I/O Trap Cause Register

Number	TBRegisterNumber::$DA
Readable	TBRegisterReadable::Yes
Writable	TBRegisterWritable::No
Short Description	ShortDesc::experimental

  0 = none (zero at the same time nextreg 0x02 bit 4 is 0)
  1 = port_2ffd read
  2 = port_3ffd read
  3 = port_3ffd write

• If nextreg 0x02 bit 4 indicates an i/o cycle was trapped, this register indicates the cause