A C64 Game - Step 47

Poor Sam got nerfed. Now his force range starts out really low (about 5 characters), but can be improved by picking the proper extra. And finally cheat keys appear. Press 1 to advance to the next stage.



We modify the existing SamUseForce routine to check for a max range (stored in PLAYER_FORCE_RANGE):

;------------------------------------------------------------
;sam uses power
;returns 1 when holding an enemy
;------------------------------------------------------------
!zone SamUseForce
SamUseForce
		  lda SPRITE_HELD
		  beq .NoSpriteHeldNow
		
		  lda #1
		  rts
		
.NoSpriteHeldNow		
		  stx PARAM6
		
		  ldy SPRITE_CHAR_POS_Y,x
		  dey
		  lda SCREEN_LINE_OFFSET_TABLE_LO,y
		  sta ZEROPAGE_POINTER_1
		  lda SCREEN_BACK_LINE_OFFSET_TABLE_HI,y
		  sta ZEROPAGE_POINTER_1 + 1
		
		  ldy SPRITE_CHAR_POS_X,x
		  lda #0
		  sta PARAM7
		
.ShotContinue
		  lda PARAM7
		  cmp PLAYER_FORCE_RANGE
		  beq .OutOfRange
		  inc PARAM7
		  ;y contains shot X pos
		  ;PARAM6 contains x sprite index of player
		  ldx PARAM6
		  lda SPRITE_DIRECTION,x
		  beq .ShootRight
		  ;shooting left		
		  dey
		
		  lda (ZEROPAGE_POINTER_1),y
		  jsr IsCharBlocking
		  beq .CheckHitEnemy
		  ldx PARAM6
.ShotDoneMiss  
.OutOfRange
		  lda #0
.ShotDoneHit		
		  rts
		
.ShootRight
		  iny
		
		  lda (ZEROPAGE_POINTER_1),y
		  jsr IsCharBlocking
		  bne .ShotDoneMiss
		
.CheckHitEnemy		
		  ;hit an enemy?
		  ldx #0
		
.CheckEnemy		
		  stx PARAM2
		  sty PARAM1
		  lda SPRITE_ACTIVE,x
		  beq .CheckNextEnemy
		  tax
		  lda IS_TYPE_ENEMY,x
		  beq .CheckNextEnemy
		  ldx PARAM2
		  ;is vulnerable?		
		  lda SPRITE_STATE,x
		  cmp #128
		  bpl .CheckNextEnemy
		  ;sprite pos matches on x?
		  lda SPRITE_CHAR_POS_X,x
		  cmp PARAM1
		  bne .CheckNextEnemy
		
		  ;sprite pos matches on y?
		  ldy PARAM6
		  lda SPRITE_CHAR_POS_Y,x
		  cmp SPRITE_CHAR_POS_Y,y
		  beq .EnemyHit
		  ;sprite pos matches on y + 1?
		  clc
		  adc #1
		  cmp SPRITE_CHAR_POS_Y,y
		  beq .EnemyHit
		  ;sprite pos matches on y - 1?
		  sec
		  sbc #2
		  cmp SPRITE_CHAR_POS_Y,y
		  bne .CheckNextEnemy
		
.EnemyHit		
		  ;enemy hit!
		  stx SPRITE_HELD
		  inc SPRITE_HELD
		
		  ;call enemy hit behaviour
		  ldy SPRITE_ACTIVE,x
		  ;enemy is active
		  dey
		  dey
		  lda ENEMY_HIT_BEHAVIOUR_TABLE_LO,y
		  sta ZEROPAGE_POINTER_1
		  lda ENEMY_HIT_BEHAVIOUR_TABLE_HI,y
		  sta ZEROPAGE_POINTER_1 + 1
		
		  ;set up return address for rts
		  lda #>( .ShotDoneHit - 1 )
		  pha
		  lda #<( .ShotDoneHit - 1 )
		  pha
		
		  ;1 as return value
		  lda #1
		  jmp (ZEROPAGE_POINTER_1)
		
.CheckNextEnemy	
		  ldx PARAM2
		  ldy PARAM1
		  inx
		  cpx #8
		  beq .NoEnemyHit
		
		  jmp .CheckEnemy
.NoEnemyHit
		  jmp .ShotContinue
		

If the extra is picked up the range is simply increased to a max of 38 (Remember, screen width is 40 characters):

.EffectIncForceRange
		  cpx #0
		  beq .DeanDoesNotUseForce
		
		  lda PLAYER_FORCE_RANGE
		  clc
		  adc #2
		  sta PLAYER_FORCE_RANGE
		
		  cmp #38
		  bcs .NotTooLong
		
		  lda #38
		  sta PLAYER_FORCE_RANGE
		
.NotTooLong		
		  jmp .RemoveItem

Adding cheat keys is way easier. Since the Kernal (yes, with an 'a') comes with a keyboard check routine we just call that. Notice that for this to work you must not have the Kernal disabled (remember the memory layout in the beginning):

		  JSR $FFE4 ;GETIN
		  BEQ .NOCHEAT
		  CMP #49
		  bne .NOCHEAT
		
		  ;jump to next level
		  jsr StartLevel
		
		  inc LEVEL_NR
		  jsr BuildScreen
		
		  jsr CopyLevelToBackBuffer
		  jsr DisplayGetReady
.NOCHEAT

And still something new: Element Areas. A new primitive that fills an area with m/n repeats of an element:

!zone LevelElementArea
LevelElementArea
		  ;!byte LD_ELEMENT_AREA,24,16,5,1,EL_SN_BROWN_ROCK
		
		  ;X pos
		  iny
		  lda (ZEROPAGE_POINTER_1),y
		  sta PARAM1
		  sta PARAM10
		
		  ;Y pos
		  iny
		  lda (ZEROPAGE_POINTER_1),y
		  sta PARAM2
		  ;x count
		  iny
		  lda (ZEROPAGE_POINTER_1),y
		  sta PARAM7
		  sta PARAM9
		  ;y count
		  iny
		  lda (ZEROPAGE_POINTER_1),y
		  sta PARAM8
		  ;type
		  iny
		  lda (ZEROPAGE_POINTER_1),y
		  sta PARAM3
		  ;store y for later
		  tya
		  pha
		
.NextElementRow
		  jsr DrawLevelElement
		
		  dec PARAM7
		  beq .RowDone
		
		  lda PARAM1
		  clc
		  adc PARAM4
		  sta PARAM1
		  jmp .NextElementRow
		
.RowDone		
		  lda PARAM2
		  clc
		  adc PARAM5
		  sta PARAM2
		
		  lda PARAM9
		  sta PARAM7
		
		  lda PARAM10
		  sta PARAM1
		
		  dec PARAM8
		  bne .NextElementRow
		  jmp NextLevelData

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