The Stack

HP Calculators use a four-element stack.  The stack is made up of X, Y, Z, and T values.  The X value is the main working value.  Certain operations (shift left/right, increment/decrement, not, and clear) work only on the X value.  When you press Enter the stack is rolled up and the X value is recopied into the X element.  Swap switches the values of the X and Y elements.