Arithmetics on Virtual Signals in Modelsim

Hi guys,

Can I in some way do arithmetic on signals I'm viewing in ModelSim?

F.x. I have two signals

signalA(7 downto 0)

signalB(7 downto 0)

and in ModelSim I want to view a wave form

mySignal = unsigned(signalA) - unsigned(signalB)

i don't want to change the vhdl code. this is only for testing and simulation

my idea was to use virtual signals and then do arithmetics on them....

virtual signal {sim:/testbench/DUT/DataChannelInst/signalA} signalA

virtual signal {sim:/testbench/DUT/DataChannelInst/signalB} signalB

# Then somehow

mySignal = signalA - signalB

# and

add wave mySignal

Any ideas and sugestion greatly appreciated q:o)

Cheers

Stefan