BRK                          BRK Force Break                          BRK

  Operation:  Forced Interrupt;                         N V - B D I Z C
              PC + 2 to Stack, P to Stack               . . . 1 0 1 . .

  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied/Stack |   BRK                 |   $00   |    1    |    7     |
  +----------------+-----------------------+---------+---------+----------+
  1. A BRK command cannot be masked by setting I.
  2. Although even official documents say BRK uses implied addressing-mode,
     it is rather an immediate one, but the operand is thrown away. This is
     why PC+2 is pushed onto stack rather than PC+1. So, better use a NOP
     directly after BRK to avoid trouble.
  3. To be very precise: The B-flag is not set by BRK. It is rather set all
     the time, only when an external IRQ is recognized it will be cleared for
     some cycles, so you can distinguish between BRK and IRQ.