A C64 game - Step 18 | GameDev.net

This time we add some enemy animation and a path based movement enemy type. The movement path is stored in a table of delta X and delta Y values. Values with the highest bit set are treated as negative.

The animation of the bat is also stored in a table (it's a simple ping pong loop).

Every objects get an animation delay (SPRITE_ANIM_DELAY), animation pos (SPRITE_ANIM_POS) and movement pos counter (SPRITE_MOVE_POS).

Remember, adding a new type means just adding the new constant and entries to the startup value tables.
If you wonder about the flickering white border on the bottom half: It's an easy way to see how long the actual per frame code runs. You'll notice more complex code taking quite a bit more time.

Here's a detailed look at the path code. It's actually pretty straight forward. Read the next byte. Check if the high bit is set and use the result to either move left/right. Rinse and repeat for Y.

;------------------------------------------------------------
;move in flat 8
;------------------------------------------------------------
!zone BehaviourBat8
BehaviourBat8
          ;do not update animation too fast
          lda DELAYED_GENERIC_COUNTER
          and #$3bne .NoAnimUpdate
          inc SPRITE_ANIM_POS,x
          lda SPRITE_ANIM_POS,x
          and #$3
          sta SPRITE_ANIM_POS,x
          tay
          lda BAT_ANIMATION,y
          sta SPRITE_POINTER_BASE,x
.NoAnimUpdate
          inc SPRITE_MOVE_POS,x
          lda SPRITE_MOVE_POS,x
          and #31
          sta SPRITE_MOVE_POS,x
          ;process next path pos
          tay
          lda PATH_8_DX,y
          beq .NoXMoveNeeded
          
          sta PARAM1
          and #$80beq .MoveRight
          
          ;move left
          lda PARAM1
          and #$7f
          sta PARAM1
          
.MoveLeft
          jsr MoveSpriteLeft
          dec PARAM1
          bne .MoveLeft
          jmp .XMoveDone
          
.MoveRight
          jsr MoveSpriteRight
          dec PARAM1
          bne .MoveRight
          
.NoXMoveNeeded
.XMoveDone
          ldy SPRITE_MOVE_POS,x
          lda PATH_8_DY,y
          beq .NoYMoveNeeded
          sta PARAM1
          and #$80beq .MoveDown
          
          ;move up
          lda PARAM1
          and #$7f
          sta PARAM1
          
.MoveUp
          jsr MoveSpriteUp
          dec PARAM1
          bne .MoveUp
          rts
          
.MoveDown
          jsr MoveSpriteDown
          dec PARAM1
          bne .MoveDown
          
.NoYMoveNeeded
          rts

The tables themselves are handmade. For the planned path we just need to make sure we end up where we started:

PATH_8_DX
          !byte $86
          !byte $86
          !byte $85
          !byte $84
          !byte $83
          !byte $82
          !byte $81
          !byte 0
          !byte 0
          !byte 1
          !byte 2
          !byte 3
          !byte 4
          !byte 5
          !byte 6
          !byte 6
          !byte 6
          !byte 6
          !byte 5
          !byte 4
          !byte 3
          !byte 2
          !byte 1
          !byte 0
          !byte 0
          !byte $81
          !byte $82
          !byte $83
          !byte $84
          !byte $85
          !byte $86
          !byte $86
          
PATH_8_DY
          !byte 0
          !byte 1
          !byte 2
          !byte 3
          !byte 4
          !byte 5
          !byte 6
          !byte 6
          !byte 6
          !byte 6
          !byte 5
          !byte 4
          !byte 3
          !byte 2
          !byte 1
          !byte 0
          !byte 0
          !byte $81
          !byte $82
          !byte $83
          !byte $84
          !byte $85
          !byte $86
          !byte $86
          !byte $86
          !byte $86
          !byte $85
          !byte $84
          !byte $83
          !byte $82
          !byte $81
          !byte 0

step18.zip

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