field splitting

[Ben Elliston]

   I have an instruction that looks something like this:
   [opcode][4-bits][src][12-bits]
   
   Where the 4 and 12-bit fields are really a part of the same 16-bit
   offset.  I've been unable to think of any way to define the field in
   such a way that I can make it a 16-bit operand that just happens to
   contain bits at different spots.

You need to define a multi-ifield, designed explicitly for this purpose.  
Check out the CGEN manual for details.

Ben