Tetris DX

Not to be confused with Tetris DS, a game released for the Nintendo DS.

Tetris DX is a Game Boy Color game that is an enhancement of Nintendo's original Game Boy Tetris, with two added gameplay modes (Ultra, lasting 3 minutes; and Vs. CPU, a match against the computer), a profile/save feature, and color graphics on Game Boy Color and above. In addition, the game's rotation system had a feature which lets a player move any tetromino except O upward by holding the direction against a wall while rotating in the opposite direction.

Modes

 * Marathon (score maxes at 9,999,999)
 * True 40 lines mode (not 40 "points" like in Tetris Worlds)
 * Vs. CPU opponent

Rotation system
The rotation system is an early version of what would become SRS. Very interestingly, L, J, T spawn pointing down, while I, S, Z spawn pointing up. TDX first tries rotation about the center of the box (the blue circle). If this fails, it tries rotation about an alternate center displaced by one cell (the gray circle). For I, S, and Z, this center starts out one cell above the ordinary center; for L, J, or T, it starts out one cell below.

Wall climbing
This rotation system is exploitable, especially on low levels: This particular exploit was fixed in SRS, because it uses a different rotation system. It returned in Tetris Best 2, this time the game trying to constantly floorkick the piece.

Infinity
Ordinarily, Tetris DX lock delay follows step reset behavior: only downward motion under gravity resets the lock delay timer. But rotation about the alternate center does reset the timer used for gravity. This can be exploited by placing a piece in a corner and rotating it back and forth about the alternate center: