Emulator of the ancient 8 bit microprocessor ISP-8A 500D/600D or more common "SC/MP" from National Semiconductor.
The download contains all sources plus an executable Java Jar file to start right away. Some SC/MP hex dump examples are provided in directory "/etc" as well.
The download also contains unit tests and some helping documentation (also in /etc).
You may slow down the CPU operations, displaying the CPUs status register as LEDs, to watch the CPU working with its registers...