variable width instructions

[David Carney]

I'm trying to write a .cpu file for a simple, non-pipelined (CISC) chip.  The 
instructions and data are all 16 bits (little endian).  Defining the 
instruction fields was relatively straight forward, until I realised that 
instructions involving 'immediate' datum are, in fact, 32 bits...

My question is: how I should go about defining the instruction fields for this 
architecture?  I.e. what value should I use for the "start" fields in 
"(define-ifield ...)" for the msb so that instruction fields are compatible 
for both 16-bit and (the effectively) 32-bit instructions (15 or 31) ?

My confusion stems from observing the line:

	(dnf f-i32       "32 bit immediate"      (SIGN-OPT) 16 32)

in fr30.cpu.  Doesn't the "16 32" denote that the start of the opcode is at 
bit 16, but the length is 32?  For my ISA I need something equivalent to 
"start" = -1, "length" = 16.  How do I go about this?

Dave Carney