More gameplay action, whee!

We add a new enemy, the beloved zombie. The zombie can appear from underground, walks about slowly, and if shot, dives in the ground again.

And while underground the zombie will move about. With the effect, that the zombie will probably appear some where else, not where the player is waiting with the shotgun.

Adding a new enemy is pretty easy now, a new entry in the behaviour table. There are a few more entries in other tables (start color, start sprite, etc.) which we don't go into detail now.

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

First off the generic counter is checked to slow the zombie movement. The most interesting part in this step is the added state for the enemies. The zombie makes good use of it, as it comes with several states:
-normal walk
-collapsing (2 part)
-reappearing (2 part)
-hidden (walk)

!zone BehaviourZombie
BehaviourZombie
          lda DELAYED_GENERIC_COUNTER
          and #$3
          beq .MovementUpdate
          rts
          
.MovementUpdate
          lda SPRITE_STATE,x
          bne .OtherStates
          jmp .NormalWalk
          
.OtherStates;collapsing?cmp #128
          beq .Collapsing1
          cmp #129
          beq .Collapsing2
          cmp #131
          beq .WakeUp1
          cmp #132
          beq .WakeUp2
          cmp #130
          bne .NotHidden
          jmp .Hidden
          
.NotHidden
          rts
          
.Collapsing1
          lda #SPRITE_ZOMBIE_COLLAPSE_R_2
          clcadc SPRITE_DIRECTION,x
          sta SPRITE_POINTER_BASE,x
          inc SPRITE_STATE,x
          rts
          
.Collapsing2
          lda #SPRITE_INVISIBLE
          sta SPRITE_POINTER_BASE,x
          inc SPRITE_STATE,x
          
          ;generate hidden time
          jsr GenerateRandomNumber
          clcadc #25
          sta SPRITE_MOVE_POS,x
          
          ;normalise position on full char
          ldy SPRITE_CHAR_POS_X_DELTA,x
          sty PARAM5
          
.CheckXPos
          beq .XPosClear
          jsr ObjectMoveLeft
          dec PARAM5
          jmp .CheckXPos
          
.XPosClear
          ldy SPRITE_CHAR_POS_Y_DELTA,x
          sty PARAM5
          
.CheckYPos
          beq .YPosClear
          jsr ObjectMoveUp
          dec PARAM5
          jmp .CheckYPos
          
.YPosClear
          rts
          
.WakeUp1
          lda #SPRITE_ZOMBIE_COLLAPSE_R_1
          clcadc SPRITE_DIRECTION,x
          sta SPRITE_POINTER_BASE,x
          inc SPRITE_STATE,x
          rts
          
.WakeUp2
          lda #SPRITE_ZOMBIE_WALK_R_1
          clcadc SPRITE_DIRECTION,x
          sta SPRITE_POINTER_BASE,x
          lda #0
          sta SPRITE_STATE,x
          sta SPRITE_MOVE_POS,x
          rts
          
.NormalWalkinc SPRITE_MOVE_POS,x
          lda SPRITE_MOVE_POS,x
          and #$7
          sta SPRITE_MOVE_POS,x
          cmp #4bpl .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
          
.Hidden;are we apt to wake up?dec SPRITE_MOVE_POS,x
          bne .RandomMove
          
          ;wake upinc SPRITE_STATE,x
          lda #SPRITE_ZOMBIE_COLLAPSE_R_2
          clcadc SPRITE_DIRECTION,x
          sta SPRITE_POINTER_BASE,x
          rts
          
.RandomMove;move randomly left/right
          jsr GenerateRandomNumber
          and #$1
          beq .MoveLeft
          
          ;move right if possible
          jsr CanWalkRight
          beq .Blocked
          inc SPRITE_CHAR_POS_X,x
          ldy #8
          sty PARAM5
          
.MoveSpriteRight
          jsr MoveSpriteRight
          dec PARAM5
          bne .MoveSpriteRight
          rts
          
.MoveLeft
          jsr CanWalkLeft
          beq .Blocked
          dec SPRITE_CHAR_POS_X,x
          ldy #8
          sty PARAM5
          
.MoveSpriteLeft
          jsr MoveSpriteLeft
          dec PARAM5
          bne .MoveSpriteLeft
          rts
          
.Blocked
          rts

Since shooting the zombie has a different effect than for any other enemy the hurt table acquires a new entry:

;behaviour for an enemy being hit
ENEMY_HIT_BEHAVIOUR_TABLE_LO
          !byte <HitBehaviourHurt ;bat LR
          !byte <HitBehaviourHurt ;bat UD
          !byte <HitBehaviourHurt ;bat8
          !byte <HitBehaviourHurt ;mummy
          !byte <HitBehaviourCrumble ;zombie          
          
ENEMY_HIT_BEHAVIOUR_TABLE_HI
          !byte >HitBehaviourHurt ;bat LR
          !byte >HitBehaviourHurt ;bat UD
          !byte >HitBehaviourHurt ;bat8
          !byte >HitBehaviourHurt ;mummy
          !byte >HitBehaviourCrumble ;zombie

HitBehaviourCrumble is completely tailored to the zombie, it merely sets the collapse state. The collapsing itself is done in the behaviour code.

!zone HitBehaviourCrumble
HitBehaviourCrumble
          lda SPRITE_STATE,x
          bne .NoHit
          lda #SPRITE_ZOMBIE_COLLAPSE_R_1
          clcadc SPRITE_DIRECTION,x
          sta SPRITE_POINTER_BASE,x
          lda #128
          sta SPRITE_STATE,x
          
.NoHit
          rts

step26.zip

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