When I want to put a constant into a multi-ifield, the
exact instructions how to set the subfields are apparently ignored, and the
total value is copied into the subfields.
I have found this while working on my machine desscription,
and I could reproduce this with a small synthetic testcase, attached as
foo.cpu.  I've put tis file into the cgen/cpu directory, and
then with the cgen directory as the cwd, invoked guile and evaluated:
(load "dev.scm")
(load-sim)
(cload #:arch "foo" #:machs "foo")
(cgen-decode.c)

note that the expected value to check entire_insn against is 0x1324,
but generated test is:
        if ((entire_insn & 0xffff) == 0x12424)