Super Game Boy - Hardware Info

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

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.