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Step 3.3.4: The program loop

We have reached the final step in our game development, the outermost loop! This loop is responsible for starting the game loops endlessly. The algorithm is super simple:

  1. Wait until the player presses fire
  2. Clear the outdated messages on screen
  3. Initialize the game, e.g set score to zero, initialize the playfield, etc
  4. Get the shape of the next piece
  5. Do the game loop until game is over
  6. Update the hiscore if score is higher than current hiscore
  7. Jump to the beginning

Here is the code:

REM --
REM -- The program loop
REM --
WHILE 1 = 1

  TEXTAT 27, 6, "press fire"

  REM -- Wait for fire button
  UNTIL joy_1_fire!() = 1

  REM -- Clear messages
  TEXTAT 27, 4, "         "
  TEXTAT 27, 5, "           "
  TEXTAT 27, 6, "          "

  REM -- Start new game. Initialize variables
  level! = 1
  delay! = 10
  TEXTAT 26, 11, "  "
  TEXTAT 26, 11, level!
  CALL clear_playfield
  game_status! = 0
  ttl_rows_cleared! = 0
  score% = 0.0
  REM -- Get next shape and display preview
  nxt_shape_no! = CAST!(RND%() * 7.0)
  nxt_shape = get_shape(nxt_shape_no!, 0)
  CALL draw_preview
  REM --
  REM -- Paste the game loop here
  REM --
  REM -- Game over. Update hiscore and messages
  TEXTAT 27, 4, "game over"
  IF score% > hiscore% THEN
    hiscore% = score%
    TEXTAT 27, 5, "new hiscore"
    TEXTAT 27, 17, hiscore%
REM -- Program loop ends here

I have started with a command that may be unknown for you, namely DISABLEIRQ. It is identical to the assembly language instruction SEI. It tells the processor to not accept any intterrupt requests. The reason why we use this command here is to make the gameplay a little smoother by turning out the default interrupt service of the C64, that is responsible for flashing the cursor, updating the system clock and watching the keyboard. Since we don't need any of the them in our game, it is safe to turn it off, saving a little CPU time every 1/50 of a second.

This loop is a WHILE ... ENDWHILE loop, which is a pre-test loop. Since its entry condition always evaluates to true, the loop will run endlessly. This is the same as

UNTIL 1 = 0

but I wanted to show you that XC=BASIC features both kinds of loops.

The rest should be really straightforward. If there is a mistake and your program can't be compiled, compare it to my version: here is the full code after all steps completed.

Congratulations, you have just coded a Tetris game in XC=BASIC! Compile, run and enjoy!

If you have questions, something doesn't work or you have improvement ideas, please write to me to feketecsaba [at] gmail [dot] com or mention @news_xc in your tweet!

Thanks for reading!