Gives you the current exception number. This provides a way of
    qualifying exception breaks.

    Example:

    DBG> EVALUATE %EXC_NUM
    12
    DBG> EVALUATE/COND %EXC_NUM
    %SYSTEM-F-ACCVIO, access violation at PC !XL,
         virtual address !XL
    DBG> SET BREAK/EXC WHEN (%EXC_NUM = 12)