macro xxx [opt]
{
  local rcaddr_reg, bh_zero
  rcaddr_reg equ bp
  bh_zero equ 0
  is_set equ 0

  match =rcaddr:reg_n, opt \{
    match reg==n, reg_n \\{
      rcaddr_reg equ reg
      match =bx, reg \\{
        bh_zero equ 1
      \\}
      if n <> 0
        display "Only value '0' is allowed for register in option 'rcaddr'!"
        err
      end if
      is_set equ 1
    \\}
    match =0, error \\{
      rcaddr_reg equ reg_n
    \\}
    is_set equ 1
  \} ; rcaddr
}

xxx rcaddr:bx=0