FW: Problem with instruction description

Hello,

I am working on the porting of binutils to a new architecture.
This processor has an instruction of the form: ld rx,[r1] + imm6 [load the 32 bit word at the address
calculated from the contents of r1 plus the 6 bit immediate value (multiplied by 4) ]
In summary:    rx <- mem[r1 + imm6 * 4]

I need to generate code for this instruction, but only in the case where the code generator in the compiler
generates a standard load, but with an offset appropriate for the imm6 value. That is to say the immediate
value would be less than 256 and be a multiple of 4 (word access).

This is the description that I am using at the moment:
(df​​-f imm6x4 "imm6x4" ()
    11 6 UINT
  ((value pc) (sra WI value (const 2)))
  ((value pc) (sll WI value (const 2))))
(dnop imm6x4 "imm6x4" () h-uint f-imm6x4)
(dni ldstack-r1-imm6x4 "ldstack r1#imm6x4"   ()   "ld $r1,[r1]+$imm6x4"   (+ (f-stack-opcode
#05) r1 imm6x4)   ()   ())   
However this description causes the "ld rx,[r1] + imm6" to be used whenever the immediate vallue is less
than 256, and not JUST when the offset is also a multiple of 4.
How do I alter this description so that it only operates when the immediate value is less than 256 AND ALSO a
multiple of 4.

I have already tried.... but this generates code with this instruction in these cases ;;; which are not
appropriate.How do I achieve this? 
I guess that I'll have to create an attribute to handle this, but I could not get an
attribute that suits me. is there a way to create an attribute of type integer, for example,
but with only specific integers not necessarily following.