UrJTAG aims to create an enhanced, modern tool for communicating over JTAG with flash chips, CPUs, and many more.
Is a serial interface for testing devices with integrated circuits. The problem that the JTAG interface was designed to solve is checking if connections between ICs are OK. Therefore you can set and check in- and outputs of ICs. In order to save pins and logic a very simple serial design was invented.
The control pin (together with clock) allows to switch device states. A state machine inside each chip can be controlled, e.g. to reset the device. This control machine also allows to have two internal shift registers in each device (although we only have on in- and one output-pin).
The registers are called instruction register (IR) and data register (DR). The current UrJTAG tool allows you to set the IR and set and get the DR. It doesn't allow you to directly control the statemachine (yet).