Five or six weeks ago David Carney asked how to handle instructions with
fields beyond the usual instruction size, which is rather CISC-ish. While
Frank and Doug responded quickly, how to handle this situation, by
defining just fields starting at the next bit (and the trick to define
lsb0? to #f), he did not succeed. As he told me, he had to move on to
other things, and so got distracted.

However, I am in a similar situation, trying to accomplish more or less
the same thing. While for my microcontroller (Mitsubishi M16C, if anybody
is interested) instructions may be as short as 8 bits, with operands,
immediates and prefixes they may grow to up to 80 bits or so. I was able
to assemble and disassemble the first instructions with different
addressing modes (and correspondingly different lengths) by using
xxx-insn-bitsize of 32 (instead of the 8 as suggested in the documentation
for the smallest instruction), but for instructions even longer than that
my attempts were not successful. While I can define instruction fields
that late, I can not fill them with registers or immediates, this data
(register indices etc.) gets stuck in the first portion of the
instruction.

Doug mentioned the fr30 port as a guidline, which has a single instruction
taking a 32 bit immediate and thus being longer than the others. However,
this functionality seems to be gone. The same problem is there: The
immediate data does not get filled in the appropriate slot. I took this
file:

; test file for ldi32
        ldi:32 $0x80000000, r0
        nop
        ldi:32 $0x00000000, r1
        nop
        ldi:32 $0xffffffff, r2
        nop
        ldi:32 $0x7fffffff, r3
        nop
.end

assembled it, and objdump of the result told me:

a.out:     file format elf32-fr30

Disassembly of section .text:

00000000 <.text>:
in disassemble_bytes
   0:   9f 80 00 00     ldi:32 0x0,r0
   4:   00 00
   6:   9f a0           nop
   8:   9f 81 00 00     ldi:32 0x0,r1
   c:   00 00
   e:   9f a0           nop
  10:   9f 82 00 00     ldi:32 0x0,r2
  14:   00 00
  16:   9f a0           nop
  18:   9f 83 00 00     ldi:32 0x0,r3
  1c:   00 00
  1e:   9f a0           nop

As you can see, the immediate data vanishes somewhere in between.

I am kind of stuck now. Is there still a trick I have been missing, or are
variable width instructions currently broken, or only fields beyond the
first instruction portion? 
I hope desperately there is a simple fix to this problem.

Thanks for your time.

Ciao,
	Manuel

PS: For the curious I have attached the current state of my m16c.cpu
file (I am not sure if it will make it to the list). Feel free to comment
on it, and find further information and a CVS repository of the will-be
binutils port at
http://savannah.nongnu.org/projects/m16c/