Tetris (Electronica 60)

Alexey Pajitnov's original Tetris was programmed in Pascal on an Electronica 60 - an unauthorized Soviet clone of a Digital Equipment Corp. PDP-11 computer. The game features monochrome graphics, and in the first revision of the game, the blocks in the tetrominos are represented by a pair of delete/rubout characters (character code 177); however, the rendering of this character code as a rectangle was a feature specific to the Soviet clone machines, an actual PDP-11 would instead display nothing. A later revision was made where the blocks are represented by a pair of square brackets instead.

Scoring

 * Points are awarded for each tetromino successfully dropped into place.
 * At level 0, the potential point score for each tetromino starts at 19, and decreases by 1 every time the tetromino descends a row due to gravity. As can be seen, an I tetromino falling freely from the top of the playfield to the bottom will descend 19 rows and score 0; it is never possible to score less than 0.
 * Playing at higher levels awards an additional 3 x (Level + 1) points per tetromino.
 * Playing with the preview disabled awards an additional 5 points per tetromino.
 * 3 digits are provided for score display; when 999 is exceeded, the counter rolls back to 0 and a tally of symbols appear in order to keep track of thousands.

Bugs
It is sometimes possible to manipulate a tetromino after performing a hard drop, even though it is supposed to be locking. Doing so will leave a trail of blocks behind, which, although visible to the player, are not recognized by the game as actually being there. This can lead to a variety of unusual scenarios, for example, visibly complete rows will not clear, and tetrominos can be dropped through the stack.