How to define variable size ISA

Hi,

I trying to rewrite Zilog Z80 CPU support using CGEN. Currently I have
implemented one byte instructions. But trying to implement two-byte ones I
have an issue: CGEN just check for ORed ifields of first and second bytes
(it is not correct), instead of checking for first and than for second one:

(dnf f-0  "whole byte 0" ((MACH z80) all-isas) 7 8)
(dnf f-1  "whole byte 1" ((MACH z80) all-isas) 15 8)
(dnf f-1x "byte 1 field x, bits 7-6" ((MACH z80) all-isas) 15 2)
(dnf f-1y "byte 1 field y, bits 5-3" ((MACH z80) all-isas) 13 3)
(dnf f-1z "byte 1 field z, bits 2-0" ((MACH z80) all-isas) 10 3)
;RLC (HL) has opcode: 0xCB 0x06
(dni rlc-mhl    "rotate left cyclic" (all-isas) "rlc (hl)" (+ (f-0 #xCB)
(f-1x 0) (f-1y 0) (f-1z 6)) () ())
(dni rlc-r      "rotate left cyclic" (all-isas) "rlc $rs1" (+ (f-0 #xCB)
(f-1x 0) (f-1y 0) rs1) () ())
(dni rrc-r      "rotate right cyclic" (all-isas) "rrc $rs1" (+ (f-0 #xCB)
(f-1x 0) (f-1y 1) rs1) () ())

And disassembler disassemble all CB prefixed instructions as RLC <r>. So it
looks like explicit field value is not work. Next I see, what is generated
for these instructions in the z80-opc.c file:
/* retn */
  {
    { 0, 0, 0, 0 },
    { { MNEM, 0 } },
    & ifmt_retn, { 0x132 }
  },
/* rlc $rs1 */
   {
     { 0, 0, 0, 0 },
    { { MNEM, ' ', OP (RS1), 0 } },
    & ifmt_rlc_r, { 0xcb }
   },
 /* rlc (hl) */
   {
     { 0, 0, 0, 0 },
     { { MNEM, ' ', '(', 'h', 'l', ')', 0 } },
     & ifmt_rlc_mhl, { 0xd1 }
    },

I have two ideas how to workaround it:
1. Switch ISAs on each prefix opcodes (and back after instruction parse
finish) and use much of macro instructions for assembler
2. Use fixed 32-bit opcodes and much of C-code hooks and helpers
I do not investigate deeply these two cases. I want to know first, it is my
mistake, CGEN bug, or undocumented limitation.

Best regards,
Sergey Belyashov