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There are 200 different dot positions that can be individually pro- grammed onto your TV screen in the Y direction. The sprite Y position registers can handle numbers up to 255. This means that you have more than enough register locations to handle moving a sprite up and down. You also want to be able to smoothly move a sprite on and off the screen. More than 200 values are needed for this.

The first on-screen value from the top of the screen, and in the Y direction for an unexpanded sprite is 30. For a sprite expanded in the Y direction it would be 9. (Since each dot is twice as tall, this makes a certain amount of sense, as the initial position is STILL calculated from the top left corner of the sprite.)

The first Y value in which a sprite (expanded or not) is fully on the screen (all 21 possible lines displayed) is 50.

The last Y value in which an unexpanded sprite is fully on the screen is 229. The last Y value in which an expanded sprite is fully on the screen is 208.

The first Y value in which a sprite is fully off the screen is 250.


 10 PRINT"{CLEAR}"                :REM CLEAR SCREEN
 20 POKE 2040,13                  :REM GET SPRITE 0 DATA FROM BLOCK 13
 40 V=53248                       :REM SET BEGINNING OF VIDEO CHIP
 50 POKEV+21,1                    :REM ENABLE SPRITE 0
 60 POKEV+39,1                    :REM SET SPRITE 0 COLOR
 70 POKEV+1,100                   :REM SET SPRITE 0 Y POSITION
 80 POKEV+16,0:POKEV,100          :REM SET SPRITE 0 X POSITION


Positioning in the horizontal direction is more complicated because there are more than, 256 positions. This means that an extra bit, or 9th bit is used to control the X position. By adding the extra bit when necessary a sprite now has 512 possible positions in the left/right, X, direction. This makes more possible combinations than can be seen on the visible part of the screen. Each sprite can have a position from 0 to 511. However, only those values between 24 and 343 are visible on the screen. If the X position of a sprite is greater than 255 (on the right side of the screen), the bit in the X MOST SIGNIFICANT BIT POSITION

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This page has been created by Sami Rautiainen.
Read the small print. Last updated May 12, 2002.