field splitting

[Eric Christopher]

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.

Currently the field is just defined as 2 separate fields.

Any ideas?

-eric