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 markSega logo with TM mark



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