It is a program for development and simulation of software for the PIC16F84 microcontroller made by Microchip. With this program you can write the code, build it and test if it works in a few seconds. While debugging, you see the internal content of all registers.
PIC Development Studio is not just only a PIC16F84 simulator. You can simulate entire digital projects provided that the needed components are available. You connect the components together just as they would be in reality and start the simulation. There are included keyboards, simple displays, basic logic circuits, a tone generator and an oscilloscope probe. If these don't fulfil your needs, writing own components isn't difficult.
Getting PIC Development Studio
Source : http://picdev.sourceforge.net/webpage/