$00/0:   6510 On-chip Data Direction Register

   +----------+---------------------------------------------------+
   | Bit  7   |   Undefined                                       |
   | Bit  6   |   Only available on C128, otherwise undefined     |
   | Bits 0-5 |   1 = Output, 0 = Input (see $01 for description) |
   +----------+---------------------------------------------------+
   Default Value is $2F/47 (%00101111).

Kernal-Reference:

 LDA $00,X   : $F5E2
 STA $00     : $FDDB
     $00,X   : $B4D7

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