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31107: Alien data structure
This structure contains an assortment of values used to manage the aliens.
_ALIEN_ROW_PTR 31107 DEFB 0,0 Pointer to an entry in _ALIEN_ROWS_DATA
_NEXT_ALIEN_ROW_TO_DRAW 31109 DEFB 0 Next screen row to populate with aliens
_LOWEST_ALIEN_YPOS 31110 DEFB 0 cy of the bottom of the lowest row on screen. Used and updated by alien movement and other code
_ALIEN_ROW_LEFT_MARGIN 31111 DEFB 0 How far the aliens are from left side
_ALIEN_ROW_BANK_WIDTH 31112 DEFB 0 Width of this row, starts at 11 aliens, reduces
_ALIEN_ROW_RIGHT_MARGIN 31113 DEFB 0 How far aliens are from the right side
_ALIEN_SPRITE_DATA_PTR 31114 DEFB 0,0 Points to the left or right looking alien sprites
_SPRITE_SCR_DEST 31116 DEFB 0,0
_SPRITE_DRAW_SCAN_COUNTER 31118 DEFB 0
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