠁⢀⡰⠎⠱⢆⡀⠈⠁⢀⡰⠎⠱⢆⡀⠈⠁⢀⡰⠎⠱⢆⡀⠈⠁⢀⡰⠎⠁⢀⡰⠎⠁⢀⡰⠎⢹
⡇⠈⠱⢆⡰⠎⠁⢀⡰⠎⠁⢀⡀⠈⠱⢆⡰⠎⠁⢀⡀⠈⠱⢆⡀⠈⠱⢆⡰⠎⠁⢀⡀⠈⠱⢆⡰⠎⠁⢀⡀⠈⠱⢆⡰⠎⠁⢀⡀⠈⠱⢆⡰⠎⠁⢀⡰⠎⠁⢀⡰⠎⠁⢀⣸
⡷⢆⡀⠈⠱⢆⡀⠈⠁⢀⡰⠎⠱⢆⡀⠈⠁⢀⡰⠎⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠁⢀⡰⠎⠁⢀⡰⠎⠱⢆⡀⠈⠁⢀⡰⠎⠱⢆⡀⠈⠁⢀⡰⠎⠁⢀⡰⠎⠁⢀⡰⠎⢹
⡇⠈⠱⢆⡀⠈⠱⢆⡰⠎⠁⢀⡀⠈⠱⢆⡰⠎⠁⢀⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡰⠎⠁⢀⡰⠎⠁⢀⡀⠈⠱⢆⡰⠎⠁⢀⡀⠈⠱⢆⡰⠎⠁⢀⡰⠎⠁⢀⡰⠎⠁⢀⣸
⡷⢆⡀⠈⠁⢀⡰⠎⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠁⢀⡰⠎⠱⢆⡀⠈⠱⢆⡀⠈⠁⢀⡰⠎⠁⢀⡰⠎⠁⢀⡰⠎⢹
⡇⠈⠱⢆⡰⠎⠁⢀⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡰⠎⠁⢀⡀⠈⠱⢆⡀⠈⠱⢆⡰⠎⠁⢀⡰⠎⠁⢀⡰⠎⠁⢀⣸
⡇⢀⡰⠎⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠁⢀⡰⠎⠱⢆⡀⠈⠁⢀⡰⠎⠁⢀⡰⠎⠱⢆⡀⠈⠁⢀⡰⠎⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠁⢀⡰⠎⠁⢀⡰⠎⢹
⡷⠎⠁⢀⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡰⠎⠁⢀⡀⠈⠱⢆⡰⠎⠁⢀⡰⠎⠁⢀⡀⠈⠱⢆⡰⠎⠁⢀⡀⠈⠱⢆⡀⠈⠱⢆⡀⠈⠱⢆⡰⠎⠁⢀⡰⠎⠁⢀⣸
⠓⠒⠒⠚⠓⠒⠒⠚⠓⠒⠒⠚⠓⠒⠒⠚⠓⠒⠒⠚⠓⠒⠒⠚⠓⠒⠒⠚⠓⠒⠒⠚⠓⠒⠒⠚⠓⠒⠒⠚⠓⠒⠒⠚⠓⠒⠒⠚⠓⠒⠒⠚⠓⠒⠒⠚⠓⠒⠒⠚⠓⠒⠒⠚⠛ 

An implementation of CHIP-8 in Hardcaml that can be executed in simulator or on physical hardware.

Architecture

The programmable CPU core is the root circuit. Through inputs to this circuit we can initially program the board-ram to prepare a program for execution and we can also use it to read the frame buffer.

CPU core holds all of the CHIP-8 implementation, calling out to different circuits like draw, bcd, xor_shift for instructions that are more complex.

Board ram is used for system memory and instructions all assume one cycle to read or write which is hardcoded into their implementation.

Testing

This project is tested visually using ppx_expect and the Hardcaml simulator. To start a visual test we initialize an instance of Programmable_cpu_core in sim and then program a test supplied ROM into board RAM by cycling the simulator. A test harness then cycles the simulator for a specific number of cycles and then uses debug inputs to Programmable_cpu_core to read the framebuffer from board RAM. The data in the framebuffer is then rendered to a string using the drawille library and printed to stdout so that ppx_expect can spot when the framebuffer changes between executions of tests.