====== NEC PC-9801 - I/O Ports ======

===== 02h =====

TODO

===== 35h =====

TODO

===== 37h =====

TODO

===== 71h =====

TODO

===== 73h =====

TODO

===== 77h =====

TODO

===== 7Ch (GRCG - Control) =====

  * bit 7 - enable (1)/disable (0) GRCG
  * bit 6 - enable (1)/disable (0) read/modify/write mode
  * bit 4-5 - select plane exposed to CPU (0-3)
  * bit 3 - enable (1)/disable (0) plane 3
  * bit 2 - enable (1)/disable (0) plane 2
  * bit 1 - enable (1)/disable (0) plane 1
  * bit 0 - enable (1)/disable (0) plane 0

===== 7Eh (GRCG - Tile) =====

This port accepts sequential (in order of enabled planes) 8-bit tile data.