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Sega Master System Store Display Unit BIOS
Dumped by Omar Cornut (Bock) on 2004/10/01
http://www.smspower.org
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Note: This dump will not show anything on any SMS emulator. It is
meant to function with dedicated hardware that is not emulated and
may never be emulated.

This is from Sega Master System Store Display units. These are heavy
units used to promote the console in North American shops, back in
the days. The unit must be opened to access the cartridge/card slots
inside. They allow up to 16 cartridges and 3 (or 16, depending on the
unit) cards to be plugged in, and the game to play is selected via a
front panel.

The unit resets itself after a few minutes, based on dipswitch
settings.

I dumped this BIOS image from an EPROM chip located on both models
(16-3 and 16-16 models). Their content is the same.

Those units have two Z80 CPUs onboard. My guess is that the first one
is part of a standard SMS circuitry, while the second one is in
charge of checking the front panel, switching/start games, and
maintaining a timer to reset the software periodically. I think this
BIOS code is run on the second Z80 but I'm not sure.

Memory Map:
   BIOS Code    $0000-$3FFF   (16 Kb)
   RAM          $4000-$47FF   (2 Kb)
   RAM          $C000+        (? Kb)

It accesses memory locations $4000 and $8000 in a way suggesting that
they have special behavior.

What puzzles me is that this BIOS ROM contains code to write certain
values to RAM at $C000 and jump back to address $0000, which is the
booting behavior of a standard SMS BIOS. This kind of code should be
run on the main Z80 running the game.

Hypothesis: the second Z80 has access to the main system RAM (mapped
at $C000+) and copies a booting procedure for the main Z80 to execute.

Sorry if this is incomplete. I did not spend much time working on
this. There's still more to study and solve if someone aims to
emulate the behavior of this unit. The information above is
incomplete and may be inaccurate.


-Omar Cornut
http://www.smspower.org