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A C64 game - Step 19

Posted by Endurion, in C64 19 August 2011 · 570 views

Nothing really new this time, just constant use of existing parts. A new enemy, some sort of mummy, which slowly walks back and forth. And takes a few more hits than the bats.

It's a new behaviour added to the table and some extra code that checks for gaps before walking.



Attached Image


Adding the enemy itself is nothing more than adding new entries to the existing tables


;object type constants
TYPE_PLAYER             = 1
TYPE_BAT_LR             = 2
TYPE_BAT_UD             = 3
TYPE_BAT_8              = 4
TYPE_MUMMY              = 5


ENEMY_BEHAVIOUR_TABLE_LO          
          !byte <PlayerControl
          !byte <BehaviourBatLR
          !byte <BehaviourBatUD
          !byte <BehaviourBat8
          !byte <BehaviourMummy
          
ENEMY_BEHAVIOUR_TABLE_HI
          !byte >PlayerControl
          !byte >BehaviourBatLR
          !byte >BehaviourBatUD
          !byte >BehaviourBat8
          !byte >BehaviourMummy

IS_TYPE_ENEMY
          !byte 0     ;dummy entry for inactive object
          !byte 0     ;player
          !byte 1     ;bat_lr
          !byte 1     ;bat_ud
          !byte 1     ;bat 8
          !byte 1     ;mummy
          
TYPE_START_SPRITE
          !byte 0     ;dummy entry for inactive object
          !byte SPRITE_PLAYER_STAND_R
          !byte SPRITE_BAT_1
          !byte SPRITE_BAT_1
          !byte SPRITE_BAT_2
          !byte SPRITE_MUMMY_R_1
          
TYPE_START_COLOR
          !byte 0
          !byte 10
          !byte 3
          !byte 3
          !byte 8
          !byte 1
          
TYPE_START_MULTICOLOR
          !byte 0
          !byte 1
          !byte 0
          !byte 0
          !byte 0
          !byte 0
          
TYPE_START_HP
          !byte 0
          !byte 1
          !byte 5
          !byte 5
          !byte 5
          !byte 10


The behaviour code looks quite simple, as most of the new code is inside subroutines ObjectWalkLeft/ObjectWalkRight.

;------------------------------------------------------------
;simply walk left/right, do not fall off
;------------------------------------------------------------
!zone BehaviourMummy
BehaviourMummy
          lda DELAYED_GENERIC_COUNTER
          and #$3
          beq .MovementUpdate
          rts
          
.MovementUpdate
          inc SPRITE_MOVE_POS,x
          lda SPRITE_MOVE_POS,x
          and #$7
          sta SPRITE_MOVE_POS,x
          
          cmp #4
          
          bpl .CanMove
          rts

.CanMove
          lda SPRITE_DIRECTION,x
          beq .MoveRight
          
          ;move left
          jsr ObjectWalkLeft
          beq .ToggleDirection
          rts
          
.MoveRight
          jsr ObjectWalkRight
          beq .ToggleDirection
          rts
          
.ToggleDirection
          lda SPRITE_DIRECTION,x
          eor #1
          sta SPRITE_DIRECTION,x
          rts


The new subroutines are a more complex version of ObjectMoveLeft/ObjectMoveRight. They add a new check, so the enemy only walks forward, if it's not blocked and there is no gap in the floor in front:


;------------------------------------------------------------
;walk object right if not blocked, do not fall off
;x = object index
;------------------------------------------------------------
!zone ObjectWalkRight
ObjectWalkRight
                    
          lda SPRITE_CHAR_POS_X_DELTA,x
          beq .CheckCanMoveRight
          
.CanMoveRight
          inc SPRITE_CHAR_POS_X_DELTA,x
          
          lda SPRITE_CHAR_POS_X_DELTA,x
          cmp #8
          bne .NoCharStep
          
          lda #0
          sta SPRITE_CHAR_POS_X_DELTA,x
          inc SPRITE_CHAR_POS_X,x
          
.NoCharStep          
          jsr MoveSpriteRight
          lda #1
          rts
          
.CheckCanMoveRight
          ldy SPRITE_CHAR_POS_Y,x
          iny
          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
          iny

          lda (ZEROPAGE_POINTER_1),y
          jsr IsCharBlockingFall
          beq .BlockedRight
          
          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
          iny
          lda (ZEROPAGE_POINTER_1),y
          
          jsr IsCharBlocking
          bne .BlockedRight
          
          tya
          clc
          adc #40
          tay
          lda (ZEROPAGE_POINTER_1),y
          jsr IsCharBlocking
          bne .BlockedRight
          
          jmp .CanMoveRight
          
.BlockedRight 
          lda #0
          rts

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