Immediate
  Operand is the data to be provided to CPU.
    The contents of program counter is the effective address.

    6502: LDA #42

  Data is immediately available upon fetch of operand.

  By default, data is a read-only constant.

  Not all systems support Immediate mode.

  But often can be simulated
    IBM360: LA R1,4(R0,R0) - side effect of how R0 handled.

    LA calculates an effective address by added the contents of a index
      and base register and an unsigned displacement and stores it in the 
      specified register.

      However, when R0 used, it's contents ignored and 0 used instead.

    This can also be used to as a way to add 2 24-bit unsigned
      values and a constant. But it won't affect flags.