Question

The code that is causing problems looks like a normal xnor operation as you can see below:

S(1) <= L(16) xnor L(26);

This line causes the following error:

ncvhdl_p: *E,EXPSMI (HDL/aes_sbox_enc_depth16.vhd,169|14): expecting a semicolon (';') [9.5.1].
ncvhdl_p: *F,MAXERR: maximum error count reached (1).
TOOL: ncvhdl 10.20-s075: Exiting on Feb 14, 2012 at 12:56:05 GMT (total: 00:00:01)

Anyone an idea what is going wrong here, the semicolon is clearly there. Is it possible that VHDL
does not support xnor, if so, how do I have to rewrite it?

Many thanks!

Answer 1

I believe that xnor is defined for bits and booleans, but not std_logic. I think it actually depends on which version of VHDL (e.g. 98 / 2002 / 2008) you’re using. It’s certainly commented out of some versions of the std_logic_1164.vhd files I’ve seen.

How about just inverting an xor?

S(1) <= not (L(16) xor L(26));