[DSP-10] Spectral Inversion

[Bob Whelan]

Hi
 
Can anyone advise on the following
 
I want to spectrally invert a LPF into an HPF
 
 To do this I read (Smith etc) that I can 
 
- invert the order ( or sign ?) of the filter kernel
- reverse the order of calculation in the FIR filter sub-routine such that the coefficients are used in the reverse order to normal.
 
I can do the former with a bit of manipulation with a text editor etc. but the second would be easier since the same coeff set would be used for both the HPF FIR and the LPF. However in the AD assembly language syntax  how do you rewrite the DO  .....UNTIL CE phrase so that the multiplication would use the coeff in the reverse order ?
 
I have consulted the AD Instruction Set Ref but although there are quite a few permissable terms I dont see how you could structure the reverse to the normal.
 
Any ideas ?
 
Thanks
 
Bob G3PJT