The register window displays the CPU register values of the function call selected by the Debug Location toolbar or the call stack window. The located function will display all the CPU registers then as you select other function calls the saved register state of that function is displayed.
The format the register values are displayed in can be changed using the popup menu on the right mouse button.
The value of a register can be modified by clicking on the value column and modifying the text or alternatively by choosing one of the default assignments that are on the right click popup menu. Note that when you modify integer register values you need to prefix hexadecimal numbers with 0x, binary numbers with 0b and octal numbers with 0. Note that modifying the saved register value of a function is not supported.