Problem with instruction description

[youssef fahmi]

Hello,
I'm sorry for the previous mail, I do not know why there were "Â" everywhere.
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.  
Thanks in advance  for your answer.Youssef