Sega Master System / Mark III / Game Gear
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This game contains code designed to detect whether it is running on a Japanese or export Master System and only display "TM" on the initial SEGA logo screen in the latter case.
Unfortunately the method used for region detection, testing bit 6 of the value read from I/O port $00, only works on the Game Gear. On the Master System, reading this port is usually unsupported, but happens to return different values on different hardware revisions of the Sega 8-bit family.
The code in question is:
0719: DB 00 INA (00h) 071B: CB 77 BIT 6,A 071D: 28 0C JR Z,+0Ch (072Bh)
From Charles MacDonald's SMS/GG hardware notes, Section 3.) Z80 I/O ports:
SMS port map: $00-$3F : Reads return the last byte of the instruction which read the port. SMS 2 port map: $00-$3F : Reads return $FF. Game Gear port map: (MS mode) $00-$3F : Reads return $FF. Genesis port map: (with PBC) $00-$3F : Reads return $FF.
The game therefore does not show "TM" when run on an original SMS. "TM" is displayed when run on an SMS 2, a Game Gear in Master System mode or a Genesis with Power Base Converter.
|Sega logo without TM mark||Sega logo with TM mark|
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