Logisim is a free tool used to design (and simulate) digital logic circuits.
http://www.cburch.com/logisim/
This repository contains a WIP processor I'm designing in Logisim.
In order to load this project, simply start up Logisim, and open CPU.circ.
Range:
0x0000 - 0xFFFF
Keyboard:
0xFAFE
Screen:
0xFAFF - 0xFFFF