A C64 game - Step 8 | GameDev.net

Of course a game isn't a game without some challenge. Therefore we need enemies. Since we have some neat little level build code why not use it for enemies as well?

We add a new level primitive type LD_OBJECT which adds objects (= sprites). We use it for both player and enemies. A new table SPRITE_ACTIVE is added to see if a sprite is used (and which type).

One central function to this is FindEmptySpriteSlot. It iterates over the sprite active table and looks for a free slot to use. If there is a free slot we set the object active, apply the object startup values and use the previously created CalcSpritePosFromCharPos to place the sprite.

Note that we don't plan to use more than 8 objects so we can directly map object to sprites.

Find an empty slot:

;------------------------------------------------------------
;Looks for an empty sprite slot, returns in X
;#1 in A when empty slot found, #0 when full
;------------------------------------------------------------
!zone FindEmptySpriteSlot
FindEmptySpriteSlot
          ldx #0
.CheckSlot
          lda SPRITE_ACTIVE,x
          beq .FoundSlot
          inx
          cpx #8
          bne .CheckSlot
          lda #0
          rts
          
.FoundSlot
          lda #1
          rts

How we add an object during level buildup:

.Object
          ;X pos
          iny
          lda (ZEROPAGE_POINTER_1),y
          sta PARAM1
          
          ;Y pos
          iny
          lda (ZEROPAGE_POINTER_1),y
          sta PARAM2
          
          ;type
          iny
          lda (ZEROPAGE_POINTER_1),y
          sta PARAM3
          
          ;store y for later
          tya
          pha
          
          ;add object to sprite array
          jsr FindEmptySpriteSlot
          beq .NoFreeSlot
          
          lda PARAM3
          sta SPRITE_ACTIVE,x
          
          ;PARAM1 and PARAM2 hold x,y already
          jsr CalcSpritePosFromCharPos
          
          ;enable sprite
          lda BIT_TABLE,x
          ora VIC_SPRITE_ENABLE
          sta VIC_SPRITE_ENABLE
          lda #SPRITE_PLAYER
          sta SPRITE_POINTER_BASE,x
          
.NoFreeSlot
          jmp .NextLevelData

step8.zip

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