1.0 (547 bytes)

There is one ball, two paddles.
Ball travels diagonally, bouncing off of paddles, top or bottom.
A scoreboard appears on the top line of the screen.
Paddles are colored. The scoreboard line has a different colour than the rest of the field.
When a goal is scored, the ball moves to the centre horizontal position and a random vertical position.
At game start and when a goal is scored the ball pauses at its new starting location for a half second (to give the player time to ready).
The left paddle is controlled by the CTRL and ALT keys, the game is exited by any other key.
The right side paddle has a simple AI (if the ball is far enough to the right, the paddle will move toward it).

1.0 to 1.1 (547 - 482 = 65 bytes)

Discovered XOR reg,reg is smaller than MOV reg, 0
Moved input and game drawing into main loop (CALLs were used only once in code).
Created a put_pixel subroutine that leaves the GP registers intact (turned out to be quite versatile).
Created a draw_paddle subroutine (now reuses same code for both paddles).
Folded the random number generation into the new_ball subroutine (minor change to function, but still acceptably random).

1.1 to 1.2 (482 - 470 = 12 bytes)

Two INC bx instructions are shorter than ADD bx, 2.
Created clamp_paddle subroutine to keep both player and AI paddles within the playing field (reused for both paddles).

1.2 to 1.3 (470 - 469 = 1 byte )

Replaced a MOV reg, mem; CMP reg, immed with CMP mem, immed.

1.3 to 1.4 (469 - 450 = 19 bytes )

Created subroutine collide_paddle which collides the ball with either paddle (used twice).
Moved update_game subroutine into main loop (only called once).
Saved a MOV bx, mem in AI code by changing ADD bx, immed; CMP bx, ax immed to SUB bx, immed; CMP bx, ax. (The results of ax were not reused.)

1.4 to 1.5 (450 - 306 = 144 bytes)

Removed scoreboard feature.
Removed coloured paddles.
Removed delay on score.
Changed some 16 bit comparisons on Y coordinates to 8 bit.
Moved new_ball subroutine into collide_paddle subroutine.
Moved paddles to screen edges (replaces the left paddle X coord constant with 0).
Moved game state update after vsync in order to interleave it with the drawing code:
	Eliminates need to store last position of ball for clearing.
	Saves a few MOV reg, mem by keeping values alive in their registers.

1.5 to 1.6 (306 - 300 = 6 bytes)

Moved player paddle movement code closer to player paddle collision code to save register reloading.

1.6 to 1.7 (300 - 296 = 4 bytes)

Tweaked random number generation to save a few bytes. Still acceptably random.

1.7 to 1.8 (296 - 288 = 8 bytes)

Placed ball Y position on stack between updates instead of storing it in memory (also push/pop instructions are much smaller than MOV reg, mem instructions). (Note this destroys one of the optimizations in 1.4 which clobbered ax, where this position is now stored.)

1.8 to 1.9 (288 - 276 = 12 bytes)

Placed ball X position on stack between updates.

1.9 to 2.0 (276 - 264 = 12 bytes)

Changed MOV ax, 0013h to MOV al, 013h, assuming that AX begins as 0. (May not be portable? Saves a byte though.)
Placed player Y position on stack between updates.
Changed ball Y and player Y position calculations to 8 bit where possible.

2.0 to 2.1 (264 - 255 = 9 bytes)

Moved input code into the middle of the code that updates the player position, saving storage of the movement position and associated memory addressings.


-- Brad Smith
http://switch.to/unleaded
http://rainwarrior.thenoos.net
rainwarrior@gmail.com