Super GameBoy Hardware Description

            by Jeff Frohwein, 2-Oct-97



 Since few people understand how the Super GameBoy (SGB)

works internally I have decided to write this document.

Here's how a normal GB is layed out:



                GameBoy or Pocket GB Internals



                       +--------------+

                       | GB game cart |

                       +--------------+

                              |

                              V

                Keypad  +-------------+ video

      +------+  control |             |  out  +-----------+

      |Button|--------->| GameBoy CPU |------>|LCD Display|

      |Inputs|<---------|             |       +-----------+

      +------+          +-------------+     

                       

 Here's how a SGB is layed out:



      + - - - - - - - - - - - - - - - - - - - - - - - +  

                        +--------------+               

      |  SGB            | GB game cart |              |

        package         +--------------+               

      |                        |                      |

                               V                       

      |         Keypad  +-------------+ video         |

                control |             |  out           

      |    +----------->| GameBoy CPU |----------+    |

           | +----------|             |          |     

      |    | |          +-------------+          |    |

      + - -|-|- - - - - - - - - - - - - - - - - -|- - +

           | |          +--------------+         |

           | |          |              |         |

           | +--------->| SNES Console |<--------+

           +------------|              |

                        +--------------+



 The GB CPU in a SGB cartridge is stamped SGB CPU but

for all practical purposes it appears to be identical

to a standard GB CPU.



 For the most part, the main function of the SGB

cartridge is to supply video signals from the GB CPU

to the SNES console. The SNES basically has to do a

screen capture of raw video signal from the GB CPU

in order to write this data to SNES video ram so that

this data is viewable on a television screen.



 In order to do SGB borders, SNES sounds, and SGB

extended colors, these enhanced SGB features are

enabled/disabled over the keypad control interface.



 Major data exchanges from the SGB CPU to the SNES

(for such things as SGB border picture data and

native SNES code and data) are performed by doing

special screen captures. The data to be transfered is

moved to SGB CPU video memory and then is displayed.

The SNES is then informed (over the keypad control

lines) to capture the data on the SGB CPU video out.



 The video out interface (shown above) on the SGB

CPU is the same as the video out interface on a

standard GB. There are only 4 shades of grey at

this point. Extended colo(u)rs in a SGB game that

appear to be coming from this video interface are

really caused by the SNES. Every 8x8 position on

the SNES screen can be set for different colo(u)r

palettes to map the 4 shades of grey in that

position to actual Colo(u)rs.