It's a tiny calculator icon in the status bar. Click it, to expand it and use it. Enter the expression to calculate and hit enter to evaluate.
Right click on the icon to change modes: hex,dec, bin or complex.There is also an option for any base b/w 2 and 24. You can also use page-down key to change base. Angle can be switched b/w radians and degrees.
In real mode operations supported are +, -, *, /, %, ^, !(factorial&...